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Opportunities in High Magnetic Field Science (2005)

Chapter: 2 Scientific Challenges and Opportunities with Higher Fields

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Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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2
Scientific Challenges and Opportunities with Higher Fields

Magnetic fields are powerful tools for studying the properties of matter because they couple directly to the electronic charge and magnetic moments of the protons, neutrons, and electrons of which matter is made up. The properties of most materials are only weakly dependent on the strengths of the magnetic fields to which they are exposed, and for these substances, magnetic fields can be used analytically to determine fundamental properties such as their characteristic electronic energy scales and the band structures of metals and insulators, the placement of atoms in molecules, or even the internal structure and dynamics of living creatures. On the other hand, in some materials the magnetic field couples strongly and dramatically influences their properties: for example, in quantum Hall devices, magnetic materials, and superconductors. For these substances, magnetic field strength is as important a thermodynamic parameter as temperature or pressure. Included in this category are many materials important for the production, control, and measurement of high magnetic fields such as high transition temperature (Tc) superconductors. As the committee argues elsewhere, improved understanding of these superconducting materials, which will derive in part from experiments done using state-of-the-art high-field magnets, will lead to the construction of better magnets.

Research using high-field magnets has been remarkably fruitful in the past.1

1  

For additional historical context, see National Research Council, High-Magnetic-Field Research and Facilities, Washington, D.C., National Academy Press, 1979; National Science Foundation, Final

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

(See Appendix A for a list of Nobel prizes awarded for research that used or significantly affected the development of high magnetic fields.) There is every reason to believe that it will continue to be so, especially if the field strengths of the magnets available to the scientific community continue to increase. In this connection, it is important to note that charged particles move in circular orbits in a magnetic field, the radius of which shrinks as the magnetic field strength increases. Similarly, the smallest size resolved by magnetic moment or spin probes shrinks with increasing field strength. Thus the need to study and characterize ever smaller objects, both those that exist in nature and those fabricated artificially, will not be satisfied unless magnets are fabricated that deliver fields of ever increasing strength and instrumentation is developed that supports their effective use.

Paralleling the distinction made above, this chapter is divided into three sections. It begins with a discussion of high magnetic field research in condensed-matter and materials physics that emphasizes new phenomena that are likely to be revealed and known phenomena that would be better understood if higher fields were available. The chapter continues with a discussion of the impact of high-field magnets on the disciplines of biology, chemistry, biochemistry, and physiology as a result of their use in instruments that exploit nuclear magnetic resonance (NMR). In particular, the committee highlights the impact high magnetic fields have had, and continue to have, on the study of the solution structures of biological macromolecules by NMR, on solid-state NMR of biological and inorganic materials, and on electron paramagnetic resonance (EPR) of metal centers in proteins and catalysts. The committee discusses the impact high magnetic fields have had on two forms of magnetic resonance spectroscopy that have developed since the Richardson report—namely, magnetic resonance imaging (MRI) and ion cyclotron resonance (ICR) mass spectroscopy. In all these areas, magnets that operate at higher fields than those available today would yield large scientific dividends.

CONDENSED-MATTER AND MATERIALS PHYSICS

High-field research in materials science is intrinsically multidisciplinary, merging ideas from physics, chemistry, biology, and engineering, and integrating both theory and experiment. It is pursued predominantly by condensed-matter physicists, the largest subfield within physics today. Materials science, the dominant activity at the world’s high magnetic field laboratories, utilizes techniques as diverse as thermal and electrical transport, thermodynamic characterization, magnetization, optical spectroscopy, and magnetic resonance. Many classes of materials are

   

Report of NSF Panel on Large Magnetic Fields, Arlington, Va., National Science Foundation, 1988 (also known as the Richardson report).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

investigated, and measurements are done over a wide range of temperatures, pressures, and magnetic fields.

In the 1920s, when high magnetic fields first became available in Europe, they were used initially to investigate simple metals and, later, semiconductors. This work resulted in the first experimental determinations of how individual electrons behave in solids and was extremely influential in the development of the theory of solids between 1930 and 1950. Among the many successes of this synergistic enterprise must be counted the first microscopic explanations of how electrical and thermal transport occur in metals and insulators, and why certain metals become magnetic. This work led ultimately to the development of the science that enabled invention of the first solid-state electronic device, the semiconductor transistor.2 It would be hard to overstate the impact of these developments on the economies of the industrialized nations in the second half of the 20th century.

Electronic correlations are at the intellectual heart of modern condensed-matter physics. Interactions within populations of electrons lead to emergent collective properties that transcend those of individual electrons, such as superconductivity, magnetic order, and even the formation of the electronic gaps that distinguish metals from insulators. These properties reflect a balance of interactions among the electrons in a population and are strongly affected by differences in dimensionality, crystal symmetry, the spin of constituent atoms, and chemical bonding. Research on correlated-electron systems deals with issues ranging from the most fundamental (e.g., determination of the mechanism responsible for high-transition-temperature superconductivity in copper oxide layered compounds) to the most highly applied (e.g., learning how to control the microstructure of materials so that high-Tc superconductors with the highest possible critical fields can be produced for superconducting magnet construction).

Historically, this field has been constantly refreshed and reinvigorated by the discovery of new materials, such as copper oxide superconductors, heavy fermion magnets and superconductors, organic conductors, and nanoscopic materials such as fullerenes. It has also benefited tremendously from the availability of low-dimensional semiconductor structures of improved quality and purity. As was the case for the noninteracting electron science of the early 1900s, experimental discoveries in this field have had a significant impact on the development of theoretical understanding, which in turn has led to fruitful suggestions about new directions to pursue in materials development. In the past three decades, eight Nobel prizes have been awarded for work in this field (2003, 1998, 1996, 1987, 1985, 1977, 1972,

2  

Indeed, it was a combination of cyclotron resonance and the Hall and de Haas–van Alphen effects that helped characterize the electron transport properties of solids that enabled these inventions.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

1970), most recently in 2003 to A. Abrikosov, V. Ginzburg, and A. Leggett for their work on superconductivity and superfluidity. High magnetic field research in advanced semiconductor structures in particular led to the discovery of the integer and fractional quantum Hall effects, resulting in the physics Nobel prizes awarded to K. von Klitzing in 1985 and to R. Laughlin, H. Stormer, and D. Tsui in 1998.

In addition to its intellectual importance, research in correlated-electron systems has already led to numerous technological advances, such as improvements in the sensitivity of the magnetic read heads used for information storage, which depend on the giant magnetoresistance of hybrid magnetic/metallic systems, and the improvements in communication that have resulted from the superior signal-to-noise ratios and interference rejection of high-Tc superconductor filters. The economic promise of this research area is enormous. Improvement in the properties of permanent magnets would impact both the efficiency of electric motors and the density and reliability of magnetic storage media. The quest to understand materials that become superconducting at high temperatures and to discover new materials that superconduct at even higher temperatures has already had important practical results. Improvements in magnetic field sensors and in key electronic components have resulted, as well as the development of high-field inserts for superconducting magnets, which will soon be used for research but may also have bioimaging applications. While currently only at the demonstration stage, superconducting power cables could have a huge economic and environmental impact by reducing power losses in electric transmission networks. Finally, electronic correlations induced by the collapse of metallic screening and finite size effects become increasingly important as the size of electronic components decreases. The trend toward miniaturization has naturally led to an increased interest in nanoscale devices that have novel electronic properties because the devices combine superconducting and magnetic components with more conventional semiconducting components.3 In every case, progress will be linked to the discovery of materials with improved collective properties.

Understanding how electronic correlations are manifested in the macroscopic behavior of correlated-electron materials is key to the rational design of future generations of advanced materials. This task will require the skills of both experimentalists and theorists from a broad range of disciplines and will need the most advanced tools and techniques. The next section outlines the most important classes of correlated-electron materials and highlights the role high-field measure-

3  

Parallel advances in the speed and miniaturization of electronics have allowed greater exploitation of high fields by enabling experiments in the compact, transient environments offered by pulsed-field magnets.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

ments play in developing our understanding of them at both the fundamental and the technological level.

Superconductors, heavy fermion compounds, and organic molecular metals are classes of complex materials in which magnetic, electronic, and structural properties are strongly related. When temperature, pressure, and doping are varied, the existence of multiple phases is often revealed. Paramagnetic, long-range, magnetically ordered, and superconducting phases are seen, which sometimes coexist. In high-Tc superconductors, the reference scale is the transition temperature Tc. In heavy fermion systems, it is the single-impurity Kondo temperature that competes with intersite magnetic couplings, while in organic conductors it is the coupling between chains or planes that often governs other properties.

High-Temperature Superconductivity

The discovery of high-temperature superconductivity in La2-xBaxCuO4 ceramics by J. Bednorz and K. Muller in 1986 inaugurated a new era in solid-state physics. Within the next 6 years, the family of high-temperature superconductors had expanded to include Y-, Bi-, Tl-, and Hg-based systems with maximum Tc ranging from 90 to 130 K, respectively, and more than 10,000 scientific papers had been published. Thus in the last decade of the 20th century, high-temperature superconductivity emerged as a major area in physics. Experiments done at high magnetic fields have contributed much to the characterization and elucidation of high-temperature superconductivity and indeed have revealed many of its more remarkable features.

Why is high-temperature superconductivity so important, or, more precisely, why do so many condensed-matter physicists choose to work on this subject? Both fundamental and practical considerations come into play. On the fundamental side, the essence of the challenge is to solve the strong correlation problem. What happens when the electrons in a metal can no longer be described using the noninteracting electron paradigm, L.D. Landau’s theory of Fermi liquids? How do electron-electron interactions change a half-filled band, which Landau’s Fermi liquid theory indicates should make an excellent metal, into an insulating antiferromagnet?

Nevertheless, the great theoretical challenges presented by the strong correlation problem do not in themselves explain the high level of international activity in this area. There is an additional ingredient—namely, the prospect that we might someday be able to make superconductors that work at room temperature and above. The idea of practical, room-temperature superconductors, with their distinctly quantum mechanical properties, such as the Meissner and Josephson effects, is tantalizing. What we now know about the mechanism of superconductivity in

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

the materials being investigated, which necessarily arises from Coulomb interactions and quantum statistics, suggests that transition temperatures of several hundred kelvin might be possible. It is this combination of fundamental theoretical importance and exciting practical potential that drives the field.

Avenues of Research

All high-temperature superconductors share a key feature that appears to be responsible for their high-temperature superconductivity: the presence of planes containing Cu and O atoms separated by bridging materials that act as charge reservoirs for those planes. These materials become superconducting at temperatures significantly higher than those of the previously known highest-Tc compounds, which are now called low-temperature superconductors: Nb compounds, for instance, have a maximum Tc around 23 K. High-Tc materials also have extraordinarily high upper critical magnetic fields (Hc2)—for example, 170 T for the widely studied YBCO and maybe 500 T for bismuth- and thallium-based compounds. On the one hand, the high critical fields of these materials make them attractive as conductors for use in high-field magnets, but on the other, their high critical fields are a serious barrier to their full characterization. The critical fields of many of these materials are so high that their normal (nonsuperconducting) states cannot be studied using even the most powerful magnets available today.4

Superconductivity in conventional materials is explained by a theory proposed by J. Bardeen, L. Cooper, and R. Schrieffer (BCS theory) and is understood to result from an interaction between electrons and phonons that causes an effective attraction between the electrons, allowing them to pair up. When a conventional superconductor becomes superconducting, the transition to this new, paired state causes a reduction in the potential energy of its charge carriers and a slight increase in their kinetic energy. The net amount of energy released is defined as the condensation energy. In many materials, the electron pairs that result have a fully symmetric internal symmetry, which is a natural consequence of phonon-mediated pairing. In the cuprate high transition temperature superconductors, condensed pairs have a different symmetry, which is indicative of an entirely different pairing mechanism.5 Thus models for superconductivity in these materials propose a different “glue” for binding carriers together.

4  

That is, studies of the normal state at relatively low temperature; even the HTS materials available today with the highest Tc are not superconducting at room temperature.

5  

The working fluid of superconductors consists of pairs of electrons (or pairs of the holes left behind in a crystal when an electron moves somewhere else). These Cooper pairs form a coherent state with specific symmetry properties.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

There are several reasons for the extraordinary focus on high-temperature superconducting (HTS) materials in recent years: their intrinsic scientific interest; the cross-disciplinary nature of the field, which reaches across boundaries that often divide materials scientists and chemists from experimental and theoretical physicists; the potential applications of materials that superconduct at temperatures above the boiling point of liquid nitrogen (77 K); and, finally, the possibility of finding a superconductor that has a critical temperature above room temperature. Applications for HTS materials include filters for cellular phone systems; superconducting transmission lines, generators, motors, transformers, and fault current limiters; higher field MRI instruments and NMR spectrometers; microwave systems; and (of course) magnetically levitated transportation systems.

The scientific challenge posed by HTS materials is more fundamental than simply understanding why they superconduct. The oxide high-Tc superconductors are a family of materials in which even the properties of the normal state are not as well understood as they are for metals like aluminum, lead, or niobium. The identities of the carriers of charge and spin—that is, the HTS equivalents of the electrons and holes in metals, semiconductors, and low-temperature superconductors—are still being debated. Thus one of the key challenges posed by these materials is understanding the physics of their normal states, either at temperatures above Tc or at fields high enough to quench their superconducting states. Since the low-temperature/high-field regime is inaccessible for many of these compounds because current magnets do not deliver fields high enough, most measurements of the HTS normal state have been done above Tc. This approach is often unsatisfactory because thermal energies are so large at those temperatures that the details of the physical phenomena of interest are obscured by thermal fluctuations.6

The competing phases of magnetism and superconductivity that exist in HTS materials are illustrated in the phase diagram provided in Figure 2.1. As the carrier doping (number of holes) is increased in these materials (usually by raising the oxygen content), they are transformed from an antiferromagnetic insulator into a metallic superconductor that has a Tc that is also dependent on carrier concentration and peaks at a level termed “optimal doping.” While this behavior has been understood for some time now, more recently a pseudo-gap regime has been added to the phase diagram. It is believed that in this range of doping and

6  

Note that transition temperature is not the only driving parameter. One might think that research in the low-temperature/high-field limit might best be done with HTS materials that have low transition temperatures. This is not necessarily so, because these materials are often less amenable to analysis using techniques such as photoemission and optical spectroscopy. Finally, sample purities for the different families of cuprate HTS compounds can vary widely.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

FIGURE 2.1 Generic phase diagram for CuO2 (cuprate) superconductors, showing temperature versus doping concentration (the latter variable maps onto the order parameter for the material). The properties of the cuprates vary with temperature (y axis) and the doping per unit cell of CuO2 (x axis). Theorists are unable to explain why the superconducting transition temperature (thick black line) is so high in the cuprates. However, if they could understand the behavior of the cuprates in the pseudogap region (blue), they might be able to explain high-temperature superconductivity.

temperature (above Tc), the carriers are paired but the pairs do not yet form a superconducting state.

As described in greater detail below, research using high magnetic fields has been critical to uncovering the secrets of high-temperature superconductivity. It is expected that high magnetic fields will continue to be essential as understanding of this phenomenon grows and potential applications are realized.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

Although the primary goal of HTS research is to understand the origin of the superconducting state in materials with the highest transition temperatures, much can be learned from the study of HTS materials with lower transition temperatures, because there is every reason to believe that the underlying physics is the same in all of them. The most promising material of this kind is YBa2Cu3O7-x (x ≈ 0.5, Tc ~60 K). It has approximately the same transition width, and the same level of intrinsic disorder as optimally doped YBa2Cu3O7-x (x ≈ 0.05, Tc ~92 K). The lower-Tc form has a lower carrier concentration, which should make it an easier material in which to observe quantum oscillations and cyclotron resonance. These phenomena can be used to characterize the shape of the Fermi surface in the normal state and to yield the effective masses of the carriers at the Fermi energy. Experiments with YBa2Cu3O6.5 in the normal state have additional advantages; the temperature can be stabilized using liquid nitrogen (~77 K).

It is fundamentally important to understand the behavior of the upper critical field, Hc2, of HTS materials as a function of temperature, which is why determination of the Hc2 of HTS materials at low temperatures has become an important issue in HTS research. So far most studies have been limited to fields less than 20 T, which are well below the zero-temperature values of Hc2 (~100 T). In addition, high magnetic fields may prove invaluable in revealing the nature of the pseudo gap in high-temperature superconductivity, a potential key to understanding the microscopic mechanism of the HTS state. These experiments will require very high magnetic fields—around 100 T—and at lower temperatures, even higher fields may be required.

High magnetic fields (>35-40 T) are a requirement for studying the H-T (magnetic field and temperature) phase diagram of the recently discovered two-gap superconductor MgB2. (For more discussion of MgB2, please see the section “Emerging Superconducting Materials” in Chapter 3.) The superconducting energy gap is essentially the energy needed to break the Cooper pairs apart: It also determines the thermodynamic properties of the material and is directly related to the superconducting transition temperature. Most superconductors have just one energy gap, but experiments suggest that magnesium diboride (MgB2) has two. The gaps correspond to transition temperatures of 15 K and 45 K and combine to give an overall transition temperature of 39 K. A spectacular increase in the upper critical field of MgB2 was achieved recently by selective alloying of s and p bands with nonmagnetic impurities. Hc2(0) values have increased tenfold: from 3 to 5 T for single crystals to 35 T for H∥c and to 50 T for H∥ab for dirty MgB2 samples. These advances offer an exciting opportunity for studying the novel physics of two-gap superconductivity, nonequilibrium interband phase textures, and vortex dynamics and pinning at high magnetic fields greater than 50 T. Because such studies require sweeps of magnetic field strength, extended averaging times for sensitive measurements, and carefully controlled conditions so that samples can be compared, steady-state fields are generally required.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×
Opportunities in Vortex Physics

Work on the mechanisms of superconductivity, particularly high-temperature superconductivity, has revealed a rich new scientific landscape. One of the important areas opened up by these activities, vortex physics, is described here in some detail because of its intellectual vitality.

There are two classes of superconducting materials: Type I and Type II. Type I superconductors correspond loosely to the pure element low-temperature superconducting (LTS) materials Hg, Sn, and Pb, in which superconductivity was first discovered. External magnetic fields are fully excluded from the bulk of Type I superconductors by surface currents flowing within the London penetration depth, and the critical fields at which superconductivity is lost (quenched) are very low, less than 0.1 T, which is why Type I superconductors have few commercial applications. Type II materials have much higher critical fields because they enter a mixed state at a lower critical field Hc1 of ~0.01 T (see Figure 2.2). Above Hc1, Abrikosov vortices form, which consist of flux tubes containing a quantum of flux 0 = 2 × 10–15 Wb. Each field filament is encircled by a supercurrent flowing to a depth equal to the London penetration depth. The centers of these vortices can be viewed as normal cores where the superconducting order parameter is suppressed. When a current is driven through the material, the flux lines experience a Lorentz force that tends to push them perpendicular to the current. If they so move, the process is dissipative and introduces resistance. This phenomenon is well understood, but exactly how magnetic fields penetrate into superconductors, how the flux lines move, and how they interact with defects in the material are not well known. The flux lines can also repel each other, so flux flow is a complex, many-body effect. We are now coming to understand that flux flow is like other dissipative effects such as earthquakes and avalanches, and the physics of vortex motion has many parallels to other areas of physics.

This quantized flux-tube state was first conceived of by A. Abrikosov to describe the situation that occurs when the superconducting coherence length ξ is much shorter than the penetration depth λ. He found that the vortex state was both quantized and characterized by a lattice structure, the existence of which was subsequently confirmed by neutron diffraction, magnetic decoration, and magneto-optical and transmission electron microscopy experiments. A particular curiosity of Type II superconductors is that the interface energy between their normal and superconducting regions is negative, making fine-scale subdivision of the vortex state energetically favorable. The vortex density depends on magnetic field as (0/B)0.5. Bulk superconductivity is destroyed when the normal cores of the flux tubes in a material overlap, which occurs at a field Hc2 of . Hc2 values can be remarkably high. For LTS materials such as Nb-Ti or Nb3Sn, the values are

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

FIGURE 2.2 Phase diagram for penetration of the magnetic field into a Type II superconductor. The green mixed-state region is where Abrikosov vortices are formed and vortex physics comes into play.

about 15 T and 30 T, respectively, but for cuprate superconductors with high Tc, Hc2 can exceed 100 T.

LTS and HTS Type II superconductors differ in the degree of interaction among the vortices with each other and with defects in the material; HTS materials offer a much richer spectrum of physics. In the essentially isotropic LTS (Type II) metallic superconductors, vortices are line objects with significant line tension and are thus effectively pinned by even dilute microstructural defect arrays, provided

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

that their size is optimum for interacting either with the vortex core size of diameter 2ξ or the screening currents that flow on a scale of the penetration depth λ. Since ξ << λ for all useful Type II superconductors, the engineering of pinning centers became an exercise in nanotechnology years before the word “nanotechnology” passed into common use. In optimized Nb-Ti wires, about 20 percent of the volume of ~3 nm-thick nonsuperconducting ribbons of α-Ti are dispersed in a superconducting matrix of Nb-Ti solid solution. These normally conducting ribbons pin vortices very effectively, allowing transport currents to develop that are about 10 percent of the depairing current density that would destroy superconductivity altogether. The strong vortex pinning in such wires allows highly stable currents to flow with virtually no dissipation, making them ideal for the fabrication of superconducting magnets.

By contrast, the highly anisotropic nature of the layered cuprate HTS materials makes the line tension of their vortices weak. Indeed in many of the cuprates, vortices break up into pancakes that lie on the cuprate planes, which are strongly superconducting. Correlations along the vortex line tend to be very weak, which makes thermal activation easy and allows a wide range of excitations that are essentially absent in the metallic superconductors. For instance, vortices can be set in motion by the Lorentz force from an externally applied transport current, enabling studies of driven phases, steady-state motion, and the new area of dynamic phase transitions.

In traditional superconductors, thermal energy is limited to about 20 K by the superconducting transition temperature, and the vortex tubes form an elastic solid. High-temperature superconductors offer a new possibility: Thermal energies up to about 100 K may melt the vortex solid, creating a novel liquid state with dramatically different properties that arise from the relative motion of vortices. As early as 1988, motion of vortices below Tc was found to create undesired dissipation in high-Tc cuprates. Theorists soon realized that vortex phases and phase transitions embody many fundamental features of condensed-matter physics, including reduced dimensionality, entanglement of flexible line objects, and the role of disorder in elastic media.

The nature of the vortex physics therefore depends not only on static variables such as pinning energy (controlled by defects in the material) and coupling energies (controlled by the anisotropy of the material) but also on dynamical variables, including temperature, magnetic field, and applied current. For instance, the density of vortices in a superconductor depends on the applied magnetic field; for an average HTS material, the separation between vortices is about 80 nm at 1 T, 6 nm at 50 T, and about 4 nm at 100 T. Adjustments to magnetic field and temperature can scan a broad range of potential interactions. The HTS materials therefore exhibit a wide range of vortex interactions that give rise to many new phenomena.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

Vortices control many aspects of the electromagnetic response of HTS materials. The richness of their vortex phase diagrams is caused by the fact that four energy scales (tilt, shear, pinning, and thermal energies) are of the same order of magnitude in the field regime determined by the properties of HTS materials. Virtually all studies so far have been made in weak fields of up to about 10 T, but much of the most interesting vortex physics, for instance, occurs in concentrated vortex arrays in YBCO at 25 to 50 T.

The nontrivial and nonlinear pinning of vortex dynamics—vortex creep—and vortex lattice melting at an irreversibility field Hirr much smaller than Hc2 has determined many aspects of the direction of HTS research. Even though many features of the macroscopic electromagnetic behavior of HTS materials have been successfully explained using vortex concepts, new theoretical predictions (including states of vortex matter such as vortex liquids, quasi-ordered lattices, and Bose glasses) continue to open avenues for experimental research. Understanding and controlling vortex pinning is particularly important for applications since strong pinning is required to develop the high critical current densities needed for driven-and, especially, persistent-mode NMR and MRI magnets.

These research avenues are addressed in greater detail in Appendix H.

Outlook for High-Temperature Superconductivity

It follows that the challenge presented by HTS materials today is both scientific and technological. The scientific challenge persists, despite the vast improvements in measurement and instrumentation capabilities that have been made since, say, the 1960s, when LTS materials were explored. For example, the BCS electron-phonon coupling can be measured quantitatively in tunneling spectroscopy experiments carried out between a polycrystalline film of Al and one of Pb. In contrast, the Bi HTS surface has been studied using scanning tunneling microscopes so stable that the same atoms can be measured over periods of weeks or months, with tunneling spectroscopy carried out atom by atom. Nevertheless, despite the power of such modern experimental techniques, HTS materials give up their secrets only very slowly.

For those interested in understanding HTS materials, it is an unhappy fact that over the past 50 years, field strengths of the magnets available for experimental work have not kept pace with the increase in the critical fields of the superconductors being discovered. The critical fields of Nb-Ti and Nb3Sn were measured using Bitter magnets in the 1960s, but the critical fields of many of the HTS materials being investigated today are not accessible with the magnets available today. One approach to the limitations imposed by this mismatch has been to study versions of the materials of interest, e.g., copper oxides, that have lower Tc

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

and Hc2. YBCO, which is widely studied, can be modified to have more accessible Tc and Hc2 by reducing its oxygen content. For obvious reasons, such work-arounds are only partly satisfactory; it is the materials with the highest Tc and highest Hc2 that offer the ultimate challenge and the ultimate opportunity. It is paradoxical that the development of the HTS materials that may make it possible to someday manufacture magnets delivering unprecedented high fields is being hindered because only magnets built of such HTS materials are likely to deliver fields high enough for their characterization.

Neutron experiments have played a crucial role in HTS work by providing insights into momentum- and energy-dependent spin gaps, as well as an increasingly precise overview of the spin fluctuations, which if not at the root of the superconductivity, are responsible for many of the anomalous normal state properties of HTS cuprates. Recently, a series of experiments done in Europe provided a remarkably clear demonstration of the value of linking the best available superconducting magnet technology with modern neutron instrumentation. These experiments revealed the antiferromagnetism of the vortex state in underdoped cuprates, as well as the sensitivity of the spin fluctuations in optimally doped samples to the vortex melting line, which had previously been seen only using bulk measurements sensitive to very long range phenomena. Additional work showed that a prominent singlet-triplet excitation (the resonance peak) for YBCO is also very sensitive to the presence of vortices. NMR experiments provided important early hints about the d-wave nature of high-Tc superconductors as well as their pronounced antiferromagnetic fluctuations. Most recently, work following the neutron experiments revealed the antiferromagnetic nature of the vortices in LSCO, suggesting similar behavior in YBCO.

Higher magnetic fields would make it possible to perform experiments beyond the upper critical fields of some of the higher Tc cuprates. These experiments would lead to an improved understanding of at least the phenomenology of materials that are more useful technologically. In addition, it would be tremendously valuable to enhance the capabilities available for doing various spectroscopies and microscopies at higher fields. Much has been learned from neutron and scanning tunneling microscopy at relatively modest fields in the last decade, and the big challenges of the next decade will be observing how vortices merge to form either a strange metal or an insulator using either position-resolved tunneling or momentum-resolved magnetic neutron scattering, or both. Other interesting opportunities lie in microwave and optical spectroscopies. Experiments of this kind would not just produce information similar to that already available, they would also make it possible to visualize terra incognita, the quantum phase transition believed to separate the high-temperature superconductor from its true zero-temperature parent phase. The committee points out that beyond providing a new

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

window on a still unsolved problem, these capabilities would greatly facilitate experimentation with other materials of high current interest: most quantum magnets, which might be hosts for protected qubits, as well as narrow bandgap semiconductors of potential use in magnetoelectronics, applications that take advantage of not only the electron’s charge but also its spin.

Ultimately, while the specific compounds discussed above provide good examples of exciting new materials that need to be investigated using high magnetic fields, it must be emphasized that they are not the only ones. All high-temperature superconductivity is ripe for high-field experimentation, and the higher the fields, the better. Generally speaking, pulsed fields with lengths on the order of 10 ms are required to provide sufficient measurement time for many studies; many other analyses will require the precision and stability obtainable only using steady-state fields. The opportunity to unlock the science of high-temperature superconductivity using high-field magnets is matched only by the opportunity to exploit this knowledge to design HTS materials that can meet the demands of the industrial marketplace.

Heavy Fermion Systems

The ability to understand and, ultimately, to predict the circumstances under which materials become magnetic is important at both a fundamental and an applied level. Nowhere is this more starkly evident than in the class of metallic magnets called Kondo lattices, or heavy fermion systems. Heavy fermions are compounds containing rare earth elements such as Ce or Yb or actinide elements such as U. Their (inner shell) conduction electrons often have effective masses (known as quasi-particle masses) several hundred times as great as that of free electrons, resulting in low Fermi energies. This property makes them reluctant superconductors. Yet at cryogenic temperatures, many of these materials are magnetically ordered, others show strong paramagnetic behavior, and some display superconductivity through mechanisms that transcend traditional BCS theory. Research suggests that Cooper-pairing in the heavy fermion systems arises from the magnetic interactions of the electron spins rather than from lattice vibrations. In this section the committee briefly describes the key topics in this quickly growing area of research and comments on the role that high magnetic fields can play.

All heavy fermion systems contain an ordered lattice of localized magnetic moments, generally due to the presence of rare earth or actinide elements, but systems based on transition metals are also studied. These moments interact with itinerant electrons contributed by other constituents of these intermetallic compounds. Much of the interest in these materials is spurred by the belief that their collective properties are well described by a generic phase diagram that is intriguingly similar to the generic phase diagram that characterizes high-Tc materials,

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 2.3 Generic phase diagram for heavy fermion systems in the plane of temperature (T) versus the order parameter (Γ). The superconducting phase (SC) surrounds the quantum critical point (Γc); on the right is the normal-state Fermi liquid phase (FL), with the pink region describing the non-Fermi liquid regime. Finally, the left-hand green area maps out the ferromagnetic and antiferromagnetic regimes.

despite the inherently different natures of these two classes of materials (see Figure 2.3).7 The phase diagram describes the stability of magnetic order as a function of an external variable G (also called the order parameter) such as pressure, chemical composition, or magnetic field. The dominant feature of the phase diagram is its quantum critical point at Γ = ΓC and 0 K, where magnetic order gives way to a mass-enhanced, nonmagnetic, metallic state from which this class of materials gets its name: heavy Fermi liquids, or simply heavy fermions. The quantum critical point in this phase diagram has attracted interest because some materials are superconductive when they are in this part of the phase diagram. The proximity of this superconducting state to the magnetically ordered phase suggests

7  

The interested reader may find more detail in two excellent reviews: G.R. Stewart, Non-Fermi liquid behavior in d- and f-electron metals, Rev. Mod. Phys. 73, 797 (2001) and A.C. Hewson and D. Edwards, The Kondo Problem to Heavy Fermions, London, England: Cambridge University Press, 1997.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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that the superconductivity observed is unconventional, mediated perhaps by magnetic rather than lattice fluctuations. Further, a new type of universal, but still unconventional, “non-Fermi liquid” behavior can be found in the vicinity of the quantum critical point that is associated with anomalous critical fluctuations.

Measurements done at high magnetic field have contributed substantially to our understanding of these systems by providing a means for studying the underlying electronic structures and by serving as a tuning parameter to adjust the underlying energy scales and, finally, as a tool for forcing the material into states that cannot be accessed at zero field. Since the experimental record is vast, the committee focuses here on a few illustrative examples.

The forces responsible for the suppression of magnetic order, and the generation of the quantum critical point in the phase diagram of Figure 2.3, remain a topic of debate. It is of central importance to determine whether the electrons responsible for the localized magnetic character found at small values of Γ leave the local moment sites and become itinerant as Γ approaches ΓC.

The remarkably robust superconductivity of the metal alloy (and heavy fermion system) CeCu2Si2 has been a long-standing mystery to condensed-matter physicists. One of the many triumphs of the BCS theory of superconductivity was its simple explanation for the strong suppression of the superconducting transition temperature Tc by magnetic impurities in metals. The local moments act with opposite sign on the two electrons of a spin singlet Cooper pair and hence are pair breaking. A by-product of this idea is the expectation that metals such as heavy fermions, whose low-temperature properties are strongly renormalized by magnetic effects, would not become superconducting. It was a shock, then, when superconductivity was discovered in CeCu2Si2 in 1979 by F. Steglich and colleagues in Dresden, Germany, only shortly after the existence of heavy fermion materials themselves was recognized.8 The discovery ignited research into the physics of these heavy fermion metals.

Magnetization experiments performed on CeRu2Si2 determined that a magnetic field of 7.8 T would drive this material from a nonmagnetic metallic state (Γ > ΓC) into a magnetically ordered state (Γ < ΓC).9 Quantum oscillations have been observed in the magnetization as the field varies, establishing the presence of itinerant electrons at all fields.10 However, there is a sudden change in the frequency

8  

F. Steglich, J. Aarts, C.D. Bredl, W. Lieke, D. Meschede, W. Franz, and H. Schafer, Superconductivity in the presence of strong Pauli paramagnetism: CeCu2Si2, Phys. Rev. Lett. 43, 1892-1896 (1979).

9  

F.S. Tautz, S.R. Julian, G.J. McMullan, and G.G. Lonzarich, The nature of elementary excitations below and above the metamagnetic transition in CeRu2Si2, Physics B 206-207, 29-32 (1995).

10  

H. Aoki and S. Uji, Transition of f-electron nature from itinerant to localized: Metamagnetic transition in CeRu2Si2 studied via the de Haas-van Alphen effect, Phys. Rev. Lett. 71, 2110 (1993) and Tautz et al., 1995 (see above).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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of the oscillations as the field exceeds 7.8 T, indicating that previously itinerant electrons have become localized on Ce moment sites at high field. While the 7.8 T is a field easily achieved by superconducting magnets in the laboratory, the success of such quantum oscillation studies depends critically on the availability of a wide range of magnetic fields for experimental use, both above and below the critical field. Subsequent experiments in which compositional variation was used to vary Γ through ΓC suggest that the Fermi surface restructuring seen is a general feature of quantum critical points and is responsible for the stabilization of magnetic order in a number of metallic magnets.

Ongoing research seeks to establish whether this result can be extended to different sorts of magnetic phase, and this information can only be obtained from quantum oscillation studies, particularly if the quantum critical point must be generated using external magnetic fields. Higher fields will be required for such quantum oscillation studies as more highly itinerant systems based on transition metals begin to be investigated. For instance, the unconventional magnetic superconductor UPt3 undergoes a similar moment delocalization transition at 20 T, and the same transition occurs in URu2Si2 near 40 T. Studies of these systems will have to await the availability of more powerful magnets equipped to study samples at dilution refrigerator temperatures.

As the experiments on CeRu2Si2 imply, magnetic fields can also be used to tune the stability of magnetism in intermetallic compounds, where electronic correlations are extremely strong and emergent energy scales are very small. Recent measurements of the YbRh2Si2 system provide perhaps the most dramatic demonstration of this tuning effect.11 In zero field, YbRh2Si2 is antiferromagnetically ordered below 0.065 K. Application of a 0.05-T field suppresses the magnetic order to 0 K and produces a nonordered state with anomalous properties. Its specific heat diverges logarithmically with temperature. Its resistance is linear at temperatures between 10 mK and 10 K, and its magnetic susceptibility is weakly divergent. At higher fields, normal nonmagnetic and metallic behavior is regained in YbRh2Si2. These observations suggest that materials tuned to the vicinity of a quantum critical point have special properties, perhaps originating with the critical fluctuations of the incipient magnetic phase. Indeed, it has been possible to show that resistance and magnetization are jointly controlled by the ratio of the field to the temperature, confirming this hypothesis. The committee notes in passing that

11  

P. Gegenwart, J. Custers, C. Geibel, K. Neumaier, T. Tayama, K. Tenya, O. Trovarelli, and F. Steglich, Magnetic-field induced quantum critical point in YbRh2Si2, Phys. Rev. Lett. 89, 056402 (2002) and J. Custers, P. Gegenwart, H. Wilhelm, K. Neumaier, Y. Tokiwa, O. Trovaerlli, C. Geibel, F. Steglich, C. Pepin, and P. Coleman, The break-up of heavy fermion electrons at a quantum critical point, Nature 424 (6948), 524-527 (2003).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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if a material is found that is naturally tuned so that in zero field and at low or zero pressure it is at its quantum critical point, its linear magnetoresistance could be very attractive for sensor applications. Although the field required to tune YbRh2Si2 to the quantum critical point is modest, similar observations have been made with different Kondo systems at fields as large as 35 T. As the field scale is material-specific, it is virtually inevitable that new quantum critical systems will be discovered as higher fields become available and as a wider range of high-field measurements makes more detailed analysis of high-field states possible.

In order to understand quantum criticality it is imperative that we discover and examine different types of magnets. These investigations will require an arsenal of instruments that operate at ever higher fields and ever lower temperatures. The instruments available are being pushed to the limit today, and to meet future needs, improved magnets, both DC and pulsed, and improved ancillary instrumentation will be required. Pulsed fields can provide access to different parts of the phase diagram, but transient effects (such as sample heating due to the magnetic field) and the need for precision measurements still drive the need for steady-state measurements in DC fields. The most detailed work requires excellent and simultaneous control of both temperature and field—very difficult to achieve with pulsed fields. However, such demanding conditions are often not required to establish properties of phase diagrams, to determine global features in insulating compounds, or (most notably) to get Fermi surface information via quantum oscillation measurements.12 A complementary approach using both pulsed and steady-state fields is necessary to identify new phenomena and then to study them in appropriate detail.

High magnetic fields are a particularly powerful means for suppressing collective instabilities that make it difficult to study the underlying disordered state from which the ordered state emerges. This approach is particularly illuminating when applied to investigations of the superconducting ground state, especially if the electrons in the superconducting condensate have unconventional pairing, but is also crucial for understanding the response of superconductors to high fields, which is essential if practical use is to be made of them. The way magnetic fields suppress unconventional superconductivity is unusually interesting. One example of a novel, field-driven state is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. In Type II superconductors, the field–temperature phase diagram shows a steady penetration of vortices into the superconducting material, leading to the termination of the superconducting state at a characteristic critical field. In superconductors of

12  

During the brief lifetime of the 60-T long-pulse magnet at NHMFL in Los Alamos it was convincingly demonstrated that experimental measurements of properties such as heat capacity and magnetostriction are very possible in pulsed fields of this sort.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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high purity and large electronic mean free paths, at sufficiently high fields, the superconductor may accommodate the increasing Zeeman energy of the Cooper pairs through a spatial modulation of the superconducting order parameter, involving layers of superconducting material separated by magnetic domain walls. Two different groups have recently reported signs of a transition from a conventional Type II superconductor to this novel FFLO superconducting state, as well as transitions in applied field between states of differing orbital angular momentum in the layered Kondo superconductor CeCoIn5 at temperatures around 0.8 K and fields over 10 T.13 Although there is mounting evidence that the superconductors in the Kondo magnets are unconventional, their superconducting transition temperatures are very low, limiting their technological importance. A more comprehensive description of superconductors in high fields appears above in the section on high-Tc superconductors.

High fields can also be used to destabilize other types of collective states in metallic magnets. For instance, an insulating ground state has been observed in zero field transport and inelastic neutron-scattering measurements on the Kondo lattice system Ce3Bi4Pt3. It was unknown whether the insulating gap implied by these measurements was a simple band structure effect, or whether it was the result of interactions among electrons quasi-localized near the Ce moments. In a first experiment of its kind, heat capacity measurements carried out using the Los Alamos 60-T long-pulse magnet showed that magnetic field suppressed the insulating gap, yielding a high-temperature state that was a normal metal.14 This experiment unambiguously demonstrated that the Kondo gap state is collective, resulting from magnetic interactions between itinerant electrons and localized moments.

Finally, there are certain collective states in metallic magnets that occur only in high magnetic fields. One striking recent example is provided by the heavy fermion system URu2Si2, where in zero field there is an unusual orbital ordering transition at 17 K, followed by a superconducting transition at lower temperature. When high magnetic fields are applied it is found that at 36 T, the Zeeman splitting of the conduction electron states suppresses the orbital order, and a reentrant magnetic state is stabilized between 36 and 39 T. This material is unique among metallic

13  

A. Bianchi, R. Movshovich, C. Capan, P. G. Pagliuso, and J. L. Sarrao, Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5, Phys. Rev. Lett. 91, 187004 (2003), and H.A. Radovan, N.A. Fortune, T.P. Murphy, S.T. Hannahs, E.C. Palm, S.W. Tozer, and D. Hall, Magnetic enhancement of superconductivity from electron spin domains, Nature 425 (6953), 51-55 (2003).

14  

M. Jaime, R. Movshovich, G.R. Stewart, W.P. Beyermann, M.G. Berisso, M.F. Hundley, P.C. Canfield, and J.L. Sarrao, Closing the spin gap in the Kondo insulator Ce3Bi4Pt3 at high magnetic fields, Nature 405 (6783), 160-163 (2000).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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systems in that a relationship between orbital order and superconductivity may be inferred, and unique also in that this relationship apparently protects the system against the establishment of itinerant magnetic order, a common instability for this class of materials. The most direct assessment of orbital and magnetic order can be obtained from x-ray and neutron scattering measurements, and they have been used to great effect in other classes of materials, most notably transition metal oxides, where the instabilities are present in zero field.

Research on heavy-fermion and related systems constitutes a major part of the National High Magnetic Field Laboratory (NHMFL) research portfolio. Researchers from around the world come to use the magnets, but it is less often that U.S. researchers go overseas to conduct this sort of research. The leading exception is for critical scattering experiments with high fields. The high-field capabilities for x-ray and neutron scattering are vastly superior in Europe, so U.S. researchers must go there for these sorts of experiments, at a considerable investment of their time and resources. These sorts of experiments are impossible in the United States today for systems such as URu2Si2, where magnetic and orbital instabilities are driven by high fields and can then be examined with beams from scattering sources.

Low-Dimensional Semiconductors

High magnetic fields have traditionally played an important role in the study of electronic properties of semiconductors. Even today, decades after the first breakthroughs, high magnetic fields play a crucial role in investigations of the electrical and optical properties of heterostructures, quantum dots, and superlattices. The semiconductor physics community now sees a clear need for magnetic fields well in excess of 30 T, combined with low temperatures, to explore the new physics associated with new materials and structures processed on the nanoscale. The availability of these high fields will also open up new fundamental areas for study—for example, systems where electron-electron interactions dominate—and will be of great assistance in the characterization, further development, and exploitation of new semiconductor materials. In this section, the committee focuses on compelling opportunities in the strongly correlated electron systems offered by low-dimensional semiconductors.

Research in low-dimensional electron systems in semiconductors is driven by more than just scientific curiosity. Many of the structures being investigated could become the building blocks for new solid-state device technologies. They might be used, for example, to produce useful devices based on novel electronic phenomena, such as Bloch oscillations in modulated structures or Coulomb blockades in quantum dot systems. They are also prime candidates for use in the currently exciting fields of spintronics and quantum computing. Studies of low-dimensional

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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semiconductor systems at high magnetic fields provide information regarding their charge transport, energy level structure, and spin states and the way electronic interactions engender these properties. Such information is crucial for the successful use of such systems in any potential application.

In the last 15 years, there have been many exciting developments in the science of low-dimensional semiconductors. In low-dimensional semiconductors charge carriers are confined to the interface between two semiconductors that have different bandgaps. They are often modulation-doped, which is to say that they have dopant atoms that are placed in the higher bandgap material at some distance from the interface, while their mobile charge carriers (electrons or holes) reside in the smaller bandgap material in a plane very near the interface. Since their charge carriers can essentially move freely in that plane, the system is effectively two-dimensional (2D). The spatial separation between the 2D carrier plane and the dopants (ionized impurities) significantly reduces the scattering of carriers by dopant atoms, making such systems ideal for studying electron-electron interactions in two dimensions. This effect is particularly interesting at low temperatures and high magnetic fields, where the kinetic energy of the carriers is quenched and Coulombic interactions between carriers dominate the properties of the system.

Investigation of the properties of 2D semiconductors at low temperatures and high magnetic fields is one of the richest areas in condensed-matter physics today. Such studies have already led to the discovery of the integral and fractional quantum Hall effects (IQHE and FQHE) and to two Nobel prizes in physics (1985 and 1998). As elaborated below, electrons in such “flatlands” continue to reveal unanticipated phases and new phenomena, which derive from subtle, nonintuitive electron-electron interactions (see Figure 2.4). By confining the carriers in the lateral directions, various one-dimensional (quantum wire) and zero-dimensional (quantum dot) carrier systems can also be formed, which are equally interesting.15

Over the years, studies of 2D carrier systems in semiconductors at high magnetic fields have led to the discovery of new states of matter. The FQHE is a good example. The carrier system involved is an intrinsically many-body, incompressible quantum liquid that, in the limit of zero temperature, can flow without dissipation. It is described by the Laughlin wave function, a many-electron function that has the Coulomb repulsion between the electrons built in, keeping them far apart. The electrons described by the FQHE wave function exhibit short-range correlation

15  

For additional background on these topics, please see S.D. Darma and A. Pinczuk, eds., Perspectives in Quantum Hall Effects: Novel Quantum Liquids in Low-Dimensional Semiconductor Structures, John Wiley and Sons, New York, 1997, or S. Girvin, The quantum Hall effect: Novel excitations and broken symmetries, Topological Aspects of Low Dimensional Systems, A. Comtet, T. Jolicoeur, S. Ouvry, and F. David, eds., Springer-Verlag, Berlin, 2000.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 2.4 Magnetotransport data for a state-of-the-art 2D electron system in a GaAs/AlGaAs heterostructure. The FQHE is a fascinating manifestation of collective behavior in a 2D system of strongly interacting electrons. At particular magnetic fields, the electron gas condenses into a remarkable quantum liquid state, which flows without dissipation in the limit of zero temperature: It has vanishing resistance and a quantized Hall voltage. This state is very delicate, requiring high-quality material with a low carrier concentration and extremely low temperatures. The key feature is the strong systematic dependence of the resistance on the applied magnetic field; the detailed structure is attributable to different collective phases of the electrons. Above: Data taken in a superconducting magnet showing a wealth of features, many of which provide new insights and twists in the many-body physics of 2D electrons. Below: Data on the same sample at higher fields and higher temperatures, providing information on the transition from a Wigner solid state to a liquid of fractional quantum Hall states. Together, these plots show how magnetic fields can be used to control and explore new phenomena in low-dimensional semiconductor systems. [See W. Pan, H.L. Stormer, D.C. Tsui, L.N. Pfeiffer, K.W. Baldwin, and K.W. West, Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor, Phys. Rev. Lett. 88, 176802 (2002) for details.] Figures courtesy of W. Pan, Sandia National Laboratory.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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in their positions (hence they behave as a liquid), but they do not form a crystalline solid phase and do not possess long-range order. Until its experimental discovery in 1982, this phase of matter was entirely unknown.

When the Final Report of NSF Panel on Large Magnetic Fields appeared in 1988, only two states of 2D electrons at high field were known: the IQHE and FQHE states. The report emphasized the need for higher magnetic fields to find out if other 2D phases exist. A particular example cited was the Wigner crystal state, a state in which the electrons minimize their Coulombic repulsion by forming an ordered array. This state was expected to occur in extremely high-quality (low-disorder) samples at very low Landau level fillings (ν 1/5) and very high magnetic fields ( 30 T). Since 1987, remarkable progress has been made in the production of semiconductor crystals by molecular beam epitaxy at Bell Laboratories (Lucent Technologies) and elsewhere. Among other things, this progress has resulted in the production of GaAs/AlGaAs-based heterostructures of superb quality with very high mobilities. The study of these materials in instruments that combine higher DC magnetic fields and lower temperatures than were previously available has led to the discovery of a remarkable number of new electron states and phenomena (see Figure 2.4 (bottom) for examples of the magnetoresistance data that can be obtained from a state-of-the-art GaAs/AlGaAs 2D electron system). These phenomena arise from the collective interactions of the electrons in 2D systems and include the following:

  • Composite fermions,

  • Skyrmions,

  • Even-denominator FQHE states,

  • Ferromagnetic phases and transitions between QHE states, both in the integer and the fractional regimes,

  • Striped phases at large half-integer fillings,

  • Reentrant QHE and insulating phases at high fillings,

  • Insulating states at very low fillings, suggestive of a pinned Wigner crystal phase,

  • Radiation-induced zero-resistance states at low fields, and

  • Various phenomena in bilayer systems, including bilayer QHE states stabilized by interlayer interaction and phase coherence.

These tremendous discoveries, some completely unexpected, have far exceeded the expectations of most experts. Each has added a new twist to the rich physics of interacting electrons in a high magnetic field. For example, a new electron state with a highly anisotropic in-plane conductivity was discovered very recently at half-integer Landau level fillings (e.g., at ν = 9/2). This is believed to be an interaction-

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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induced “stripe” phase, composed of narrow, parallel regions of QHE states with ν = 4 and ν = 5. Another example is the emergence of skyrmions near the ν = 1 filling factor. These are low-energy excitations of the ferromagnetic QHE ground state at ν = 1 and are charged, finite-size, electron spin textures. Again, the Coulomb interaction is responsible for bringing about these excitations.

These discoveries have depended on the availability of high-quality samples and high magnetic fields. Equally crucial has been the availability of electronically and mechanically quiet environments for making observations. Progress in this field will continue in parallel with improvements in sample quality. It is clear, however, that as the quality of samples improves, lower temperatures and better instrumentation will be needed to explore their ever more subtle properties. Ample access to state-of-the-art magnets will also be important, because the experiments necessary to reveal new phenomena will be time-consuming explorations of the properties of these materials as a function of many variables—for example, sample and carrier density, temperature, and sample orientation with respect to the direction of magnetic field.

Much can be learned from the study of one-dimensional (1D) and zero-dimensional (0D) systems at high magnetic fields and low temperatures. Experiments of this kind will be especially revealing when the magnetic length becomes comparable to the system size. Since the length scale of a magnetic field is inversely proportional to the square root of its field strength, fields on the order of 100 T are needed to probe scales of about 2.5 nm. These measurements can provide information about the interplay between electron-electron interactions, confinement, and magnetic energies in small systems containing only a few electrons. Electronically quiet environments will be even more important for this research than they are for 2D systems, because 1D and 0D systems are unusually sensitive to mechanical and electronic noise.

The very high fields offered by pulsed magnets are helpful in uncovering entirely new phenomena in these materials, but longer-term magnetic field stability is required for detailed studies of the underlying mechanisms. In summary, the advance of research in 2D electron systems as well as 0D and 1D systems hinges importantly on (1) substantially improved access to the fields presently available, (2) improved environmental stability (including lower electronic noise levels), (3) greater access to lower temperatures, and (4) availability of magnetic fields beyond 30 or 40 T.

Organic Conductors and Superconductors

Organic superconductors are interesting experimental systems because they are well understood chemically and because their low Fermi energies and lower

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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dimensionality favor correlated electron ground states and novel types of superconductivity. Experimentally, using equipment available today, one can observe their properties at magnetic fields well above those necessary to quench their superconducting—or other correlated electron—states and thereby explore quantum oscillations and probe their Fermi surfaces. In this way, the fundamental properties of the charge carriers can be explored and current theories of correlated electrons can be tested.

Charge transfer salts composed of organic molecules are among the most interesting electronic materials ever discovered.16 Many of their unusual properties are only evident at high fields. Their molecular architecture leads directly to lower electron densities, smaller bandwidths, and larger electronic anisotropies than those found in their inorganic cousins. Nevertheless, their strong Coulomb interactions make them strongly correlated electron systems, and they exhibit virtually all of the cooperative ground states associated with such systems: superconductivity, magnetism, non-Fermi liquid behavior, as well as additional instabilities usually found in low-dimensional systems—for example, QHE, spin and charge density waves, and localization from disorder and from interactions. Their electrochemical growth yields clean materials with very large mean free paths (1-10 mm) and hence strong orbital effects in magnetic field.17

Early experiments on the quasi-1D TMTSF (tetramethyltetraselenafulvalene) salts showed that in open-orbit electronic systems, the magnetic field anisotropically shrinks the electron wavefunction, resulting in a field-induced dimensional cross-over. This is manifest as a metal-antiferromagnetic insulator transition, or field-induced spin density wave. The anisotropy of these materials made it possible to separate the orbital and spin effects associated with their superconductivity and pointed to a triplet spin pairing state.

The discovery of a class of quasi-2D charge transfer salts (known as bis(ethylenedithio)tetrathiafulvalene, or BEDT-TTF, salts) with closed orbits has led to a flurry of high magnetic field studies. In both quasi-1D and quasi-2D materials, long mean free paths lead to qualitatively new magnetotransport

16  

For an introduction to these topics, see J. Singleton and C. Mielke, Quasi-two-dimensional organic superconductors: A review, Contemp. Phys. 43, 63-96 (2002), or S. Chakravarty and P.W. Anderson, Interlayer Josephson tunneling and breakdown of Fermi liquid theory, Phys. Rev. Lett. 72, 3859 (1994).

17  

S. Uji, T. Terashima, H. Aoki, J.S. Brooks, M. Tokumoto, N. Kinoshita, T. Kinoshita, Y. Tanaka, and H. Anzai, Fermi-surface reconstruction in the organic conductor (BEDT-TTF)2(TlHg(SCN)4), J. Phys. Condens. Matter 6, L539-L547 (1994); O. Klein, K. Holczer, G. Gruner, J.J. Chang, and F. Wudl, Electrodynamics of the superconducting state of kappa (BEDT-TTF)2Cu(NCS)2, Phys. Rev. Lett. 66, 655-658 (1991).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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phenomena. In particular there are enormous angular-dependent magnetoresistance oscillations, which allow direct measurements of the Fermi surface shape and parameters. A wealth of Fermi surface instabilities and reconstructions allow quantitative investigation of nesting vectors, spin effects, and coupling parameters. There are, additionally, giant de Haas-van Alphen and Shubnikov-de Haas oscillations, which can be fit in sufficient detail so that electron-phonon interactions can be characterized and the nature of the superconducting state explored. With these clean, low-electron-density, anisotropic samples, it has even been possible to observe the quantum Hall effect in bulk crystals.

Recent discoveries are as exciting as those that preceded them. For example, field-induced superconductivity has been observed in a BETS salt (bis(ethelynedithio)-tetraselenafulvalene), a most beautiful example of the Jaccarino-Peter effect, i.e., compensation of an externally applied magnetic field by an internal exchange field. There are also several hints that the inhomogeneous FFLO superconducting state exists in BEDT salts. There are even experimental indications of the existence of a new state of a field-induced charge density wave, analogous to the FFLO state, where spin splitting competes with gapping of the Fermi surface.

These experiments have typically pushed the capabilities of magnet facilities to their limits of high field, low temperature, high pressure, and exact alignment of the field with the anisotropic crystal axes.

Combining High Fields with X-Ray and Neutron Scattering

Neutron scattering has been at the forefront of research into magnetic materials since 1948, when the first direct observation of antiferromagnetism was made by C.G. Shull and J.S. Smart.18 Beginning in the 1960s, the observation of collective magnetic excitations by neutron scattering provided direct information on exchange interactions, soft magnon modes and phase transitions, magnon-phonon interactions, and other phenomena. Collective magnetic excitations were observed in many different materials, including metals, which were itinerant ferromagnets where the existence of such excitations had been questioned. More recently, neutron-scattering experiments at high magnetic fields have provided information about quantum magnetic systems, especially at low dimensionality, leading to new insights into many theoretically predicted phenomena, such as the collective effects leading to a gap in the excitation spectrum of 1D spin-1 chains (the Haldane gap). Work in Europe at the highest fields available has shown that high magnetic

18  

C.G. Shull and J.S. Smart, Detection of antiferromagnetism by neutron scattering, Phys. Rev. 76, 1256 (1949).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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fields induce striped antiferromagnetic order in some high-Tc materials, a key observation for the theory of these materials.19

While the use of x-rays for magnetic scattering studies is of more recent vintage, the rapidly increasing brightness of synchrotron sources has made many new kinds of experiments possible. In 1988, D. Gibbs et al. showed the resonance and polarization behavior of x-ray scattering by studying the spiral magnetic structure in Ho.20 Resonant magnetic x-ray scattering was later used by M.B. Salamon to investigate the magnetic properties of the induced moment of Lu in a Dy-Lu alloy, where the atomic selectivity afforded by the energy dependence of the resonance allowed direct observation of the Lu scattering in the presence of the much stronger scattering from Dy. High photon fluxes also have allowed studies of surface magnetism and the change in magnetic behavior as a function of depth below the surface.

When higher magnetic fields become available at synchrotron light sources, many new scientific opportunities will open up. For example, the hole-doped cuprates have high critical fields, and the instruments available today do not allow observations to be made on these materials at interesting ratios of applied to critical field. In the area of heavy fermions, where the range of fields currently available is not adequate for exploring the quantum critical points of many systems, instruments operating between 20 and 25 T would open important new possibilities. The same range of fields would make it possible to study induced magnetic moments in nonmagnetic materials, both molecular and metallic (such as Lu). In this case, there are exciting opportunities for x-ray studies because the atomic specificity provided by such resonant scattering should make it possible to isolate the magnetic signal component—for example, to observe the Lu contribution in a Lu-Dy alloy. Furthermore, there are 3D quantum systems where the interesting physics begins at 20 T, a field that is not available at an x-ray- or neutron-scattering facility anywhere in the world. An equally important opportunity would be the pursuit of structural and dynamic properties in complex organic and biological systems. High fields, when coupled with neutron and x-ray sources, will provide a new variable-contrast agent for pursuing these studies. The possibility of polarizing unpaired electrons is similarly exciting.

19  

B. Lake, H.M. Rønnow, N.B. Christensen, G. Aeppli, K. Lefmann, D.F. McMorrow, P. Vorderwisch, P. Smeibidl, N. Mangkorntongstar, T. Sasagawastar, M. Noharastar, H. Takagistar, and T. E. Mason, Antiferromagnetic order induced by an applied magnetic field in a high-temperature superconductor, Nature 415, 299-302 (2002).

20  

D. Gibbs, D.R. Harshman, E.D. Isaacs, D.B. McWhan, D. Mills, and C. Vettier, Polarization and resonance properties of magnetic x-ray scattering in holmium, Phys. Rev. Lett. 61, 1241-1244 (1988).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Current Capabilities

During the past two decades, major user facilities for x-ray and neutron scattering were constructed in the United States and older user facilities were upgraded. The Advanced Photon Source at Argonne National Laboratory and the Advanced Light Source at Lawrence Berkeley National Laboratory (LBNL) are world-class, third-generation synchrotron radiation sources equipped with a wide range of instrumentation. The Stanford Synchrotron Radiation Laboratory at the Stanford Linear Accelerator Center, the National Synchrotron Light Source at Brookhaven National Laboratory, and the Cornell High Energy Synchrotron Source have also been steadily upgraded. Instrumentation at the Center for Neutron Research of the National Institute of Standards and Technology has improved significantly since 1989, instrumentation at the High Flux Isotope Reactor at Oak Ridge National Laboratory is being upgraded now, the source and the instrumentation at the Lujan Scattering Center at the Los Alamos Neutron Scattering Science Center in Los Alamos National Laboratory have been greatly enhanced, and the Intense Pulsed Neutron Source at Argonne National Laboratory continues to operate reliably with a first-class suite of instruments. The Spallation Neutron Source (SNS), now under construction at Oak Ridge National Laboratory, will be the world’s most intense neutron source when it begins operation, and it will eventually be equipped with a comprehensive suite of instruments. These developments will place the United States at the forefront of scattering research.

Some provision has been made at U.S. scattering facilities for users who want to study the effect of magnetic fields on their samples, but as Table B.2 in Appendix B documents, the United States has yet to take advantage of the research opportunities that would be created by combining the world-leading capabilities of the NHMFL in magnet design and construction with the outstanding x-ray and neutron capabilities it already has in place. In user surveys done at U.S. scattering facilities, the top concern of the materials science and condensed-matter physics community is sample environment and, in particular, the availability of magnetic fields. Current capabilities limit the range of properties that can be studied at these photon- and neutron-scattering facilities and are especially confining in the field of high-Tc systems, where researchers cannot conduct studies at fields near the critical field strength.

At both the European Synchrotron Research Facility in Grenoble (x rays) and the Hahn-Meitner Institute in Berlin (neutrons), the facilities for doing scattering/diffraction research at high magnetic fields—both those now in place and those planned—are far superior to any in the United States. For example, the neutron-scattering center at the Hahn-Meitner Institute, which already has capabilities unmatched at any U.S. facility, has submitted a proposal for a 40-T magnet21 and

21  

Installation of the 40-T magnet has been delayed for funding reasons.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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is proceeding with plans for a 25-T superconducting magnet, and the European neutron-scattering community has identified the 40-T magnet as its highest priority. Similarly, the FELIX user facility in the Netherlands already combines a high-power subpicosecond laser with 16-T fields. (See Table B.2 in Appendix B for a survey of selected efforts around the world to incorporate high-field instruments with premier scattering centers.) Using these capabilities, European researchers have recently made important advances in probing the mechanisms of high-temperature superconductivity. For instance, neutron experiments in the presence of magnetic fields have revealed the antiferromagnetism of the vortex state in underdoped cuprates.

Seizing the High-Field Scattering Opportunity

The United States would have a world-leading capability for studying the responses of materials to high magnetic fields with neutrons if it developed a suite of instruments with magnetic field capabilities at SNS. Ideally this set of instruments would enable users to do single-crystal diffraction, powder diffraction, small-angle neutron scattering, engineering (residual strain and preferred orientation) studies, neutron reflectometry, and inelastic scattering, all at high magnetic fields. By carefully planning the locations of such instruments, it should be possible to keep stray magnetic fields from interfering with other instruments at SNS. In considering needs at SNS, one should not forget the opportunities that exist for improving high magnetic field instrumentation at existing neutron facilities. Capabilities comparable to or exceeding those now available in Europe could be achieved at those facilities also. Both at SNS and at existing facilities, many of the needs of the user community could be met with off-the-shelf 20-T superconducting magnets, and more aggressive goals might be satisfied with resistive DC magnets operating in the 25-T range. In any case, these developments will require close interagency cooperation and planning, which will have to begin immediately if high-field instrumentation is to be available at SNS when it becomes operational. The White House Office of Science and Technology Policy interagency working group on neutron-scattering facilities is one entity organizing and prioritizing these efforts.

U.S. photon sources are beginning to provide magnetic field capabilities, and requests for such capabilities are beginning to turn up. The Advanced Photon Source at Argonne is planning a workshop where the opportunities provided by high magnetic fields will be explored. Although, unlike neutrons, x-ray photons do not couple directly with the magnetic properties of the materials with which they interact, their disadvantage in this regard is compensated for by the extraordinary brightness of the photon beams now available at synchrotron light sources. Even

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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though the study of magnetic systems with x rays is a relatively young enterprise, it is clear that new opportunities exist.

A significant investment would have to be made to make state-of-the-art magnetic-field experimentation possible at U.S. x-ray and neutron user facilities, but as has already been made clear, the payoff would be more than commensurate. Not only would the capital costs for constructing the equipment required be considerable, it would require a long-term commitment of new operating funds. The committee heard estimates that it might take $1 million or $2 million per year to staff a state-of-the-art magnet facility for 24-7 operation at a neutron or x-ray facility, while electric power costs for a steady-state magnet at the highest fields would be on the order of $100,000 per day, assuming the need for a 30-40 MW power supply and corresponding cooling facilities. While a pulsed magnet would be cheaper to build and operate, the mismatch between neutron pulsing rates (20-200 Hz) or x-ray source pulsing rates (MHz) and magnet pulsing rates (1 Hz) would result in severe performance penalties. The trade-offs will have to be studied in detail before they can be properly evaluated. Furthermore, the costs of making high magnetic fields available at neutron and x-ray facilities are probably so high that the number of such systems that can be built will be limited, and care will have to be taken to ensure they are installed where they can be used most effectively. It is important to note that although the number of high-field magnet suites that can be built at U.S. scattering facilities is certain to be limited by economic considerations, there is a strong need for building at least a few of them. The kinds of measurements they would enable cannot easily be made by scientists who can make only occasional, short-duration visits to facilities in other nations; such measurements require much time. Additionally, these high magnetic field tools would leverage investments that have already been made at existing U.S. resources and would greatly expand domestic science capabilities.

The committee notes that interest has been expressed in solving this problem by buildng a new light source, either a synchrotron or a free electron laser, at NHMFL. In its estimation, this approach is unlikely to be a wise one. First, the nation should invest in high-field instrumentation at its neutron sources before it turns to high-field instrumentation at synchrotron sources. Second, it is likely to be much cheaper to bring high-field instrumentation to existing synchrotron and neutron sources than to build an entirely new synchrotron light source at NHMFL, both in terms of construction cost and long-term operating costs.

Future Needs

The phenomena described above illustrate the amazing range of behaviors displayed by correlated electron systems that are either driven by, or can be probed

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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by, high magnetic fields. This research area is active and vital, and future advances will come from the availability of higher fields and a broader range of precision high-field measurement techniques.

The scale of characteristic fields in correlated-electron systems differs considerably from material to material. So far, most experimental work has concentrated on systems with low characteristic field scales and has involved experiments that use slowly varying magnetic fields. Studies of these systems would benefit greatly from higher field superconducting magnets, an expanded hybrid magnet program, and also the development of high-field, long-pulse magnets. However, equal attention must be paid to improving both the variety of techniques that can be employed at high field and the availability of these techniques to members of the materials science community. Today, we are very far from being able to study any high-field instability in the same detail as an instability that occurs in zero field, largely because of the difficulties that surround the execution of high-field experiments over a wide range of temperatures or in complex environments, such as those supplied by high-pressure cells. Particularly pressing is the need for the development of quantitative magnetometry; improved techniques for carrying out thermal conductivity and heat capacity measurements in pulsed field magnets; better methods for making low-noise, low-signal-magnitude electrical transport measurements, especially at low temperatures; and the integration of general-use pressure and uniaxial stress cells into high-field environments. Furthermore, investment in equipment for producing low temperatures in magnets of all types—superconducting, Bitter, and pulsed—would have an enabling effect on this endeavor, where strong correlations lead to intrinsically small energy scales and the current focus on quantum mechanical effects increasingly directs attention toward the lowest temperatures and the largest values of the field strength:temperature ratio. Finally, it should be noted that high magnetic fields share a strong connection with nanoscience and technology: High fields are both enablers of and enabled by developments at the nanoscale. Various magnetic lengths, such as the flux lattice constant, decrease with increasing magnetic field, allowing true nanometer-scale probes. Similarly, detailed behaviors that can be controlled at the nanoscale could be used in the generation of substantially higher field magnets.

As already noted, nowhere is the gap between the instrumentation available for experimentation at zero field and that available for high-field experimentation wider than in neutron and x-ray scattering. This is particularly unfortunate because scattering experiments provide a powerful means for elucidating atomic and magnetic structure as well as determining the nature of the spatial and dynamical correlations in materials. The development of new high-field capabilities at x-ray-and neutron-scattering centers in the United States could have an enormous impact.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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High-Field Facilities for Materials Research

Because the scientific and technological importance of magnetism is widely appreciated, there are centers focused on high magnetic field research throughout the world. The committee identified 32 such centers, including those in the United States, all of which offer access to outside users to some degree. Table B.1 in Appendix B catalogs those centers and summarizes the instrumentation available at each; Table B.2 in Appendix B outlines some of the magnetic field capabilities at selected scattering centers. Appendix B also includes short descriptions of the capabilities at these centers and their research interests.

Just by virtue of the number of its magnet stations, the NHMFL is by far the most important facility in the United States offering users access to high magnetic fields (see Appendix B). The NHMFL has led the United States to world leadership in the production and use of high magnetic fields in many areas of materials research, but it would be unwise to assume that its success is a sure sign of U.S. dominance in all other areas of high-field research. In the first place, materials science is only one of many scientific disciplines that use high magnetic fields. In the second, centers outside the United States, both those that provide services to outside users and those that do not, give stiff competition across the entire spectrum of high-field research.

MAGNETIC AND ION CYCLOTRON RESONANCE: APPLICATIONS OF HIGH FIELDS TO BIOLOGY, CHEMISTRY, AND MATERIALS RESEARCH

Over the past 50 years, analytical techniques based on magnetism have become essential tools for chemists, biomedical scientists, and physicians. While these techniques have contributed to progress in solid-state physics and materials science, their development has also given rise to a community interested in high-field magnets that is not only distinct from the solid-state and materials science community but also far larger, measured either by the numbers of scientists involved or by the resources invested. On the whole, unlike the materials science community, the analytical community does not patronize user facilities. Its members instead do their science using instrumentation available locally.

Many of the magnet-dependent analytical techniques of interest here exploit NMR. NMR is used by chemists and biochemists to determine molecular structures. Solid-state physicists and materials scientists use it to characterize materials. Physicians use an NMR-based technique called MRI to visualize the interior of the human body. An entirely different magnetic phenomenon, ICR, is the basis of a powerful technique for measuring atomic and molecular masses. The current maximum field strengths for magnets in resonance applications vary depending

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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on the physical size of the magnet, which in turn is dictated by the kinds of measurements for which it is to be used. These magnets must be extremely stable and homogeneous. For NMR, field limits are now about 21 T. For ICR, field strengths are just passing 12 T; for MRI, fields are from 4 to 9 T.

The magnet at the heart of the modern NMR spectrometer, MRI instrument, or ICR mass spectrometer is a superconducting magnet, often one having modest field strength but highly optimized with respect to stability and homogeneity. However, the performance of almost all such instruments improves with field strength so that the demands of the large communities that use them have been and continue to be an important stimulus for the development of superconducting magnets of ever-increasing field strength.

Introduction to NMR

Because the nuclei of many atoms—for example, 1H, 13C, 15N, and 17O—have magnetic moments, they absorb and emit radiation at characteristic radio frequencies when placed in magnetic fields. This phenomenon, NMR, was first detected in atomic beams at Columbia University in 1938, and NMR signals were first observed in condensed matter at Harvard and Stanford in 1946. Initially, NMR was used by physicists to study the interactions of nuclei with applied magnetic fields, with one another, and with their environment. Its subsequent use as a tool for investigating the electronic and magnetic properties of materials led to many advances in solid-state physics—for example, the first experimental verification of the BCS theory of superconductivity, by L.C. Hebel and C.P. Slichter in 1959.22 This classic experiment measured nuclear spin lattice relaxation times in normal and superconducting aluminum. The difference in the temperature dependence of nuclear relaxation and ultrasonic absorption measurements confirmed a central feature of BCS theory, namely, that electrons of opposite spin and momentum are correlated.

The frequencies of the NMR signals produced by atomic nuclei are sensitive to the chemical environment, and in the 1950s, chemists discovered that molecular structure and dynamics could be investigated fruitfully by NMR spectroscopy. It is now routinely applied to virtually everything chemists study, such as inorganic compounds, organic compounds, synthetic polymers, natural products, and fossil fuels. NMR spectrometers are essential components of the modern chemical laboratory.

22  

L.C. Hebel and C.P. Slichter, Nuclear spin relaxation in normal and superconducting aluminum, Phys. Rev. Lett. 113, 1504-1519 (1959).

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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In the late 1950s, biochemists began applying NMR to biological macromolecules like proteins and nucleic acids. The number of magnetically active nuclei in the average biological macromolecule is large, and the complex NMR spectra of these molecules could not be resolved using 1950s instrumentation, so progress was slow. Beginning in the mid-1970s, however, three important developments transformed this field. First, superconducting magnets suitable for NMR were developed that had field strengths greater than 9 T. NMR spectrometers built around such magnets had both the spectral resolution and the sensitivity required for analyzing the spectra of small macromolecules. Improved performance resulted not only from the superior resolution and sensitivity afforded by increased field strength but also from improvements in detector electronics. Second, multidimensional NMR techniques were invented that made it possible to spread the NMR signals produced by macromolecules over several dimensions, which further improved resolution. At the same time, efficient methods were developed for determining which atoms in a molecule are close together and for identifying those that are neighbors because they are chemically bonded. Third, molecular biologists and biochemists devised methods for preparing biological macromolecules in which 12C and 14N are replaced by their spin-1/2 isotopomers 13C and 15N and/or in which 1H is replaced by 2H, either to simplify proton spectra or to sharpen the resonances produced by other nuclei. The result of these developments is that the NMR spectra of macromolecules containing thousands of magnetically active atoms can now be assigned and their structures solved. The present-day molecular weight limit for complete protein structure determination is 40-50 kDa. A recent example is shown in Figure 2.5.

Along the way an entirely different use was found for NMR. If a suspension of cells, a piece of tissue, or even an intact organism is placed in an NMR spectrometer, the spectrum observed will be dominated by the resonances of the low molecular weight metabolites it contains. These metabolites tend to be present at high concentrations, and their resonances are much narrower than those from the macromolecular materials present. Since NMR is a noninvasive technique, in a properly designed experiment, which may involve isotopically labeled nutrients, metabolic events can be followed in a living organism in real time, and often processes occurring in one organ of an animal can be distinguished from those occurring in another.

Since powerful new NMR techniques continue to be developed at a rapid rate and instrumental capabilities continue to improve, it is impossible to predict the limits of NMR spectroscopy. Its scientific impact can be gauged from the number of NMR-related Nobel prizes awarded over the years. The prize in physics was given to I.I. Rabi in 1944 and to F. Bloch and E.M. Purcell in 1952, both for the initial demonstrations of NMR. In 1991, R.R. Ernst received the Nobel prize in

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 2.5 Ribbon representation of the molecular structure of a 283-amino-acid portion of the Gag polyprotein encoded by the HIV-1 genome, determined by multidimensional solution NMR methods. This structure illustrates both the complexity and the biomedical relevance of problems addressable by modern NMR. This portion of HIV-1 Gag contains two helical domains that are ultimately processed to form the capsid and matrix proteins of the mature HIV-1 virus. [See C. Tang, Y. Ndassa, and M.F. Summers, Structure of the N-terminal 283-residue fragment of the immature HIV-1 Gag polyprotein, Nat. Struct. Biol. 9, 537 (2002) for details.] Image courtesy of R. Tycko, National Institutes of Health and M. Summers, University of Maryland at Baltimore County.

chemistry for developments in Fourier-transform NMR and two-dimensional NMR, and in 2002, his colleague K. Wüthrich received the Nobel prize in chemistry for protein structure determination by NMR. In 2003, the Nobel prize in physiology and medicine was awarded to P. Lauterbur and P. Mansfield for the invention of MRI.

Today, NMR is one of the two primary means of determining complete structures of molecules (biological and otherwise), the other being x-ray crystallography. Information about the structures of biological macromolecules is essential for understanding biological processes and for the development of new drugs and new technological materials. In addition, NMR is an important tool for characterizing molecular motions in atomic-level detail, on timescales from <10–9 s to >1 s.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Molecular motions are an essential part of every biological process, and they determine the mechanical properties of materials. MRI is one of the most widely used medical imaging modalities (along with x-ray and ultrasound imaging and positron emission tomography (PET) and computer-assisted tomography (CAT) scans), and functional MRI (fMRI) is rapidly becoming the principal method for investigating both normal neurological function and neurological diseases.

NMR is typically performed on molecules either dissolved in a liquid, known as solution NMR, or in solidified samples—for example, crystallized proteins—known as solid-state NMR. The key difference between the two is that molecules in solution are freely diffusing and those in the solid state are not. In solution studies, the extraction of structural information can be hampered by dynamic processes that cause the shape of a molecule to vary on NMR timescales. However, the tumbling motions of molecules in solution average out certain systematic effects that can make it difficult to interpret the results of solid-state experiments.

The Role of Field Strength in NMR

In a typical NMR measurement, a sample that has come to equilibrium in a magnetic field is subjected to a series of radio-frequency pulses that perturb the orientations of its nuclear spins, and the radio-frequency radiation the sample emits in response is measured. The frequency of the NMR signals observed, which is proportional to the energy of the transitions they represent, depends linearly on magnetic field strength. The proportionality constant that relates frequency to field, which is called the nuclear gyromagnetic ratio, is different for each isotope. For hydrogen, the NMR frequency in megahertz (MHz) is ~42.6 times the field strength in tesla (T), but for 13C, the NMR frequency is ~10.7 times the field strength. Thus, the NMR signals produced by different isotopes are easily distinguished by frequency. State-of-the-art NMR spectrometers suitable for macromolecular applications operate at magnetic fields up to 21.1 T (900 MHz for hydrogen) and have bore diameters in the 54-89 mm range.

The magnet in a modern NMR spectrometer is invariably a superconducting magnet constructed using Nb-Ti and/or Nb3Sn wire, and it operates at 4.2 K or below.23 The magnets in NMR spectrometers designed for the analysis of liquid samples must produce fields that are much more stable and homogeneous than the magnets used for any other kind of experiment. Typical magnetic field drift rates are less than 10 parts per billion per hour, and fields are homogeneous to less

23  

High-Tc materials have not yet been used in commercial NMR magnets but are likely to be components of NMR magnets built in the future that operate at fields significantly above 21.1 T.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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than 10 parts per billion over volumes of about 1 cm3. These requirements derive from the fact that the NMR signals of individual nuclei within molecules in solution can have intrinsic frequency widths (linewidths) on the order of 0.1 to 10 Hz. Thus, even small magnetic field instabilities and inhomogeneities can broaden the signals significantly, a smearing that becomes unacceptable when the molecule being examined contains many nuclei of the same isotope, each producing a signal that differs in frequency only slightly from that of its neighbors.

NMR spectrometers that operate at high fields are generally superior to those that operate at low fields. Higher-field instruments resolve complex spectra better and have better signal-to-noise ratios. In the NMR spectra of molecules in solution, the signals from chemically distinct atoms of a given isotope have slightly different frequencies due to differences in the magnetic shielding provided by the electron clouds in which they are embedded. These site-specific variations in NMR frequencies are called chemical shifts, and if they did not exist, there would be little reason for chemists to use NMR. Chemical shift differences (in frequency units) increase in proportion with magnetic field, so the bigger the field, the better resolved the spectra obtained, provided that line widths do not increase significantly with field, which is often the case for samples in solution. In an N-dimensional NMR spectrum, the number of resolvable sites—and hence the molecular weight and complexity of the molecule that can be analyzed—in principle should increase as H0N, where H0 is the field strength of the spectrometer’s magnet.

The smallest number of atoms or molecules in a sample that can be detected by an NMR spectrometer—its sensitivity—is also a crucial issue in nearly all NMR measurements. Because the energies of the photons an NMR spectrometer must detect are very small, ~0.004 meV or less, ordinary samples must include more than 1016 molecules if they are to give NMR signals strong enough to allow detailed analysis. For many potentially interesting systems—for example, integral membrane proteins that function as hormone receptors—the required sample quantities greatly exceed what can be readily prepared. Novel techniques for sensitivity enhancement in NMR are under experimental investigation to address this problem.

In practice, signal-to-noise gains as a function of applied field strength have typically been closer to linear, implying only quadratic reductions in measurement times, largely because the efficiency of the circuitry available for signal detection tends to fall as frequency increases.

The field dependences of nuclear spin interactions can lead to less obvious but equally significant advantages for spectrometers that operate at higher fields. For example, the transverse relaxation optimized spectroscopy (TROSY) effect leads to anomalously large improvements in both sensitivity and resolution of certain classes of experiments important for protein structure determination as fields approach 20 T. Also, when quadrupolar nuclei—nuclei with intrinsic spin angular

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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momentum quantum numbers greater than 1/2—are studied by NMR in solids, the inverse dependence of quadrupolar line broadening on H0 and the linear dependence of chemical shifts on H0 produce quadratic enhancements in spectral resolution and sensitivity.

Finally, at high fields, the assumption that sample behavior is independent of field strength begins to break down even for solutions of diamagnetic molecules like proteins. The magnetic susceptibility of most biological macromolecules is sufficiently anisotropic so that their tendency to orient in solution in external magnetic fields is detectable in high-field NMR spectrometers. When molecules orient this way, through-space nuclear magnetic dipole-dipole interactions, which at lower fields are averaged to zero by molecular tumbling, become large enough to measure. Valuable information about molecular structure can be obtained from the analysis of residual dipolar couplings.

Recent Developments in Solution NMR

Although the development of spectrometers incorporating magnets of ever-increasing field strength has been the leitmotif of NMR technology since its inception, other advances, such as those already mentioned in electronics and computation, have also contributed to progress. Another advance is exemplified by the recent introduction of cryogenically cooled probes, which are NMR-signal detection devices in which the radio-frequency coil and preamplifier are cooled to very low temperatures to reduce the contribution of instrument noise to the signals detected. Sample quantities and measurement times are several times lower for NMR spectrometers equipped with such probes than they are for spectrometers that operate with conventional room-temperature probes.

The molecular weight limit for NMR structure determination will probably increase twofold or more in the next few years. It has been discovered that both the sensitivity and the resolution of certain NMR measurements crucial for characterizing proteins, specifically those that involve protein backbone nitrogen sites and aromatic side-chain carbon sites, can be greatly enhanced by exploiting the cancellation between nuclear magnetic dipole-dipole interactions and anisotropic chemical shifts that occur at fields near 20 T. This cancellation greatly reduces certain nuclear spin relaxation rates (decoherence rates), which leads to sharper NMR lines and reduced signal losses in multidimensional NMR experiments. New radio-frequency pulse sequence techniques such as TROSY take advantage of this effect. When TROSY techniques are combined with other recent developments such as protein chemical shift databases, residual dipolar couplings in aligned systems, and cross-correlated relaxation effects, full-structure determinations should become possible for proteins having molecular weights around 80 kDa.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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It should be emphasized that although the phenomenon that makes TROSY possible has been understood for years, its significance for biomolecular NMR was entirely unanticipated and was discovered only as a result of observations made once NMR spectrometers operating at fields near 20 T became available. The lesson of this experience is that the development of magnets that operate at higher fields can foster progress by leading unexpectedly to new ideas, new observations, and new approaches that promote scientific progress.

Solid-State NMR

The molecular NMR spectroscopy discussed above is done on substances dissolved in water or some other liquid. A different type of NMR, called solid-state NMR, is used to characterize solids of all kinds, including insoluble aggregates or assemblies of biopolymers—for example, the amyloid deposits associated with Alzheimer’s disease and proteins embedded in biological membranes.

Solid-state NMR is a well-established discipline in chemistry. It has been particularly important for investigating the molecular structures of polycrystalline and noncrystalline inorganic materials, such as the zeolites used as industrial catalysts and the glasses used as stabilization media for nuclear waste. For example, the distributions of local structures and bonding geometries within glasses, determined from solid-state NMR measurements, provide critical tests of theories of glass structure. Solid-state NMR is much less widely used for biological research than solution NMR and has been slower to develop because the resolution and sensitivity of most existing solid-state NMR spectrometers are too low to permit routine determination of the structures of macromolecules in the solid state. One reason resolution is poor in solid samples is that the 1H NMR signals on which structure determinations largely depend are broadened by strong nuclear magnetic dipole-dipole interactions that in solution are normally averaged to zero by molecular tumbling, as noted earlier. A solid-state technique called magic-angle spinning removes broadening due to dipole-dipole and other interactions to lowest order, but higher-order effects remain, which still broaden solid-state NMR resonances. Nevertheless, techniques are being developed for obtaining structural constraints for complex biochemical systems by solid-state methods.

Higher-field magnets would help alleviate the line broadening problem in solid-state NMR because the higher-order effects of dipole-dipole interactions, which are field-independent, become less important relative to the chemical shift differences as the magnetic field increases. It is also worth noting that the magnetic field homogeneity and stability requirements for solid-state measurements are not as stringent as for biological NMR measurements. A 30- to 50-T resistive or hybrid magnet, which was unusable for conventional multidimensional

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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biological NMR, could be very valuable for solid-state NMR of biological or inorganic materials.

Improvements in field strength could lead improved performance in other ways. Many of the materials studied by solid-state NMR contain quadrupolar nuclei such as 27Al, 17O, 23Na, and 67Zn. NMR measurements on quadrupolar nuclei at low fields are notoriously difficult because, even with magic-angle spinning, the NMR line from a single site may be extremely broad (~104 Hz) as a result of orientation-dependent, second-order quadrupole shifts. In complex structures with many inequivalent sites, the NMR lines of individual sites are commonly unresolvable. However, the second-order quadrupole shifts are inversely proportional to the applied field, H0, while the chemical shifts that distinguish one site from another are directly proportional to H0. Thus, the resolution of such NMR spectrum should increase quadratically with H0. The line-narrowing that accompanies the reduction in second-order quadrupole shifts should lead to an even stronger increase in sensitivity at higher fields. Solid-state NMR measurements on inorganic materials therefore stand to benefit enormously from the availability of higher field magnets.

NMR in Condensed-Matter Physics

Since the discovery of high-Tc superconductivity in cuprate materials in 1986, there has been a resurgence of interest in NMR in the condensed-matter and materials physics community. Measurements of the dependence of 63Cu, 65Cu, 17O, and 89Y NMR frequencies, spin-lattice relaxation rates, and electron-mediated couplings and the dependences of these quantities on temperature and doping have provided important information about the electronic structure of high-Tc ceramics, the nature of antiferromagnetic electron spin fluctuations, and the nature and symmetry of the Cooper pair states that give rise to superconductivity in these materials. NMR measurements provided the first experimental evidence for spin-singlet, d-wave pairing in the superconducting state of high-Tc ceramics.

NMR has also contributed to our understanding of fullerenes (molecular forms of carbon, such as the buckyball, C60) and the superconducting alkali fullerides, which were discovered in 1990-1991. NMR measurements have provided important information about the molecular dynamics, phase diagrams, and electronic properties of these materials. 13C NMR spectra and spin-lattice relaxation measurements were the first to reveal the rapid molecular rotations in solid C60 and permitted the determination of kinetic parameters that affect sound velocity and thermal conductivity in these materials. NMR measurements on superconducting alkali fullerides provided early estimates of the electronic density of states, superconducting state energy gap, and magnetic penetration length.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Beginning in 1994, optically pumped NMR techniques were used for the first time to study the properties of 2D electron systems in GaAs/AlGaAs quantum well structures. Optical pumping, in which optical excitation of electron-hole pairs in GaAs layers leads to enhanced nuclear spin polarizations and enhanced NMR signals, permits measurements on semiconductor thin films and quantum wells containing ~1016 nuclei. The NMR data provided the first experimental evidence for skyrmion states in 2D electron systems in the extreme quantum limit of low temperatures and high fields—that is, electronic states with effective spins greater than 2. The optically pumped NMR data, which demonstrated the existence of skyrmions near a Landau level filling factor ν = 1, stimulated many subsequent theoretical and experimental studies of skyrmions in 2D electron systems and sparked renewed interest in the electron spin properties of such systems in the FQHE regime.

These examples illustrate the potential of NMR to provide information about strongly interacting electron systems that is complementary to the information obtained from the measurements of transport properties, susceptibilities, and optical properties that are commonly performed in solid-state physics. These NMR measurements depend on the availability of stable and homogeneous high magnetic fields, but as in the case of the inorganic materials discussed above, the stability and homogeneity requirements are not as stringent as in most biological NMR applications.

The development of higher fields will generally increase the sensitivity of NMR measurements in condensed-matter and materials physics. It could, for example, make NMR studies of semiconductor heterostructures possible without optical pumping. In addition, the availability of higher fields with adequate homogeneity and stability will allow intrinsically high-field phenomena to be studied by NMR. NMR measurements on skyrmion states and fractional quantum Hall states in GaAs/AlGaAs quantum wells are one example. Another impressive example is provided by recent studies of field-induced magnetic ordering in quasi-2D transition metal oxides, carried out at the Grenoble High Magnetic Field Laboratory, in France, in which 63Cu and 65Cu NMR spectra were recorded as a function of field, up to 28 T and at 35 mK. These studies revealed drastic and unanticipated changes in the NMR spectra beginning at approximately 27 T, from which the field-induced magnetic superlattice structure could be inferred.

The new fields of spintronics and quantum computation are two additional areas in which NMR measurements could have an impact. Relatively simple solution NMR experiments, quite analogous to the pulse sequence techniques commonly employed in biological NMR, have already been carried out to demonstrate the basic principles of quantum computation. In these experiments, nuclear spins represent qubits, and radio-frequency pulse sequences are used to implement

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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quantum logic gates. Future implementations of quantum computation are likely to be based on microfabricated semiconductor devices, in which the qubits may be quantum-confined electron spins, nuclear spins, optical transitions, or some combination of these. High magnetic fields and NMR-related techniques will play an important role because the ability to address individual qubits and minimize the decoherence of quantum information will increase with increasing field.

Magnetic Resonance Imaging

MRI is a noninvasive technique for determining the spatial distribution of nuclear spins in samples. In biological applications, the spin distribution studied is usually that of 1H (129Xe, 3He, and 13C are also used), and since water is by far the most abundant hydrogen-containing substance in tissue, most biological MRI images show how water is distributed in the sample, which may be a live human being. In MRI experiments, samples are placed in magnetic fields that vary in strength across the sample volume in a well-defined way. All of the nuclear spins in the sample are then perturbed using radio-frequency pulses, and the radiation emitted by the sample is detected. Provided the point-to-point variation in field strength in the sample is large compared with chemical shift differences, differences in the resonant frequencies of spins will encode differences in location, not differences in chemical state.

MRI is now routinely used by clinical radiologists instead of x rays to obtain images of soft tissue in the human body and is the preferred method for diagnosing brain tumors, multiple sclerosis, and other neurological and spinal conditions and some forms of cancer. Recent work suggests that it may be possible to determine the metastatic potential of a tumor accurately from a spectroscopic image in which the chemical composition of a tumor and its location and size are determined by MRI. MRI is also being developed as a method for determining in advance the precise location and depth of the incisions that should be made during surgery to minimize tissue damage and patient risks.

An extremely important and entirely unanticipated advance has been the development in the past 15 years of MRI methods for imaging brain activity. This technique, called fMRI, is revolutionizing the neurosciences. Psychologists use fMRI to determine the regions of the brain that participate in specific thought processes and emotional responses and to develop new classifications for cognitive tasks based on the regions of the brain they activate. fMRI may become a diagnostic tool for psychiatry.

The sensitivity of MRI images to brain activity is believed to derive from the changes in local blood flow and oxygen consumption when a specific area of the brain increases (or decreases) its level of function. These physiological changes

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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alter the local concentration of deoxyhemoglobin, a paramagnetic molecule. The relaxation rates of the nuclear spins change in response because changes in deoxyhemoglobin concentration affect the local magnetic field homogeneity. It is easy to operate MRI instruments so that fast relaxing spins are distinguished from slow relaxing spins.

MRI instruments currently account for the vast majority of superconducting magnets sold commercially. The field strengths of the magnets used in the instruments employed to image the human body are lower than those for ordinary NMR spectroscopy, but the sample volumes they accommodate are much larger. Field strengths are up to 9.4 T (in a small number of research groups), and bore diameters of 90 cm are typical.

Higher fields improve the sensitivity of MRI instruments just as they improve the sensitivity of NMR spectrometers, and in this case improved sensitivity translates into improved spatial resolution. Furthermore, in fMRI experiments, small changes in signal intensities must be detected following specific stimuli. The information content of fMRI data is thus strongly limited by the signal-to-noise ratio, so higher fields are beneficial here, too. However, issues related to patient safety may limit what can be done in this area as much as issues related to the construction of MRI instruments operating at higher fields.

Magnetic resonance “microimaging” experiments are carried out on small laboratory animals or cell cultures using instruments equipped with magnets having much smaller bores than those used in the clinic. To date, microimaging experiments have been done using magnetic fields of up to 17.6 T. The spatial resolution achieved is on the order of 10 microns, and is again limited by the signal-to-noise ratio. The imaging of amyloid plaques in transgenic mouse models of Alzheimer’s disease has been reported. Techniques are being developed to track movements of individual cells that are appropriately labeled with paramagnetic contrast agents. Development of compounds that can be used as cell-type-specific and protein-specific contrast agents for MRI and microimaging is an active area of chemical research. Microimaging at even higher fields and with new contrast agents may make it possible to follow cell movements in a developing tissue, changes in gene expression, and movements of subcellular structures noninvasively and in real time in live laboratory animals. These innovations could make important new classes of experiments possible for developmental biologists and endocrinologists, among others.

Prospects for Improvements with Still Higher Fields

Several ways in which increases in magnetic fields can improve NMR performance have already been mentioned. Over the past 20 years, the development of

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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higher field magnets for solution NMR has been an incremental process. NMR spectrometers operating at 1H NMR frequencies of 270 MHz, 360 MHz, 400 MHz, 500 MHz, 600 MHz, 750 MHz, 800 MHz, and 900 MHz have been developed in sequence, with each step requiring 2-4 years to accomplish (see Figures 2.6 and 2.7). Most of these steps took place in industry, but with an increasing degree of academic research collaboration. For example, one of the most recent 900-MHz spectrometers to come on line was developed almost entirely at NHMFL. It is likely that instruments operating at still higher fields will one day be developed in a similarly incremental way and that this will gradually increase the complexity of the systems that can be characterized successfully by NMR. It is important to realize, however, that although each step in the process has been small, the cumulative effect has been large. The 800- and 900-MHz NMR spectrometers available today are vastly superior to the 360-MHz spectrometers that came on the market 20 years ago in almost every respect.

If NMR spectrometers with stable, homogeneous fields of 30 T or higher (1.3 GHz or higher) were to become available in the future, the impact on both biological and nonbiological users would be considerable. The increased sensitivity and resolution of such an instrument would probably allow using solution NMR to determine the structure of proteins two to five times larger than the proteins whose structures can be determined today. The tendency of macromolecules to align in magnetic fields, which is detectable but quite small in today’s NMR spectrometers, would become more significant (scaling as the square of the field strength), enabling new approaches to structure determination through anisotropic nuclear spin interactions. Biological solid-state NMR measurements would be qualitatively transformed, principally because 1H NMR signals would become well enough resolved to begin to be as useful and informative as they already are in solution NMR. Progress would be made in solving the general problem of membrane protein structures. Recent studies of relatively small model proteins in microcrystalline form have demonstrated the feasibility of full structure determination by solid-state NMR. The improved spectral resolution at higher fields will facilitate the extension of these results to larger membrane proteins. Solid-state NMR spectroscopy of inorganic materials would improve dramatically as second-order quadrupole effects become small spectral perturbations rather than the dominant feature in NMR spectra of quadrupolar nuclei.

As mentioned earlier, certain types of NMR have less stringent requirements for magnet homogeneity and stability than solution NMR. In particular, many biological and nonbiological solid-state NMR measurements can be performed in fields with about 1 ppm homogeneity over 0.1 cm3 (rather than 1 ppb over 1 cm3). Furthermore, magnet instabilities are manageable if the field drift can be calibrated or monitored during experiments. Provided drift rates are not too great, it

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 2.6 The magnet of the 900-MHz NMR spectrometer in Pacific Northwest National Laboratory’s Environmental Molecular Sciences Laboratory. This picture illustrates some of the issues associated with high-field magnets. This 21-T magnet was manufactured by Oxford and has a 63-mm room-temperature bore and a stored energy of 27 MJ; it is sited inside a cylindrically shielded enclosure 24 ft in diameter and extends 15 ft above and below the main laboratory floor (the magnet itself is 8 ft in diameter and 21 ft tall). The center of the magnetic field is located at exactly floor level to preserve the desired symmetry of the magnet and shield. The person standing next to the magnet is just over 6 ft tall. Photo courtesy of William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 2.7 The growth of the field strength of NMR spectrometers in common use. The types of superconducting wire used for the highest-field NMR magnets in each era are indicated at the top of the figure. Before 1992, magnets operated at the temperature of liquid helium (about 4 K). Since then, the highest-field magnets have required pumped-helium cooling systems that provide even lower temperatures (about 2 K). Current technology (Nb-Ti, Nb3Sn, and (Nb,Ta)3Sn) may enable 1-GHz operation, but new high-field conductors based on MgB2 or HTS materials will have to be developed for higher frequencies. (The committee notes that a 600-MHz NMR magnet was introduced in 1978 at Carnegie Mellon University; the magnet was constructed of Nb3Sn tape and was nonpersistent, setting it apart from the more heavily commercialized NMR magnets. As a proof-of-principle, this design may have important value for the future.) Figure courtesy of Bruker Biospin, Inc.

is possible to correct data for magnet drift either in real time or after the fact. Thus, a 30- to 50-T magnet for NMR, with 1 ppm homogeneity over 0.1 cm3 and 1 ppm/min drift specifications, would be valuable even if it could not be used for high-resolution NMR of proteins in solution.

For any such a magnet to achieve its full potential, however, ancillary equipment would have to be developed. In particular, NMR probe and cold-probe circuitry that permits double- and triple-resonance experiments at 1H NMR frequencies of 1.3 GHz and above will have to be developed and incorporated into the probes, including those with high-speed, magic-angle spinning capabilities. In addition, capabilities for both low-temperature and high-temperature measurements must be part of the package as well as radio-frequency power amplifiers that

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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produce ~500-W pulses at the NMR frequencies of hydrogen and other relevant nuclei. Finally, the spectrometer console, which controls the execution of pulse sequences and the acquisition of signals, must have the full capabilities of a modern NMR instrument.

Strategic Considerations for Higher Field NMR

The committee believes that the construction of 1.3-GHz NMR spectrometers is a scientifically justifiable objective. Its endorsement is strongly influenced by the fact that in the past, NMR spectroscopy has shown an astonishing capacity to reinvent itself in productive but unanticipated ways in response to improvements in instrumentation. However, the committee realizes that this enterprise will present major challenges for the agencies that support research dependent on high-field NMR spectroscopy, the people who manufacture these instruments, and the user community. The highest-field NMR spectrometers available today, which operate at 900 MHz, cost approximately $5 million each, and the cost of a 1.3-GHz spectrometer, assuming that it can be built, will certainly be much higher, perhaps as much as $20 million. Thus, unlike the number of 600-MHz NMR spectrometers available in the United States today, for example, the number of 1.3-GHz NMR spectrometers will never be large, and demand for access to them is certain to exceed supply. Two conclusions follow:

  • The technical challenges that surround the construction of the magnets required for these spectrometers are so great and the potential market for them is so small that manufacturers are unlikely to undertake their construction without at least some direct public support. Thus, in addition to playing their traditional role as the main purchaser of the product, federal agencies interested in high-field NMR research will have to shoulder some of the risk up front or these instruments will not be built.

  • Although the NMR data obtained with a 1.3-GHz spectrometer is expected to be of substantially better quality than that obtained with current spectrometers, which should allow new problems to be addressed, the throughput of these instruments might only be twice that of current 900-MHz instruments. Thus the NMR community will need to address the challenge of providing fair access to these spectrometers for all interested parties while at the same time ensuring that only experiments that absolutely require the highest available fields are performed on them. Given that the NMR community is used to running experiments on locally controlled instruments rather than sharing centralized facilities, some social engineering may be required to ensure a sensible outcome.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Ion Cyclotron Resonance

The phenomenon of ICR can be used to determine atomic and molecular masses. The technique is called ion cyclotron resonance mass spectroscopy (ICRMS). In an ICRMS instrument, modest numbers of ionized molecules are introduced into an ion trap in the middle of a homogeneous magnetic field. The trajectories of such ions are circular, viewed along the direction of the field, and the frequencies at which they orbit are determined by their mass-to-charge (m/z) ratios, and are proportional to magnetic field strength. The radii of ion orbits at the time of injection are small, and since the ionized molecules in the injected sample are randomly distributed around those orbits, viewed from the outside, the circulation of these ions has no net electrical effect. Ions can be forced to enter orbits of higher radius by the application of radio frequency pulses of the appropriate frequency, duration, and polarity, and the coherent orbital motions of the ion clouds that result induce oscillating voltages in detection circuitry that surrounds the ion traps, from which m/z values can be determined. (The similarities between ICRMS and NMR experiments are many and obvious.)

Because the lifetimes of the coherent cyclotron motion of such clouds, which are limited by collisions and by magnetic field inhomogeneities, can be quite long, m/z values can be determined with remarkable precision (~1 ppm) by ICRMS. The molecular weights of the components of mixtures containing thousands of chemically distinct ions can be determined simultaneously by ICRMS. The mass accuracy can be less than that of a single electron, so that chemical compounds with the same nominal molecular weight but different elemental compositions can be distinguished by ICRMS.

ICRMS is rapidly growing in importance. One application is in the field of proteomics, where the goal is to determine the full array of proteins expressed within a given cell type or tissue (and the posttranslational modifications of these proteins)—for example, as a function of exposure to hormones, drugs, or other bioactive compounds; as a function of developmental stage; or as a function of disease state. These measurements can be done with ICRMS by extracting proteins from cells or tissue, fragmenting them into shorter peptide segments, and then determining the masses of all fragments. The sensitivity of ICRMS is remarkably high: About 100 ions of the same species can generate a detectable signal. Thus, it is entirely compatible with the usual scales of molecular biology experiments. Because of the sensitivity and resolution of ICRMS spectra, ICRMS methods may be employed in the future to establish biomarkers for disease states in humans. ICRMS is also a tool of unparalleled power for determining the composition of complex mixtures such as crude oil.

ICRMS benefits from high magnetic field. Commercial ICRMS spectrometers currently operate at fields up to 12 T, and instruments up to 14.5 T are under

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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construction. Provided that field homogeneity is not limiting, resolving power increases linearly with field, and upper mass limit and dynamic range improve quadratically. Homogeneity requirements are not as stringent as in high-resolution NMR because the orbital motions of ions tend to average out field inhomogeneities. Thus, magnets with about 10 ppm homogeneity and about 10 ppm short-term stability are suitable for ICRMS. High-field resistive magnets could conceivably be used.

ICRMS is still in its infancy, and it has benefited mightily from the many techniques that have been developed in recent years for preparing molecular ions for mass spectrometric analysis. New applications are being discovered every day, and it is likely that before long, every research university will feel it must have ICRMS spectrometers on its campus. As it happens, ICRMS is one of the big success stories at NHMFL, whose Center for Interdisciplinary Magnetic Resonance in Tallahassee, Florida, has been an important locus for the development of ICRMS ever since it was founded.

Electron Paramagnetic Resonance

The phenomenon of EPR was discovered in bulk matter by E.K. Zavoisky in 1944, before the initial observations of NMR. EPR and NMR spectroscopy are similar in many respects; both examine the response of a sample exposed to radiofrequency radiation in a magnetic field. In EPR, the species detected are unpaired electrons. In NMR, it is nuclei that have nonzero spins. The technical differences between the two kinds of spectroscopy derive mainly from the difference in gyromagnetic ratio between electrons and nuclei. In a 1-T field, for example, the resonant frequency of a simple organic free radical is about 28 GHz, but the resonant frequency for protons is only 43 MHz. The wavelength of 28 GHz radiation is around 1 cm, which implies that a 1-T EPR spectrometer must be equipped with microwave electronics, a constraint that limits what can be done. Microwave devices tend to be relatively narrow-band, so EPR spectra are usually collected by setting the electronics of the spectrometer to a single frequency and then sweeping the field strength of its magnet over a wide range so that the different paramagnetic species in the sample can be brought into resonance one at a time. EPR spectrometers available commercially today operate at 95 or 130 GHz and have 3.5- to 5-T magnets.

As is the case for NMR and ICRMS, in principle, the performance of EPR spectrometers improves with increasing magnetic field, but there are limitations. If an EPR spectrometer were built around a 21-T magnet, i.e., a state-of-the-art NMR magnet, the resonant frequencies for simple organic free radicals would rise to about 588 GHz and wavelengths fall to about 500 µm. For some of the metal

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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centers (in metalloproteins and surface-catalytic metals) interesting to chemists and materials scientists, the frequencies might be even higher, about 6 THz, and the wavelengths even shorter, about 50 µm. Radiation having these characteristics lies in an awkward part of the electromagnetic spectrum, between microwaves and the far infrared; it is hard to work with.

The development of an integrated spectrometer operating at 21 T would present major challenges, but the rewards could be commensurate. New radiation sources, phase shifters, resonators, digital switches for pulse generation, power amplifiers, and preamplifiers would all have to be devised. However, if they were, it would become possible to do EPR experiments equivalent to the multidimensional, double-resonance experiments now routine in NMR at all field strengths and in EPR only at low field (9 GHz). The impact would be transformative. The spin Hamiltonian explored by NMR spectroscopy is simple to deal with theoretically because each of its terms is three to six orders of magnitude smaller than the dominating Zeeman interaction. It is different for EPR spectroscopy, because many of the internal interactions that must be taken into account are comparable to, if not larger than, the electron Zeeman interaction. Thus, electron spin Hamiltonians often depend on a dozen or more parameters that cannot easily be determined independently. At high field, EPR spectroscopy will approach the limit where these parameters can be measured accurately from first-order spectra. In addition, many interesting metal-centered species important in catalysis and materials have EPR transitions that can be measured only at high fields because of the large internal fields inherent in the metal ion or the material containing them.

Specialized, one-of-a-kind continuous-wave EPR spectrometers have been used in Europe, Israel, and NHMFL to demonstrate that metal centers of the kind just described can indeed be observed at high field in catalysts and optoelectronic and magnetic materials, demonstrating the potential of high-field EPR. It is time to develop the full range of pulsed EPR capabilities for high field so that the measurements now made routinely on simple organic free radicals and some metal centers at conventional EPR fields can be done on systems that require high field. Development of pulsed EPR spectrometers that operate at 15 T and beyond is already under way in the university and government labs of Germany, the Netherlands, and Italy. The United States should not allow itself to fall behind in this area.

Importance of Ancillary Technological Development

As has been pointed out repeatedly above, technological and methodological developments have led to constant advancement of the capability of magnet-based measurement devices and a concomitant increase in their impact on many areas of science. The development of higher field magnets has played a central role in these

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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advances to be sure, but the importance of developments in other areas of technology should not be forgotten. In fact, without these advances in ancillary technologies, the full potential of higher-field magnets would not have been realized, and, given the expense of high-field magnets, investments in technologies needed to optimize their utility makes good economic—as well as scientific—sense. While federal funding for the application of existing technology and methods to specific scientific problems has generally been good, federal funding for the development of novel technology and methodology has been inadequate.

NMR and MRI instrument manufacturers have done a good job of advancing the ancillary technology relevant to these techniques when relatively large commercial markets for their products justified their doing so. For example, the recent development of cryogenically cooled probes for high-field solution NMR, which enhance sensitivity severalfold, was carried out entirely by instrument manufacturers, based on their expectation about the market for them. However, there are many other areas where technological advances are sorely needed but where the commercial market is not large enough to attract the attention of instrument manufacturers. Several examples follow.

  • Biological solid-state NMR measurements on integral membrane proteins, which, in principle, could be used to determine the structures of receptor-bound hormones, neurotransmitters, and other biological signaling molecules, are severely limited by the availability of appropriate samples. It is often very difficult to make such materials in quantities sufficient for the instrumentation now available. This problem would be greatly alleviated by the development of magic-angle spinning probes for solid-state NMR that operate at 10-30 K. Further sensitivity enhancements might be obtained by techniques such as dynamic nuclear polarization (DNP) and optical pumping. Recent DNP experiments, in which paramagnetic compounds are added to frozen solutions to permit cross-relaxation between electron spins and nuclear spins during microwave irradiation of the electron spins, suggest that signal enhancements of two orders of magnitude may be achievable quite generally with further developments in microwave technology, solid-state NMR probe design, and paramagnetic reagents.

  • In the area of MRI, the advent of whole-body imaging systems operating at 8 T and above creates new problems in detector coil design, largely because the wavelengths of the radio-frequency radiation detected are comparable to anatomical dimensions, making the amplitudes of the radio-frequency fields within the body highly nonuniform. Optimal coils for high-field MRI will probably not be developed unless a significant research program is undertaken by groups outside the commercial sector, and this is unlikely to

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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happen without federal support. Similarly, MRI microscopy at the highest available fields, which could be used for noninvasive imaging at the cell or organelle level within model organisms, would benefit significantly from the development of improved microcoils and field gradient coils.

  • Hybrid magnets with fields greater than 40 T may not have the homogeneity and stability required for biological NMR, but they do have considerable potential for NMR studies of inorganic materials and phenomena in condensed-matter physics. Realization of this potential will require the design and construction of appropriate NMR probes with high- and low-temperature capabilities and the development of field stabilization methods to extract the maximum possible spectral resolution and sensitivity.

  • Finally, perhaps more than any other class of experimental techniques now in common use, NMR and MRI have both benefited from methodological advances that are purely concept-based, as opposed to equipment-based. Physical, chemical, and biological scientists have benefited enormously from the design of radio-frequency pulse sequences that excite and manipulate nuclear magnetic moments in new ways. Developments in pulse sequence methodology have usually resulted from new insights into the way nuclear magnetic moments evolve in external and internal magnetic fields and from new ways to describe these evolutions mathematically. Because higher fields significantly change the relative strengths of the interactions that determine how nuclear magnetic moments evolve in response to radio-frequency pulse sequences, improvements in pulse sequences will be required if the field is to take full advantage of high-field magnet development.

Projects aimed at improving NMR and MRI capabilities, from which large numbers of investigators are certain to benefit, are surely as worthy of support as the specific projects based on current NMR and MRI techniques. A strong program requires a healthy balance between research aimed at improving current capabilities and research that exploits current capabilities.

OTHER SCIENTIFIC USES OF HIGH-FIELD MAGNETS

High-field magnets are critical components of instruments used in several research areas that so far have been mentioned only in passing, most notably high-energy and nuclear physics, plasma science, and fusion energy research. Advances in high-energy physics have long been strongly coupled to developments in magnet technology, because increases in magnet performance have been required for the construction of ever more power particle accelerators and particle detectors.

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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The U.S. high-energy physics community established an early dominance in superconducting accelerator technology by constructing the Tevatron at Fermi National Accelerator Laboratory (FNAL), which is still the world’s highest energy accelerator. Its 4-mile circumference ring includes 1,000 Nb-Ti superconducting magnets supplying a field of 4.4 T. It began operation in 1983 and has been upgraded over time to reach an energy of 1 TeV. An even larger accelerator is the Large Hadron Collider (LHC) at CERN, designed to collide proton beams at 14 TeV. Still under construction, it will use a dual-aperture beamline 27 km in circumference, which includes 5,000 Nb-Ti superconducting magnets. These magnets will operate at 1.9 K with a peak field of 8-9 T. The expected start-up date is 2007. Because Nb3Sn is brittle, its use requires special magnet construction techniques that are more costly than those for the tough and ductile Nb-Ti. Thus, unless the higher-field performance is required, Nb-Ti is generally employed, as it was for the LHC, where it was found easier to design with Nb-Ti than Nb3Sn. A prototype 16-T dipole magnet for the next generation of accelerator dipole magnets (for proton machines) was demonstrated in 2003 at LBNL; these magnets will require Nb3Sn, MgB2, or an HTS conductor.

High-energy physics magnets are predominately dipoles, quadrupoles, and higher order configurations, but although their field and force distributions are very different from the solenoids used for most other purposes, the stresses and other engineering problems confronted in their construction are similar. The Tevatron and the LHC achieve their goals using huge quantities of Nb-Ti magnets, but it is unlikely that Nb-Ti magnets will suffice for the large circular accelerators that might succeed the LHC. The field strengths attainable by Nb-Ti magnets will simply not be enough. For this reason, the U.S. high-energy physics community has established a research program to develop Nb3Sn-based dipoles and quadrupoles. LBNL has achieved a 16-T peak field in a small dipole prototype coil using an advanced, very high critical current density Nb3Sn wire. Other laboratory partners in the DOE high-energy magnet technology effort include Fermilab and Brookhaven National Laboratory. Fields up to 20 T may be achievable with further development, but this will require a focused program devoted to the commercial production of large quantities of very high current density Nb3Sn wire. Because the different magnet user communities have been isolated from one another, developments in accelerator magnet technology have not had a broad impact outside the field. Recently, however, researchers at FNAL and LBNL have begun to form partnerships with fusion science magnet designers and superconducting materials experts outside the traditional laboratory centers.

Research into the development of fusion as a future energy source has been going on around the world for years. Fusion devices are operating in Europe,

Suggested Citation:"2 Scientific Challenges and Opportunities with Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Russia, and Japan, and new ones are being constructed in South Korea, China, and India. All the fusion devices being used or being considered require very large superconducting magnets. The United States is now operating only one superconducting fusion device, known as the Levitated Dipole Experiment. However, it has had an extensive superconducting magnet development program under way since the 1970s. In the 1980s the extremely large superconducting mirror machine MFTF-B was started up at Lawrence Livermore National Laboratory, but it never went into full operation for reasons unrelated to magnet technology.

Fusion magnets come in many shapes and sizes, including solenoids, toroids, and helical coils. The devices presently in operation use Nb-Ti magnets, but newer machines will use Nb3Sn magnets. The largest project now being planned is the International Thermonuclear Experimental Reactor (ITER), which will be a collaboration between the United States, Europe, Russia, Japan, China, South Korea, and India. The device will cost more than $5 billion and is scheduled to be constructed over 8 years beginning in 2006. This machine will require the commercial production of about 500 tons of high-quality Nb3Sn superconductor over a several-year period, a more than 10-fold increase in world production of Nb3Sn.

During the 1990s the parties involved in ITER made several large-scale prototype superconducting magnets. The largest of these was the Central Solenoid Model Coil, built jointly by the United States and Japan. Its coil has an inner diameter of 1.6 m, an operating current of 46,000 A, a peak field of 13 T, and a stored energy of 640 MJ. It can be operated as a DC magnet or ramped from zero field to 13 T in 8 s without quenching.

These examples suggest the broad utility and critical importance of high-field magnet science and technology in fields beyond condensed-matter physics, materials science, and magnetic resonance. Although the scale of application for high-field magnets in a high-energy particle accelerator is vastly different than that associated with the study of correlated-electron systems, both communities drive—and benefit from—general advances in high-field magnet technology. It is important to note that these disparate communities of users have not traditionally collaborated on magnet technology. Indeed, the recent convergence of the particle physics and fusion science magnet efforts was precipitated more by their shared source of funding (DOE’s Office of Science) than by any overlap of ongoing research efforts. It is nevertheless the case that advances in magnet design, construction, and performance made by one community can significantly benefit other communities.

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High-field magnets—those that operate at the limits of the mechanical and/or electromagnetic properties of their structural materials—are used as research tools in a variety of scientific disciplines. The study of high magnetic fields themselves is also important in many areas such as astrophysics. Because of their importance in scientific research and the possibility of new breakthroughs, the National Science Foundation asked the National Research Council to assess the current state of and future prospects for high-field science and technology in the United States. This report presents the results of that assessment. It focuses on scientific and technological challenges and opportunities, and not on specific program activities. The report provides findings and recommendations about important research directions, the relative strength of U.S. efforts compared to other countries, and ways in which the program can operate more effectively.

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