Opportunities in High Magnetic Field Science (2005) / Chapter Skim
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2 Scientific Challenges and Opportunities with Higher Fields
2 Scientific Challenges and Opportunities with Higher Fields
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From page 14... ...
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.
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From page 15... ...
. 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 macro molecules by NMR, on solid-state NMR of biological and inorganic materials, and on electron paramagnetic resonance (EPR)
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From page 16... ...
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)
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From page 17... ...
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.
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From page 18... ...
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.
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From page 19... ...
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 com pounds.
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From page 20... ...
; and, finally, the possibility of finding a superconductor that has a critical temperature above room tempera ture. 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; micro wave systems; and (of course)
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From page 21... ...
, 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.
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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 measure ments, and carefully controlled conditions so that samples can be compared, steady-state fields are generally required.
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From page 23... ...
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)
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From page 24... ...
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.
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From page 25... ...
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.
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From page 26... ...
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 super conductors 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.
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From page 27... ...
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.
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From page 28... ...
In this section the committee briefly describes the key topics in this quickly grow ing 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.
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From page 29... ...
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 7The interested reader may find more detail in two excellent reviews: G.R.
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The forces responsible for the suppression of magnetic order, and the genera tion 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 leave the local moment sites and become itinerant as approaches C
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From page 31... ...
Subsequent experiments in which compositional variation was used to vary through C 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.
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From page 32... ...
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.
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From page 33... ...
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
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From page 34... ...
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.
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From page 35... ...
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)
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From page 36... ...
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.
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From page 37... ...
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)
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and zero dimensional (0D) systems at high magnetic fields and low temperatures.
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Chang, and F Wudl, Electrodynamics of the superconducting state of kappa (BEDT-TTF)
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More recently, neutron-scattering experiments at high magnetic fields have provided informa tion 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)
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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.
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In user surveys done at U.S. scattering facilities, the top concern of the materials science and condensed-matter physics community is sample environ ment and, in particular, the availability of magnetic fields.
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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.
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From page 44... ...
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.
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From page 45... ...
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.
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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.
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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.
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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
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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
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Thus, the NMR signals produced by different isotopes are easily distin guished by frequency. State-of-the-art NMR spectrometers suitable for macro molecular applications operate at magnetic fields up to 21.1 T (900 MHz for hydrogen)
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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 H0 , where H0 is the field strength of the spectrometer's magnet. N 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.
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From page 52... ...
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.
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From page 53... ...
For example, the distributions of local structures and bonding geometries within glasses, deter mined 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.
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From page 54... ...
The line-narrowing that accom panies 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.
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From page 55... ...
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 mag netic 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.
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From page 56... ...
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.
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From page 57... ...
The field strengths of the magnets used in the instru ments 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)
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From page 58... ...
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)
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From page 59... ...
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)
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Sn ( (Nb,Ta) 3Sn 3 NbTi NbTi Nb3Sn 3 3 3 single multi Nb33Sn Nb33Sn filament filament NbTi NbTi NbTi 1500 1000 900 800 750 600 500 500 400 270 180 90 60 Date 1965 1975 1985 1995 2005 1960 1970 1980 1990 2000 2010 FIGURE 2.7 The growth of the field strength of NMR spectrometers in common use.
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From page 61... ...
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.
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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.
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From page 63... ...
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.
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From page 64... ...
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)
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From page 65... ...
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.
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From page 66... ...
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.
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From page 67... ...
The field strengths attainable by Nb-Ti magnets will simply not be enough. For this reason, the U.S.
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From page 68... ...
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 proto type superconducting magnets.
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