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3 Scientific Opportunities INTRODUCTION This chapter is based on the many responses from members of the scientific community to a letter of inquiry (see Appendix A). We are grateful for the responses and wish to thank all those who spent so much time and effort to help in formulating this report. Their names are listed, together with those of others who participated in this study, in Appendix B. Because the availability of high magnetic fields affects a wide range of research, it was necessary to select examples from among a large number of suggestions. The final selection for inclusion represents the judgment of the members of the Panel. The Panel adopted a specific definition of a high magnetic field. We con- centrated on the scientific opportunities provided by (1) continuous uniform magnetic fields up to 75 T; (2) quasi-static pulse fields up to 100 T (duration, hundreds of milliseconds); (3) nondestructive fields up to 250 T (durations of milliseconds); (4) very-short-pulse destructive fields up to 2500 T (duration of tens or hundreds of nanoseconds). The feasibility of generating such fields is discussed in Chapter 5 on Magnet Design and Materials. Significant scientific opportunities exist over the entire field range greater than 30 T. As the available field strengths are increased, the number of oppor- tunities increases. Certain unique experiments require steady-state fields of the order of 75 T. Examples are experiments employing nuclear magnetic resonance (e.g., biology and chemistry), where the resonance frequency is comparable to X-band electron spin resonance, leading to comparable sensi- tivity ;de Haas-van Alphen and cyclotron resonance in concentrated alloys and "dirty" compounds (A15 superconductors); and atomic spectroscopy, where study of low-quantum-number atomic levels becomes possible. We, therefore, set steady-state, high-homogeneity fields of 75 T as our principal objective. Unfortunately, technology appears to make this goal prohibitively expensive. We are forced, therefore, to recommend a detailed study of methods (perhaps 8
Scientific Opportunities 9 novel) that can generate these fields. We suggest that a staged program of high static field development, with intermediate levels of 45 T and 60 T, may emerge as the most likely method for achievement of this goal. A number of opportunities do exist at these intermediate fields. The Panel notes that present technology is sufficient to design and con- struct quasi-static fields in the neighborhood of 100 T. A subset of opportuni- ties identified with steady-state operations at 75 T can be fulfilled with such an approach, along with transient experiments that profit from the time- dependent field profile. Examples of the former are de Haas-van Alphen ex- periments, which have longer relaxation time orbits, and cyclotron resonance experiments in dirty systems. Examples of the latter are studies in metallurgy and relaxation time measurements. Common to the technology for generation of quasi-static fields is the requirement of a large energy storage source (ca- pacitor banks, rotating armatures, or flywheels). We call attention in this report to the cost advantages to be gained by taking advantage of such equip- ment when possible. The feasibility of construction of quasi-static field facili- ties has led to our second conclusion, that a program for the generation of quasi-static fields to and beyond 100 T is desirable. We wish to take note of the advantages of such an approach for the principal conclusion of this Panel. The diagnostic techniques developed for 100-T quasi-static fields will be of great value to the 75-T steady-state facility, once it becomes a reality. Indeed, the fabrication and operation of the former may strongly influence the design feature of the latter. Finally, very high (1000 T to 2500 T) fields for very short time periods (less than a microsecond) can be generated relatively inexpensively. The sci- ence here is new and uncharted. Large energy sources are needed for these fields, but they can be chemical as well as electromagnetic. Experiments have already been performed near 300 T, and the diagnostics necessary at these fields could be used at higher fields. The experiments that are feasible in this field range cannot require high homogeneity. However, some (e.g., relaxation measurements) may even profit from the short rise and fall times involved. Magneto-quantum electrodynamics affords an example of a feasible class of experiments; so too does the study of intense temperature and pressure changes in materials of geophysical interest. Considerable preparatory work has already been accomplished, and we conclude that the time is ripe for exploitation of available methods that explore an unknown and potentially scientifically rich field regime. CONCLUSIONS AND HIGHLIGHTS In this section we summarize our principal scientific conclusions and give an indication of the excitement and richness of the research that could be ac-
10 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES complished with higher magnetic fields. Subsequent parts of this chapter describe these scientific opportunities in greater detail. The Panel believes that the generation and use of high magnetic fields in the study of the properties of matter is of great importance to the national program for research and technology. Failure to mount and maintain a con- sistent, strong effort for the generation of higher fields will substantially impair the future research position of the United States and will result in missed opportunities for development and research. We conclude that a strong effort to accelerate production of higher steady-state magnetic fields, including vigorous support for further research and development on all aspects of the technology of high-magnetic-field con- struction, is of central importance. The current limit of about 30 T for steady-state fields should be increased as technology advances. Present tech- nology can be applied to the development of facilities with more than twice this field. The advantages of steady-state high fields are great, and every opportunity to achieve this mode of operation should be pursued vigorously. The production of quasi-static fields should also be pursued, but the important long-term effort toward the attainment of higher steady-state fields should have high priority. Research and development on the properties of high-field superconducting materials and magnets, as well as development of alternative forms of resistive magnets, are important components of this effort. In summary, we conclude that significant new scientific opportunities exist in the utilization of steady-state, highly homogeneous magnetic fields up to 75 T and that a design program to attain such fields should be undertaken. The design and construction of facilities for the production of quasi-static magnetic fields approaching 100 T is a step toward this goal, and sufficient scientific opportunities exist to warrant construction of such facilities in their own right. As a dividend, the technology generated by such a program would be of inestimable value to a steady-state 75-T program. Further, important scientific opportunities require short-pulsed magnets with fields over 1000 T, which are feasible with current technology. The development and construction of these facilities should not necessar- ily be carried out at a single center. Indeed, the complex diagnostic and experimental equipment needed to do forefront research would probably require a number of magnet installations, some at large-scale facilities such as synchrotron radiation, neutron scattering, or high-energy physics machine sites. The competition among laboratories and the possibility of exploring different approaches simultaneously would be healthy. We emphasize that a consistent, sustained, orderly approach to the genera- tion of higher magnetic fields and to their use in the study of the properties
Scientific Opportunities \ \ of matter is of great importance. Research on the properties of matter in high magnetic fields is at the forefront of biology, chemistry, metallurgy, and physics and should be aggressively pursued. A necessary concomitant to the development of new magnetic-field facilities is that sufficient research sup- port be allocated to allow the exploitation of such high magnetic fields. In addition, large energy sources exist at facilities for fusion research, weapons technology, and high-energy physics; therefore, these major resources for high-field magnets and research should be considered in developing ultra-high- field facilities. In what follows, we list some highlights of the scientific opportunities that the Panel considered. Amplified and detailed considerations comprise the remainder of this chapter. Transition to Lower Dimensionality and Wigner Crystallization Magnetic fields reduce the dimensionality of motion of an electron gas. Confinement to orbits along the magnetic-field lines results in a lowering of the electronic kinetic energy, allowing the potential energy greater influence over electronic motion. For a sufficiently high field, the Coulomb interaction can dominate (equivalent to a low-density electron gas), with correlation causing-crystal- lization for densities in the metallic regime. The effect should be most marked for two-dimensional metals (e.g., metal-oxide-silicon devices). Inter- esting model systems might be semiconductors, which under intense optical pumping form electron-hole droplets. These "metals" have low electron den- sity but high conductivity. Intense magnetic fields are expected to localize the holes in a "string of pearls" along the field direction, with the electrons in a cylindrical sheath about the line of holes. Electronic Structure of Exotic Metals Both one- and two-dimensional elec- tronic structures exhibit metallic conductivity. Because of the extreme anisot- ropy of the Fermi surface, they tend to be more susceptible to lattice defor- mations than three-dimensional materials. A number of conflicting theories have been presented to describe their electronic properties. Unfortunately, all of these systems exhibit relatively short conduction electron scattering lengths, making conventional Fermi surface probes at conventional field strengths ineffective (e.g., de Haas-van Alphen, cyclotron resonance). Large magnetic fields would be required so that the product of the cyclotron fre- quency, coc, times the electron scattering lifetime, T, could exceed unity. Under such conditions, use of Fermi surface probes could determine the origin of the complex phase transitions exhibited by these systems. High-Field Superconductors The study of these materials is intertwined very closely with the advancement of high-field magnet technology. Aside
12 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES from technological benefits, however, there are many basic scientific ques- tions for high-field superconductors that can be at least partially answered by research at magnetic fields greater than those now available. The fundamental target here is a better understanding of the role of materials parameters such as composition, ordering, vacancy concentration, and dislocation density on critical temperature, critical field, and critical current density. High-field in- vestigations are absolutely vital for the last two of these properties. Metallurgical Phase Transitions At present, pulsed high magnetic fields are not used extensively in metallurgical research. When they become more read- ily available, they are expected to have a significant impact on metallurgical studies, in particular for the study of phase transformations. Examples are some of the martensitic transformations in which a nonmagnetic phase trans- forms into a magnetic phase. The transformation can be driven by an applied magnetic field. Application of a 100-T pulse with a duration of more than a few milliseconds (long enough not to cause eddy current heating) on an a-FeNi alloy changes the martensitic transformation temperature by more than a few hundred degrees. Therefore, the nucleation and growth of martens- ite can be controlled by the field, facilitating observation of the phase change during the process of transformation. Such a study should contribute sig- nificantly to the understanding of the nucleation mechanism, which is still an open and important question. Chemical Reactions The detailed dynamics of chemical reactions in gases, in the condensed phase, and on surfaces are expected to be influenced uniquely by magnetic energies approaching kT at ambient temperatures. Sufficiently large magnetic fields will significantly modify the potential energy surfaces that dictate the approach and interaction between atoms or molecules. These same fields are sufficient to partially orient molecules in solutions through the diamagnetic interaction, thereby allowing an entirely new approach to the study of steric effects on chemical reactions. The orientation of molecules on surfaces can also be accomplished in such fields, and studies of surface re- actions or states in high fields will expose hitherto unknown properties of those complex systems. Structure of Biological Systems Increases in the static field employed in NMR spectroscopy from the current limit of 14 T to the range of 75 T will yield substantial increases in resolving power and will allow study of very-low- concentration nuclei. Lower-molecular-weight biological materials (up to 20,000), which now exhibit spectra of which only a small fraction is resolved, will generate spectra that can be interpreted in great detail. Among the impor- tant problems that would be opened to attack are antibody-antigen inter-
Scientific Opportunities 13 action, hemoglobin structure and function, transfer RNA structure in solu- tion, protein-lipid interaction, enzyme-complex assembly, and receptor- binding site mapping. The determination of the time development of metabo- lite levels in vivo and the techniques of three-dimensional NMR imaging will be applicable to assemblies as small as individual cells. Spectroscopy of Atoms and Molecules High magnetic fields alter the colli- sion cross section for resonant collisions of atoms and molecules. For ex- ample, the long-range dipole-dipole contribution to excitation transfer can be nearly eliminated in the Na-Na* resonant collision for certain Zeeman-level transitions. This allows one, in principle, to study separately the higher-order nonresonant terms of the interaction potential that are masked in zero fields by the resonant dipole term. Study of collision dynamics under high magnetic fields involves the time of collision directly and promises new insights into the collision potential. On a strictly spectroscopic level, study of atomic and molecular spectra in very high magnetic fields and at high temperatures can be used to simulate conditions at the surface of white dwarf stars (which exhibit fields of the order of 100 to 1000 T). This would enable the unscram- bling of astronomical spectra for which only theoretical models are now available. DETAILED DISCUSSION OF SCIENTIFIC OPPORTUNITIES In this section we treat the opportunities mentioned in Highlights in detail. Each subsection is classified in one of three categories so that the reader can better perceive the character of the scientific opportunities available. These categories are as follows: CATEGORY 1. Experiments that are extensions of existing ones but that are incomplete in that the desired data cannot be obtained because existing magnetic fields are too low. CATEGORY 2. Experiments that have been performed with existing fields and for which extension to higher fields would be useful. CATEGORY 3. Experiments that might lead to new and interesting results if they were performed in high magnetic fields-it is not known what these results might be, but new fields of re- search might open as a consequence. The field technology is also indicated. Steady-state fields are always useful, but the magnitude of field required will sometimes exceed even the 75 T that we have postulated. In these cases, pulse fields are required and will be explicitly noted.
14 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES Semiconductor Research This important field of basic and applied research will be profoundly affected by the availability of high magnetic fields. Although much can be done with extensions of existing fields into the 30-T range, new developments could take place at 50-75 T. The topics we consider are not meant to be exhaus- tive; rather, they illustrate the excitement that high magnetic fields can gener- ate in semiconductor research. We shall cover six topics: (1) transition to lower dimensionality, (2) mag- netic semiconductor electronic structure, (3) "sweeping" through the electron-phonon clothing of electronic motion, (4) testing the effective mass approximation, (5) shallow impurity studies, and (6) Zeeman splitting of deep impurity states. We note other interesting fields only by title: Hall angle saturation in noncompensated materials; electron-hole cylinder condensation (rather than droplets) leading to increased density, resembling behavior in neutron stars; heavy mass cyclotron resonance; the approach to the true quantum limit for degenerate Si and Ge (requiring fields near 100 T); two- dimensional magnetoplasmon and excitonic effects; and the study of negative hydrogen ions in semiconductors in high magnetic fields. All these studies will be possible with higher magnetic fields. Transition to Lower Dimensionality. Steady Fields. SOT. CATEGORY 1. Wigner showed in the late 1930's that at very low electron concentrations condensation into a lattice occurs as a consequence of the electron-electron correlation energy. This condensation has been sought under a variety of experimental conditions but has yet to be observed. Observation of the con- densation, apart from its intrinsic interest, would also improve understanding of that elusive quantity, the correlation energy of an electron gas. Magnetic fields reduce the dimensionality of the electron gas. For a three-dimensional system, the electron orbits are cylindrical for sufficiently large fields (o>cr > 1), and translational motion takes place only along a single direction. For two-dimensional systems, as for example in a metal-vacuum-helium struc- ture or a metal-oxide-silicon sandwich, a magnetic field of sufficient mag- nitude traps the electrons in their lowest Landau level. The Landau orbits restrict motion in the plane so that the translational kinetic energy is strongly reduced. Under such conditions, the system is quasi-zero dimensional, and Coulomb interactions at all densities are expected to have drastic effects on the low-temperature properties. Using the Hartree-Fock approximation, one can show, theoretically, that the system will develop a charge density wave instability as the temperature is lowered (Fukuyama, Platzman, and Ander- son). For all fractional occupations of the lowest Landau level, except one
Scientific Opportunities 15 half, the transition is first order. Similar behavior is expected for three- dimensional metals. Fukuyama has predicted a charge density wave state with wave vectors having components both parallel and perpendicular to the mag- netic-field direction. The physics underlying the increased tendency toward localization involves the reduction of the kinetic energy of the electrons afforded by the presence of the magnetic field. Wigner crystallization is pre- dicted for low electron densities (too low for these structures in the absence of magnetic fields at reasonable temperatures), because the electron kinetic energy decreases more rapidly than the correlation energy as the density is reduced. The use of a magnetic field makes observation of condensation practical at reasonable temperatures and concentrations by means of the "freezing out" of the kinetic energy inherent in low-Landau-level occupation. Typical magnetic fields predicted to lead to condensation are in the tens of tesla range. Magnetic fields near 50 T should provide sufficient reduction of the kinetic energy to make this electronic "solid" accessible at liquid helium temperatures. Electron-hole droplets in semiconductors are also strongly perturbed by magnetic fields. Their low carrier density (~ 1017/cm3) and small effective mass means that magnetic fields can have huge impact on the correlation energy. For example, a field of 60 T applied along a [100] direction in Ge is sufficient to cause the electron-hole gas to occupy only a single Landau level. Predicted structures in the "crystallized" state are curious: the holes are expected to line up like a "string of pearls" along the field direction, with the electrons moving in cylindrical sheaths centered on the holes. Observation of these examples of "electron freezing" would be the culmination of 40 years of study of the electron-electron correlation energy. Magnetic Semiconductors. Steady-State or Quasi-static Fields. SOT. CATEGORY 1. This class of solids exhibits another scientific opportunity afforded by the availability of high magnetic fields. The cyclotron resonance frequency can be observed in Landau-level resonance studies by optical absorption at high fields and frequencies, as demonstrated by Lax and his co-workers. However, to explore the electronic structure requires that WCT > 1 . For many materials, T is so short that this condition is impossible to achieve at fields currently available in the laboratory. Striking examples are the europium chalcogenides (EuO, EuS, EuSe, and EuTe), which are both magnetic and semiconducting. Impurity scattering is large, however, thus T is small at electron concentrations necessary for band conduction (1018/cm3). (At lower electron concentrations, conduction is limited to hopping.) At 10T,
16 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES COCT * 0.2, which is too low for measurements of the de Haas-van Alphen effect or of anisotropy in the magnetoresistance to be useful for band-structure studies. The situation is so serious that one does not know even where the minimum of the conduction band is located. There are two competing models: the minimum is at the T point (center of the zone) or at the X point (the 100 face of the zone). The former implies s-like character for the conduction electrons; the latter d-like character. The former means spherical energy surfaces; the latter highly nonspherical ones. This important question can be settled by experiment if fields of the order of 50 T become available. This example is but one of a large number for which the material by its nature has a short scattering time and the only alternative is increasing the magnetic field. Electron-Phonon Clothing. Short Pulse. 100 T. CATEGORY 2. Electrons are never "free" in solids. Their motion is affected by the presence of lattice vibrations. The coupling between these two systems affects the behavior of electronic motion, leading to an electronic effective mass different from that calculated by band theory for a rigid lattice. This dressing of the electron is excitation-frequency dependent. For motion of the electron faster than the lattice can follow, the electrons behave as free. Thus, an experiment that would take one from the "slow" to the "fast" regime should exhibit quite different electronic masses at the two limits. The most authoritative study of this behavior is the work of Holstein and is referred to as the polaron problem. Semiconductors are ideal candidates for study of this phenomenon. Cyclotron resonance frequencies coc = eH/m*c can be made comparable with, or greater than, the longitudinal optical phonon frequency (LO phonon), the one most strongly coupled to the electron's motion. This allows one to make the transition from slow to fast electronic motion by merely increasing the cyclotron frequency, that is, by increasing the magnetic field. Some experiments that exhibit the beginning of this "undressing" of the electron have been performed by choosing systems for which m*/m is very small. Consequently, the electron-phonon coupling is weak. However, of fundamental interest are the intermediate and strong coupling regimes that occur in polar materials when the effective mass is large. To sweep through the LO phonon frequency where resonant coupling will occur, fields of the order of 100 T are required (as well as infrared lasers to see the effect). The polaron problem represents one of the most interesting and difficult of the coupled system problems in condensed-matter science. A thorough study through the entire frequency regime where m* is expected to vary would be of fundamental interest.
Scientific Opportunities 17 Effective Mass Approximation. Steady State. 100T. CATEGORY 3. The use of a dielectric constant to describe the Coulomb potential in which the electron moves in a semiconductor depends crucially on the electron's orbit being large compared to the size of an atomic cell. When this ratio is not large, the electron experiences rapid changes in its potential, leading to a breakdown of the effective mass approximation. Large magnetic fields force the electrons into orbits whose radii are ~(fic/eH)K. Fields of 10,000 T correspond to an orbit contained within one lattice cell. 100 T leads to orbits that enclose 10-20 sites, enough to begin to introduce additional structure in the cyclotron motion, and leads to the breakdown of the effective mass approximation. In the case of highly anisotropic layered compounds, these fields are reduced by an order of magnitude or more for motion in the direction perpendicular to the layers, amplifying the magnitude of the breakdown substantially. Shallow Impurity Studies. Steady State. 100T. CATEGORY 3. As noted in the preceding paragraph, as the orbit becomes comparable to the cell size, "central cell" corrections are required, and the effective mass approximation fails. Studies of the effect of the central-cell correction will enable the nature of chemical contaminants to be better understood in the case of technologically important materials such as GaAs, InP, and ternary and quaternary alloys based on the III-V compounds. Zeeman Splitting of Deep Impurity States. Short-Pulse Fields. 100-10,000 T. CATEGORY 3. The Zeeman splitting of the rather broad, deep impurity levels in semiconductors is rarely detectable. Observation of the Zeeman splitting should be possible for fields of the order of 100 T. These measurements would assist in the assignment of the symmetry and origin of these levels. If the study could be extended to 10,000 T, the Zeeman splitting would be comparable to the band edge of the semiconductor host, enabling the nature of the impurity states to be strongly modified, as well as allowing observation of the excited states of the impurity potential. Metals Research A number of tools are available for the study of the Fermi surface of metals, including magnetoresistance and de Haas-van Alphen measurements and
18 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES magnetic breakdown. The former two require that coc?" > 1 for the extraction of significant information. The latter requires fields such that (A£)2 = HftHEp, where A/T is the bandgap, Ep is the Fermi energy, and MB is the Bohr magneton. These conditions have required metals of high purity or small bandgaps, given limitations on H of 15-20 T. The availability of high fields makes possible broad application of these tools to systems of great interest and importance. We discuss four examples, but clearly the list of possibilities is much longer. For example, one might even be able to saturate the mag- netoresistance of simple metals in very high fields (potassium is a case in point) or to study the lattice conduction of heat by reducing electron transport perpendicular to the field in the extreme quantum limit. The four areas in which unusual opportunities become available are (a) electronically driven-phase transitions, (b) charge density waves in potassium, (c) electronic structure of exotic metals, and (d) concentrated binary alloys (or disordered systems). Electronically Driven Phase Transitions. Steady State. 150T. CATEGORY 1. The relationship between structural instability and high-temperature superconductors is still not understood and is clearly of great importance. Recent experimental studies have shown that structural instabilities and anomalous defect behavior are characteristic of the high Tc A15 structure superconductors. This result is thought to originate either in a high density of states at Ep or Fermi surface nesting. In these materials, cyclotron resonance measurements require high magnetic fields for two reasons: (a) fields greater than the upper critical field Hc2 are required so that the field penetrates the superconductor uniformly (fields of the order of 20-30 T are required for many A15 compounds, for example Nb3Sn); (b) because T is relatively short in these materials, very high fields are required to satisfy the condition COCT > 1. Furthermore, interesting regions of the Fermi surface (which are flat and therefore of large mass) also require high fields for observation. With sufficiently large fields, one could examine the size and shape of the Fermi surface in both transforming and nontransforming samples, thus directly probing the interrelationship between electronic density of states, Fermi surface nesting, and structural stability. To think of actually "following" the Fermi surface change of shape through a structural instability in this manner is an interesting prospect. Preliminary studies with static-pulse fields by Lowndes and Arko, using the Amsterdam 40-T quasi-static fields, with 0.2-second decay ramp time, have shown that oscillatory effects can be observed in some A15 compounds. Fields of 150 T will be required to map fully the Fermi surface topology in these materials.
Scientific Opportunities 19 Charge Density Waves in Potassium. Steady State. 30 T. CATEGORY 1. The electronic properties of potassium continue to remain a mystery if one accepts the conventional nearly free electron spherical Fermi surface for this monovalent metal. Overhauser has argued persuasively that the (a) lack of saturation of the magnetoresistance at fields for which U>CT = 300, (b) anomalous high-field torque anisotropy, (c) intense optical absorption with a threshold of 0.6 eV of a bulk metal vacuum interface, (d) conduction electron spin resonance ^-factor anisotropy, (e) residual resistivity anisotropy, (f) Hall coefficient discrepancy, and (g) remarkable lack of reproducibility of experiments on this supposedly "simple" metal all point to the existence of charge density waves in potassium. The waves are supposed to be oriented along the [110] directions, and a "general" crystal will contain domains in which the charge density wave vector Q is along a particular [110] direction for that domain. The existence of a unique Q value implies an anisotropy in the electrical resistivity parallel or perpendicular to Q. In a crystal containing many domains, the multidomain structure leads to a residual resistivity. However, each single Q domain possesses an anisotropic magnetic susceptibility (very small, of the order of 10~8 emu). Thus, in sufficiently large fields, one could line up the domains. Rotation of the domains must compete against local strain fields, and Overhauser estimates that fields in excess of 10 T are required. If high fields are applied to wires of potassium, one predicts that the residual resistivity will change by a factor of 5 back and forth as one rotates the field parallel and perpendicular to the wire axis. This unequivocal test of Overhauser's prediction is an important one that high magnetic fields can make of our understanding of the behavior of the so-called simple metals. Electronic Structure of Exotic Metals. Quasi-static. 100 T. CATEGORY 3. Studies of exotic forms of solids, many with reduced dimensionality, have shown that metallic behavior is" more general than previously thought. Superconducting polymers are known (e.g., [SN]* ), as well as layered compounds (e.g., the transition metal chalcogenides), which have nearly one- and two-dimensional structure for their conductivities. They tend to be more susceptible to lattice deformations, and there are a number of conflicting theories regarding the character of their electronic structure and lattice coupling. The deviation of their Fermi surfaces from flatness (in the case of one-dimensional metals) or ridgelike (for two-dimensional metals) is essential for the understanding of their conductivity and stability. However, all of these systems have relatively short conduction electron scattering times. Use
20 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES of conventional Fermi surface probing techniques (see section on Electronically Driven Phase Transitions) requires large magnetic fields so that cocr > 1. High-field investigations would provide a basis for evaluating the many theories that attempt to describe structural phase transitions in these systems. In particular, one might be able to determine the extent to which electron-electron correlation affects the phase transition and associated changes in conductivity in these less than three-dimensional materials. Concentrated Binary Alloys. Quasi-static for Exploration; Steady State for Details. 75-100 T. CATEGORY 3. The availability of very high magnetic fields would allow measurements of electronic scattering rates and of dimensional changes in Fermi surfaces in binary alloys to be extended beyond the dilute limit. There are AXB\ _ x alloy systems for which it seems likely that these quantities could be measured over the entire concentration range (x = 0 to 1). This would be a significant experimental advance, for effects strongly nonlinear in x, nonrigid band behavior, and order-disorder effects could all be studied. It is conceivable that strongly disordered systems, such as amorphous metals, could be studied in the quantum limit. Underlying short-range order may manifest itself in smeared quantum oscillations. Even changes in short-range order as a function of temperature and composition could be studied in principle. At present precise quantum oscillation methods are limited to nearly pure metals. High magnetic fields will free the experimenter of this constraint and allow for precise measurements of changes in band structure as a consequence of alloying and structural changes. Low-Temperature Physics Research Two interesting prospects emerge in low-temperature physics with the advent of magnetic fields greater than 50 T. Both involve phase transitions that could not be observed otherwise. The first is the Bose condensation of spin-aligned hydrogen, and the second is the observation of a ferromagnetic moment for liquid 3He-A Spin-Aligned Hydrogen. Steady State. SOT. CATEGORY 1. A number of experimenters are attempting to observe the predicted Bose condensation in spin-aligned hydrogen. The techniques all involve lowering
Scientific Opportunities 21 the energy of the parallel-electronic-spin molecular configuration as compared with the usual antiparallel electronic singlet molecule. This might be done using an oven to generate atomic hydrogen, then allowing recombination to occur in the presence of a strong field. Unfortunately, conventional fields (15T) are just on the borderline (or perhaps too small) for success. Condensation of spin-aligned hydrogen in the presence of a 50-T magnetic field should provide sufficient stability for the aligned state to remain for times long enough to look for the predicted phase transition. This transition has unusual features that make the search for it of great scientific interest. By the very nature of the spin-aligned state, high magnetic fields are required, at least at the formative stage of the triplet molecules. Ferromagnetic Moment in 3 He-,/4. Steady State. SOT CATEGORY 1. Paulson and Wheatley have shown that a ferromagnetic moment exists in 3 He-A using an ultrasonic method, but it was impossible to obtain a texture suitable for measurement in the low field limit. A field of 50 T should orient the moment, allowing for its direct measurement. Liquid 3He is one of the richest physical systems for study in condensed-matter science. It exhibits a complex phase diagram, with two quite distinct phases in the ordered state (the so-called A and B phases). The higher-temperature phase, A, is one where the 3He atoms are paired with parallel spin. In the presence of a magnetic field, the A -phase transition line splits into two, with the parallel nuclear spins predominantly along the magnetic-field direction for the AI phase. Leggett has argued that a ferromagnetic moment exists in 3He-A because of chemical and pairing effects. It is obscured by the internal dipolar coupling, which requires a field of 50 T to decouple the ferromagnetic moment from the dipolar field. Zero sound could detect the direction of the moment. As the external field is increased, the moment should swing from perpendicular to parallel to the field, allowing for the direct measurement of the moment. It is entirely possible that other orientation effects in 3He could come into play as the external field overcomes the orientation-locking power of the dipolar field. These will develop as one gets "used to" working with this quantum liquid without the hindrance of the internal dipolar field. Magnetic Properties This field first comes to mind with the advent of high magnetic fields. A Zeeman splitting of 1.4 K results (for a g = 2 spin) in a field of 1 T. Availability of 75 T leads to Zeeman energies in the 100-K range, easily of the order of the exchange field for many ferromagnetic and antiferromagnetic
22 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES materials. This leads to a wide variety of magnetic-field-induced phase changes in magnetic systems. It also suggests the possibility of trimerization of spin-Peierls systems for one-dimensional systems that have already undergone spontaneous dimerization from the uniform antiferromagnetic state. The energies are also comparable with the difference in valence energies in intermediate valence compounds. High fields could therefore be used to stabilize one limit of the intermediate valence state, allowing for the determination of the character of that limit, as well as a measure of the binding strength. We shall discuss three other aspects, all of which require high magnetic fields: (a) critical phenomena, (b) magnetic-field-induced transitions, and (c) singlet ground-state systems. Critical Phenomena. Steady State. SOT. CATEGORY!. The multicritical points in magnetically ordered systems permit tests of modern theories of phase transitions and scaling. Great progress has been made in the past few years. Universality allows one to generalize the behavior of magnetic systems to general (nonmagnetic) phase transitions in matter. One of the most interesting regimes is the bicritical point in an antiferromagnet, where the spin-flop, antiferromagnetic, and paramagnetic phase boundaries meet. To study this region, fields of several times the critical field Hc = (2//E//A)^ are required. (Here //£ and //A are the exchange and anisotropy fields, respectively.) Typical values lie in the 15-S0T range. An example that promises interesting physics is the two-dimensional magnetic system K2NiF4 for which the spin-flop field is 27 T. A variety of lower-dimensional and crossover phenomena can be studied in the vicinity of this regime. A second important field of research on critical phenomena concerns Ising systems in transverse magnetic fields. It has been shown for the one-dimensional case that the Ising model ground state in a transverse field can be mapped onto the Ising model in the absence of a field in two dimensions at arbitrary temperature. Series expansion results indicate that this is a general situation, with the Ising model in a transverse field in n dimensions mapping onto the Ising model without a field in n + 1 dimensions. Experiments on materials such as FeF2 in 50-75 T fields offer the possibility of studying the properties of the four-dimensional Ising model! Magnetic-Field-Induced Transitions. Steady State. 75 T. CATEGORY 1. The origin of ferromagnetism in metals is still an unsolved problem. For conductors that have large exchange enhancements, the effect of the applied
Scientific Opportunities 23 field is magnified proportionally to the exchange enhancement; theories of itinerant ferromagnetism suggest that a ferromagnetic phase transition might be induced on application of a large applied field for appropriate band parameters. The magnetic response of the metal to the field involves the detailed structure of the density of states, which generally has not been calculated with sufficient precision to make predictions. Experiments using 20-35 T have been made on Pd and YCo2. Although Pd is the most exchange-enhanced element of the periodic table and shows long-range polarization as a host for 3d elements (e.g., Fe, Co), no evidence for induced ferromagnetism has been observed in Pd. There is some evidence that YCo2 might show this effect. Fields of 50-75 T would allow an increase in resolution of almost two orders of magnitude because the exchange-enhanced portion of the magnetic moment is expected to vary as D3H3, where D is the exchange enhancement. Studies of the coupling between magnetic impurities in strongly enhanced hosts would also be rewarding. As the susceptibility of the host increases, the range of impurity-impurity coupling also increases, so that a high field might be used to vary the coupling in a controlled manner. These results would be important for understanding dilute alloys, spin-glass, and nondilute magnetic systems. Singlet Ground-State Systems. Steady State. SOT. CATEGORY 1. There are many magnetic systems in which the single-ion ground state is a singlet. Magnetism occurs through the Van Vleck off-diagonal moment, which can undergo a spontaneous transition if the mutually induced exchange coupling is sufficiently strong to overcome the crystal field single-ion splitting. The excitation spectrum of such systems remains an open theoretical question. High magnetic fields can compete with the crystal field splitting, in some cases allowing magnetic ordering where none existed before. It would be of great interest to follow the development of the induced moment as the applied field reduced the splitting of the ground and excited state. In particular, the theory can only treat the limit of small excited-state occupation. Experiments that move smoothly from the region where the theory is applicable to the region where it is not might well give a clue to the way to approximate the system (even to describe it!) in the regime of excited-state occupations comparable to the ground-state occupation. Materials Research The range of application of high magnetic fields to materials research is great. We shall concentrate on three subjects: (a) high-field superconductivity,
24 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES (b) metallurgical phase transitions, and (c) field-induced changes in pressure and temperature with geophysical applications. An interesting additional application is the nuclear magnetic resonance of 3 He in the embrittlement problem. Though the number of nuclei is small, at high fields (75 T) the resonance sensitivity is comparable to electron spin resonance (see the subsequent section on Research in Biology), allowing the use of magnetic resonance to study various nucleation phenomena related to mechanical strength of materials. Other applications are of comparable interest; however, the three we have chosen for discussion should serve as representative examples. High-Field Superconductors. Steady State or Quasi-static. 75 T. CATEGORY 1. The present boundaries of the superconducting state are approximately 23 K (NbsGe films) and 60 T (PbMo6S8 ). These boundaries are not well understood from either a theoretical or an experimental viewpoint. There is little doubt that the availability of research facilities for higher dc magnetic fields would be a boon for investigators. Work at high fields should yield a better understanding of the basic material parameters that control both the critical temperature, TC, and the upper critical field He2. Materials of interest include the Chevrelle phases, Bl and A15 structures, thin films, and finely divided superconductors produced by either rapid quenching or plasma deposition techniques. Investigations of critical current densities and flux pinning at very high fields should throw new light on the electronic and magnetic interactions associated with small-scale defects in solids. Of special interest is a study at nigh fields of the empirical relationship Jc x B = constant, which holds approximately for most practical conductors. Further work is necessary on the fundamentals underlying this relationship, with the objective of discovering new methods of flux pinning to remove this serious limitation. Progress in this field would be of vital importance to future high-field magnet design. Other basic aspects of superconductivity might be advanced by the availability of fields approaching 100 T. For example, one might search for the existence of triplet pairing of the electrons, which, if it occurred, should modify the magnetic-phase boundary curve at field energies approaching the condensation energy. Investigations of the temperature variation of //c2 would also allow a test of a variety of theoretical predictions connected with the paramagnetic limit, effect of spin-orbit coupling, and other basic features of high-field superconductors.
Scientific Opportunities 25 Metallurgical Phase Transitions. Quasi-static. 200 T. CATEGORY 3. When there is a difference in the magnetic susceptibility or the magnetization between the phases involved, an applied magnetic field can induce or suppress the phase transformation. The use of magnetic fields to control the phase transformation is advantageous, for example, compared with driving the transformation with temperature changes, in that the change in the driving force can be achieved in a very short time, which is important when the transformation is very rapid. An example is the martensitic transformation in magnetic alloys, including steel. Martensite in steels is the a-Fe phase super- saturated with carbon and is obtained when the carbon steel is quickly quenched. Because of the presence of carbon in the interstitial lattice position, this phase is exceedingly hard. Even though the martensitic transformation has been known for almost 2000 years, its mechanism is not completely understood. Unlike many transformations in which atomic diffusion is a vehicle, the martensitic transformation proceeds via a collective motion of dislocations. Therefore, the mechanism of nucleation must be very different from the usual case. A high-magnetic-field pulse could induce the transformation during the length of the pulse, with the transformation ending when the pulse was over. Thus, one could stop the transformation at various stages, making it possible to observe the process (e.g., by electron microscopy) at intermediate stages of the transformation. At present, the lath martensite formation in Fe68Ni32 alloy, which occurs at around 100-200°C, would appear to be a good first candidate for study by this technique. Field-Induced Changes in Temperature and Pressure with Geophysical Applications. Short Pulse. 1000 T. CATEGORY 3. A rapidly changing high magnetic field produces large changes in temperature and pressure, by either eddy-current effects or adiabatic heating and cooling. Surface (eddy) currents are generated within the penetration depth in the conductor that exclude the flux from the interior. The surface temperatures and pressures increase proportional to B2. At 100 T, the surface melts and pressures of 400 atmospheres are reached. Much higher fields can be achieved and the states of matter under extreme conditions can be studied. Intense shock waves are generated into the bulk of the metal, with the formation of new structural phases, not feasible in conventional laboratory environments, expected. Should this possibility be realized, it would have an impact on research in geology and geophysics. The pressure at the center of the earth is
26 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES estimated to be of the order of 3 million to 4 million atmospheres, which can be generated by a field of 103 T, whereas the highest static pressure achieved to date, is about 1.7 million atmospheres. The solubility of H, S, Si, and K under such high pressures in Fe-Ni alloys (which form the core of the earth) is one of the most important quantities controlling the nature of the core. The knowledge of the reactions of the Fe-Ni alloy with oxides or sulfides of Mn, Fe, Cu, and Ni under high pressure and temperature should help to im- prove understanding of the distribution of the mineral resources on the earth's surface. In an insulating medium, a pulsed field will penetrate the entire volume. Thus, uniform changes in temperature can be produced rapidly for a short pulse in an adiabatic manner, even when the thermal conductivity of the solid is extremely low. This rapid change in temperature would quench metastable phases such as amorphous phases, which may not be obtainable by other methods. Research in Chemistry High-magnetic-field research will help to answer three outstanding problems in chemistry: (a) understanding complex reactions in condensed phases; (b) understanding the factors determining the dynamics of atoms in molecules and of atoms during the formation of molecules; and (c) understanding the electronic structures of highly excited states. The discussion that follows deals with ion-cyclotron resonance, spectroscopy and structure determination, chemical-biological reactivity, and chemical dy- namics. Each of these research areas requires more than just a high mag- nectic field; adjunct facilities, such as state-of-the-art laser systems, spec- trometers, and controlling computers are also needed. Ion-Cyclotron Resonance. Steady State. 100T. CATEGORY 1. For a given detection frequency, the mass resolution varies as B*. The state of the art is a limit of 1000 mass units at 7.5 T. A 100-T steady field would allow a mass limit of 200,000, thus introducing the possibility of studying biological systems, polynuclear complexes, and large organometallics. It will be feasible to sequence a polypeptide by conducting successive chemical reactions in the spectrometer. The direction of activity in this field is toward higher magnetic fields, laser-induced chemistry of ions, and polynuclear systems. At 100T, an ion-cyclotron resonance device can be used to study less than a picogram of material; therefore, this method would be useful for trace species in biological systems and for isotope separation.
Scientific Opportunities 27 Spectroscopy and Structure Determination. Short Pulse. 100 T. CATEGORY 3. There are a vast number of useful spectroscopic experiments that become feasible when 100-T fields are available. It will be possible to study nonlinear magnetooptics in considerably more detail than was previously possible. The Faraday effect, Cotton-Mouton effect, and higher-order magnetooptic effects have not been explored as have electrooptic phenomena. Nonlinear magnetooptics is a fresh research field for experiment and theory. This is especially true for dilute gases in which the lower-order effects are already often difficult to detect. The measurements that could be made at higher fields would bear directly on the higher-order magnetic susceptibilities of molecules, hence providing the basis for a more nearly complete description of the energies of molecular systems in magnetic fields. Fields of 100 T can decouple the electron spin from the molecular axis in many molecules such as hydroxyl or nitric oxide. It would be possible to study the effects of this decoupling. First- and higher-order Zeeman effects will be possible for many molecular excited states. The more complex Zeeman effects of high-Rydberg states will be accessible. Rotational ^-factors can be measured, and nuclear hyperfine structure in excited states can be observed without the difficulties of the radiative width or Doppler width that currently obscure many phenomena. The quartet and quintet states of molecules, previously unknown, might be explored; level crossing (and anticrossing) and optical double-resonance experiments can be contemplated in a new regime of separations between zero-order states. Second-order magnetic energy shifts could be measured even with complex molecules and other systems displaying wide linewidths. Chemical and Biological Reactivity. Steady State. 100 T. CATEGORY 3. Magnetically related research avenues that will lead to improved understanding of chemical reactivity are studies of reaction mechanisms by variations of relative reaction rates in complex systems; studies of new chemical properties brought about by changing molecular wavefunctions in the magnetic field; studies of the effects of magnetically induced anisotropy on otherwise isotropic chemically reacting systems; and studies of photochemical reaction pathways involving magnetically sensitive singlet and triplet states of molecules in high fields. Because 1 T produces an energy splitting of 1.4 K for a free electron, high fields can generate Zeeman splittings of hundreds of degrees so that chemical activation processes can be modified substantially.
28 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES Optically induced chemical reactions often involve electron spin triplet states of molecules or biradicals. Because of the anisotropy of the spin-spin and spin-orbit interaction, the three spin sublevels are not degenerate even in zero field and there is an intrinsic chemical spin anisotropy. Different spin sublevels will have quite different chemical reactivities. At normal tempera- tures the observed reaction rate is an average over the contributions from the three states. In fields of 100 T, directed along particular molecular directions, the rates of these photochemical reactions may be affected by factors of 103 as a result of the spin-orbit anisotropy. Studies of this type can lead to a substantial improvement in the understanding of the nature of chemical re- actions, cage effects, and radiationless processes. The magnetic field in this case causes both orientation and spin selectivity. The rates of chemical reactions in solutions are described in terms of rate constants that are ensemble and orientation averages of microscopic rates. The relative orientation of the chemical reactants influences the course of a reaction, and for large asymmetric molecules, many encounters may be needed before reaction occurs. Because of these effects, we can expect re- action rates to be sensitive to 100-T external fields. The opportunity to study in a direct fashion the steric factors influencing chemical reactions is thus provided. The magnitude of effects such as this depends on a number of as yet unknown molecular topological parameters and on the degree of orienta- tion of |3: 0 = (XH - Xj") ff2/kT, where X|| and Xj" are the susceptibilities parallel and perpendicular to the plane of the molecule, respectively. A mole- cule such as benzene has (xn - Xj") = 6 x 107 emu/mol, so at 300 K, |3 ~ 10"3 for H =151. The degree of orientation in the sample can be studied by means of the magnetically induced birefringence (Cotton-Mouton effect) even at relatively low magnetic fields. Larger anisotropic molecules and biopoly- meric systems are even more readily oriented since the diamagnetic suscepti- bility is an additive property. Studies of rotational relaxation in liquids and biological systems will be possible with pulsed fields of 100T, with initial orientations that are different from those achieved with electric fields. Chemical reactions that involve ion transport, as well as anisotropic molec- ular motion, such as those occurring in biological membranes, are expected to be magnetically sensitive at sufficiently high fields. Research of this type will provide a new dimension in understanding properties of membranes. Chemical Dynamics. Steady State. 100 T. CATEGORY 3. Fundamental processes in chemical dynamics are receiving intense experi- mental and theoretical study. There is need for tools that can be used system- atically to perturb the potential surfaces. Surface crossing points are parti-
Scientific Opportunities 29 cularly vulnerable to external magnetic fields. Because these are the regions of nuclear configuration space in which much of the dynamical action occurs, their study is important. Using fields of 100 T, it will be possible to explore magnetic effects on collisions and on the breakdown of the Born- Oppenheimer approximation. Laser-induced chemical reactions occurring sub- sequent to the excitation of single rotational-vibrational levels will be af- fected by the field-induced mixing of discrete rovibronic levels with con- tinuum states. Other important applications in dynamics are the study of magnetic effects on collisions at high fields, on the effects of energy excess on energy transfer, and on orientational selection rules in moderately large mole- cules. The nonradiative relaxation and autoionization of molecules will also be influenced by 50-100 T magnetic fields that will couple Rydberg levels significantly to the ionization continuum. The collision of two hydrogen atoms provides a prototype situation for understanding magnetic effects on dynamics. During the approach of two H atoms at zero field, the potential energy may be lowered when the spins are opposed corresponding to the formation of the ground singlet state. In the event of a collision with a third body, the H2 molecule would then be formed in some vibrational-rotational level of the ' 2g+ electronic state. In the pres- ence of a 500-T field, the situation is changed if the atoms are at thermal equilibrium for normal temperatures, for now the pair of atoms will most often see a repulsive potential comparable with kT so that they will slow down on approaching one another. The formation of H2 is prohibited, and the atoms will fly apart after the collision. Whereas in the zero-field case the bound singlet and repulsive triplet state of H2 are degenerate only at large separations of the atoms, the situation is changed in a magnetic field, for then the potential wells for the |0> and I -1 > spin states should "cross" at some relatively small internuclear separation and there is a threshold kinetic energy for bond formation. Transitions in the crossing region can occur by means of third-body effects or by the spin-orbit interaction, which for the case of H22lg+ and 32//~ is vanishingly small. With nonhydrogenic doublet atoms or molecules, the spin-orbit coupling is much larger and the rate of transitions between the singlet and triplet potential curves could be studied in high-field experiments. In complex molecules there are bound energetically accessible states of both singlet and triplet multiplicity, and it is the state-to-state details of the combination processes that are magnetic-field sensitive. New features of spin-relaxation dynamics leading to the thermal equilibrium can be ob- served from such experiments. Calculations for the potential curves of H2 and H2 + in strong magnetic fields have already been attempted. These calculations could be extended to molecules having three or more nuclei and to the new sets of potential sur- faces used in studies of molecular reaction dynamics in the presence of strong
30 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES fields. The symmetry properties of the states and the selection rules will be altered. In particular, there is no longer rotational invariance and the field introduces new quantization axes where space goes from spherical to cylindri- cal symmetry. This is expected to influence the motion of charged particles so that in ionization processes the states of emitted electrons are not con- tinuous. These effects influence the angular distribution of the photoelectron spectrum. Photochemical reaction paths and rates can be modified because suf- ficiently high magnetic fields will influence nonradiative relaxation processes in molecules. Here the effects could be quite large for fields of 100-500 T, and the experiments will contribute to the understanding of radiationless processes in molecules and to the development of chemical separation methods. Pulsed fields could be used for studies of many of these optically induced processes. Molecular recombination pathways may also be studied using high fields (n^H ~ kT). If photolysis of B in the presence of A produces two molecular ions, say by electron transfer: A +B* =*A~ +B~, the recombi- nation of the ions will lead to A and B in a distribution of electronic and vibrational states that will be changed by an external field. One process that might be enhanced by the field is 2A~ + 2/T => 3A + 1B producing metastable triplet states of A. Reactions of this general type might possibly occur in photosynthetic systems. Research in Biology Uses of Nuclear Magnetic Resonance (NMR). Steady State. 75 T. CATEGORY 1. In the past ten years the application of NMR spectroscopy to the study of structures of biological molecules and solutions such as proteins, nucleic acids, lipids, and carbohydrates has contributed important understanding of structure and function basic to biomedical sciences. To obtain greater res- olution and observe individual resonances from nuclei in all parts of the molecule, considerable effort has been devoted to increasing the strength of the static field. Superconducting magnets of 5.0 T were used in 1962, com- mercial spectrometers are now available with fields up to 9.4 T, and develop- ment of a spectrometer operating at 14.1 T is well under way. These advances were valuable for small molecules, but the resolution for large molecules remains frustrating^ small. Because the molecular motion in solution is faster than mechanically obtainable spinning speeds or electronically attainable forced spin-flip rates, the new techniques of magic angle spinning and dipolar decoupling do not help appreciably in increasing the resolution. Present ex- perience also does not encourage one to predict the attainment of high res- olution in spectra of solid proteins of high molecular weight.
Scientific Opportunities 31 TABLE 1 Statistical Prediction of Number of Separate a-Proton Resonances Observable in Representative Proteins at Several Field Strengths Ribonuclease Subtilisn Dehydrogenase Hemoglobin Immunoglobulin Field Wm= 13,690 Wm = 27,288 Wm = 39,805 Wm ~ 60,000 Wm ~ 150,000 (Tesla) 124 Residues 274 Residues 374 Residues 600 Residues 1500 Residues 5 12 5 0 0 0 10 24 14 8 0 0 20 56 27 20 12 0 40 74 55 40 24 0 80 93 109 80 48 20 Resolution: Whether the resolution will increase with increasing fields de- pends on whether the individual resonance lines narrow or broaden as the field increases. For cases in which magnetic dipolar relaxation is expected to dominate, as, for example, for the a-protons of proteins, resolution should increase as field is increased. Taking into account the predicted line widths, the number of resonances, and the spectral range in which the resonances occur, one can make a statistical prediction of the number that should be separately resolved at various applied fields. The number for a variety of proteins is given in Table 1. At currently available field strengths, we can resolve a small fraction of the resonances in proteins of low molecular weight, such as ribonuclease, while those of higher molecular weight are not yet open to study. Improvement in field strength, however, greatly increases the power of the method for lower-molecular-weight proteins and brings new classes of proteins into range. For 13C, chemical-shift anisotropy is important for tri- gonally bound carbon, so that resolution of carbonyl groups in proteins, for example, would worsen at sufficiently high fields, but for tetrahedral carbon (as in a-carbons) the situation would continue to improve in the range of 5-80 T. Currently there is great interest in studying the solution structure of the transfer RNA's for which crystal structures have been established by x-ray diffraction. At available fields, the low-field-shifted NHN resonances are not completely resolved, and their number (and assignments) is a subject of some controversy. Increased field strength to 20-30 T will result in sufficient reso- lution to allow complete assignments, studies of thermal folding and un- folding, and determination of the conformational energies. Other research areas opened to study at higher fields include antigen-antibody interaction, protein and lipid interaction in cell membranes, interaction of histones with nucleic acids, enzyme complex assembly, and identification and mapping of receptor binding sites. Studies of phosphorus metabolites in intact cells using 3' P resonance is currently a very important development. These studies, pursued at higher fields, would benefit by increased sensitivity and spectral spread.
32 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES Sensitivity: Sensitivity is expected to continue to improve in the 5-80 T range, though not so rapidly as at lower fields, because (T2/Tt) decreases as l///o in the high field limit. Increases in sensitivity are important because they allow studies of smaller samples of substances that are difficult to obtain and studies at lower concentrations where aggregating or insoluble species are concerned. New Phenomena-Orientation Effects, Observation of J, Coupling to Quad- rupolar Nuclei: At fields used to date, orientation of single diamagnetic molecules has had no detectable influence on NMR spectra. Orientation has been achieved using liquid crystals, where domains, rather than individual molecules, are oriented. Further, small orientations of small molecules have been detected by the Cotton-Mouton effect. Recently such effects have been used to study rigid helix lengths in DNA, orientation in phospholipid mem- branes, and local order in liquid polymers. If a molecule has an anisotropic diamagnetic susceptibility (such as ben- zene AX~ 60 x 10~6 cm3/mole), orientation will be of the order H^Ax/RT. Orientation at 10 T and 100 T will be 2.4 x 10~s and 2.4 x 10~3, respec- tively. If the dipolar structure of completely oriented benzene covered a spectral width of 10 kHz, structure would not be visible in the spectrum at 10 T but might be visible at 100 T. By choosing molecules with larger aniso- tropic susceptibilities (condensed-ring system, paramagnetic complexes) and spectra more sensitive to orientation (larger dipolar splitting or quadrupole splittings), such features might become visible at fields as low as 10 T. This would provide a method for the determination of precise interatomic dis- tances within complex molecules in solution, a determination for which no other satisfactory methods are available. In regard to observation of J coupling to quadrupolar nuclei, the formula for l/Tiq due to relaxation by a quadrupole moment suggests that l/7\ will become long in the high-field limit (COTC > 1). Under these circumstances, the splitting of signals from adjacent nuclei by the quadrupolar nucleus will be- come visible; for example, peptide proton resonances will show splitting by the I4N nuclei to which they are attached. This extra parameter should be useful in characterizing the bonding state of the peptide NH (whether it is H-bonded, nonplanar, solvent exposed, etc.). In the same way 14N broadening and eventual splitting in transfer RNA's will be detectable at modestly higher fields and could lead to unambiguous assignment of bridging NHN signals and characterization of the hydrogen- bonding scheme in these molecules. NMR Imaging: The use of higher applied fields for NMR imaging will result in greater sensitivity, which means that smaller volumes of tissue will give
Scientific Opportunities 33 detectable signals and smaller biological structures can be imaged. For typical experimental conditions, the volume of tissue giving rise to an image element at 10 T and 80 T is 3 x 1Q-8 cm3 and 1 x 10-9 cm3, respectively. Time- averaging techniques will improve these volumes somewhat, so that three- dimensional imaging of the internal structure of isolated individual cells will be possible. This enlarges the prospect for studying and understanding chemi- cal processes and material transport in the cell. Ultra-High-Field NMR Quasi-static and Shorter Pulses. Up to 300-400 T. CATEGORY 2. For conventional-pulse NMR spectroscopy it is necessary to have very steady high dc fields with great uniformity (1CT9 over cubic millimeters) over ex- tended periods of time, preferably from a few hours to continuous operation. The availability of such fields allows maximum information to be extracted from the spectrum and permits such aids to assignment in the various forms of double irradiation and cross relaxation to be applied. At the same time, it should be pointed out that somewhat limited but nevertheless quite useful experiments can be carried out under less stringent conditions-long pulsed fields on the order of 100 T for seconds, up to 300-400 T for milliseconds. Rapid-passage techniques would provide high-field information on 7\ and T2 relaxation times, /-modulated echoes, and even high-resolution spectra, pro- vided that the time evolution of the magnetic field is known and re- producible. Magneto-Quantum Electrodynamics (MQED) Short Pulse. Greater than 250 T. CATEGORY 3. The availability of high magnetic fields and high-energy electrons will make possible more precise tests of quantum electrodynamics without requiring the calculation of many other, nonelectrodynamic effects such as those due to strong interactions that result from the presence of protons in the experi- ments. Small pulses of high-energy electrons (E ~ 900 GeV) will be available in the next five years when Fermilab's "Tevatron," a 1000-GeV proton ac- celerator, is operational. The electrons will be produced by decays of muons and bremsstrahlung. Combining these beams of electrons with pulsed 250-T magnetic fields will make possible two types of experiments that are of fundamental interest. Each of these measurements would be done on a "pure" electrodynamic system-electrons in an external magnetic field. The first of these experiments would involve tests of radiative corrections to quantum electrodynamics in strong fields. Measurements of the anomalous
34 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES ^-factor of the electron in a high field and of the production of electron- positron pairs in a field will provide tests of quantum electrodynamic theory and will indicate the importance of "strong interaction" effects in electro- dynamic phenomena. The second involves the measurement of the spectrum of synchrotron radiation at high energies and high fields. These measurements are of con- siderable interest in calculating quantum corrections to the classical synchro- tron radiation formulas. For E = 900 GeV and B = 250 T, the parameter 2 = 3/2(E/mc*)B/Bc, where Bc = 4.4 x 109 T, is equal to 0.15. When 2 ~ 1, quantum effects are important, so that measurement of the spectrum at 250 T might exhibit these corrections. In addition, at these energies and fields, the radiative damping will be extremely large, for the elctron would radiate away all of its energy in less than 1 cm. This strong damping regime will provide further tests of the theory, as well as potential copious sources of high-energy photons. Spectroscopic Research The availability of high magnetic fields will expand greatly the opportunities for spectroscopic studies of atoms and molecules. Experiments have been performed in fields of the order of 15 T (maximum). Extension to 30 T would probably double the number of new experiments, with from 60 to 75 T doubling the number again. One can anticipate steady progress in this field, increasing linearly with increases in available fields. Apart from the intrinsic interest that these new experiments would gener- ate scientifically, they would also be relevant to astronomical investigations. The magnetic fields at the surfaces of white dwarfs are of the order of 100-1000 T. Current observations are fitted to theoretical spectra of atoms appropriate to this field range, which may not be a sound procedure. Not only are the theoretical treatments limited in their accuracy, but the combi- nation of high temperatures and high fields results in peculiarities in lineshape that can complicate observation substantially. This "motional Stark effect" lineshape can, however, overcome the Doppler broadening and allow studies of radiative and collisional broadening. Laboratory simulation of astro- nomical spectra might become a valuable tool in the study of white dwarfs and other objects with magnetic fields in the 100-1000 T range. We shall discuss four categories of spectroscopic experiments that would be possible with high magnetic fields: (a) high Rydberg states in which the Zeeman energy and the Coulomb energy are comparable, (b) motional Stark effect lineshape, (c) anticrossing observations, and (d) collision dynamics.
Scientific Opportunities 35 High Rydberg States. Steady State, 75 T; to Short Pulse, 1000 T. CATEGORY 1. The magnetic energy becomes comparable with the Coulomb energy for elec- trons in a principal quantum number state n when B = 105/n3 T. This requires that one work at very high quantum numbers for laboratory fields (Cal has been studied to n = 90). Fields of 1000 T would enable one to study much lower-energy states and to extend the number of systems that could be studied. The behavior of complex atoms in these field ranges is not well charted. In addition to the intrinsic interest of states in the regime where the magnetic and Coulomb energies are comparable, magnetic white dwarf stars exhibit fields at their surface between 100 and 1000 T. Determination of the spectra of atoms and ions in the laboratory under similar fields would aid substantially in the identification of spectral lines from these sources. Motional Stark Effect Lineshaper. Steady State. 75 T. CATEGORY 2. When an atom is subjected to a large magnetic field, its emission lineshape will be dominated by the motional Stark effect if its velocity (temperature) is sufficiently great. This effect arises from the velocity-dependent terms in the absorption line energy: 5£"(v) = (vy/c)E0 + (a'/c*)(vx2 + i>y2)//2. Here E0 is the absorption energy in the rest of the frame of the atom, vy is the speed of the atom in the y direction, the field H is along the z direction, the light that is absorbed propagates along the y direction, a is one half the difference in atomic polarizability in the ground and excited state, and c is the velocity of light. The first term is simply the Doppler shift; the second results from the atom experiencing an electric field E = (v x H)/c in its own rest frame and is quadratic as a consequence of the second-order Stark shift of the atomic energy levels. The remarkable aspect of the combination of both terms is that for sufficiently large fields and velocity, the absorption lineshape becomes highly asymmetric. On one side it is characterized by an abrupt cutoff, on the other by a relatively long exponential tail. This shape, together with the velocity profile, greatly complicates the absorption spectrum of an atom at high temperatures in high magnetic fields. It may well dominate the emission lineshape for sources including white dwarfs and (under some circumstances) even plasma fusion machines. The study of atoms under these conditions could, therefore, be important for analytic purposes. Further, the cutoff occurs over a width comparable with the homogeneous linewidth not the Doppler width of the transition. This finding suggests that a careful lineshape analysis can result in sub-Doppler information in high-temperature gases.
36 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES Anticrossing Investigations. Steady State. 75 T. CATEGORY 2. The magnetic-field energy can be made sufficiently great that atomatic or mo- lecular energy levels possessing large zero-field separations can be made to cross. In general, matrix elements exist between levels, so that a repulsion occurs in the crossing region and the levels "anticross." This means that if we monitor the emission of one of the levels preferentially, the intensity of emission will "switch" from one line to another as the field increases through the anticrossing region. The beauty of such measurements is twofold. They enable one to deter- mine the zero-field splitting accurately (assuming that the Zeeman energies of the two levels are known separately or can be determined from the spec- trum). By this technique, therefore, one can determine excited triplet-state energies precisely, though direct absorption from a ground or lower-lying singlet may be forbidden. Because the zero-field splittings can be substantial, this form of spectroscopy requires larger fields as the zero-field splitting becomes larger. The second aspect of this measurement is the width of the crossover regime. For sufficiently long-lived states, the transition width is given directly by the matrix element that couples the energy levels. This can be an interesting quantity, representing a highly forbidden term in the Hamil- Ionian (e.g., between states of different multiplicity) that could not otherwise be obtained. This form of spectroscopy will undoubtedly be used extensively for studies of excited levels and their couplings in the future. An example is the study of the quartet states of CN by T. A. Miller, who determined the rotational constant and bond length of these states, as well as the fine- structure constants and a rough value of the vibrational frequency. Before these experiments, nothing was known of the quartet states in CN. The availability of higher fields will considerably enhance the chances of finding similar anticrossings in molecules like N2 + and C2. Collision Dynamics. Steady State. 7ST. CATEGORY 2. The presence of an intense magnetic field generates a precession of the ground and excited moments of atoms or molecules that can change the collision cross section in a gas. This relatively young field began with the thesis of J. C. Gay and promises to yield information concerning the collision potential for like and unlike atoms and molecules. An example is Gay's study of the Na-Na* resonant transfer process. He studied the transfer of excitation from one Zeeman component (M=-3/2 sublevel of the 32/>3/2 state of Na) to another (M= 3/2 of the same sublevel) using a narrow-line laser to excite
Scientific Opportunities 37 only the former Zeeman component. Application of an 8-T field effectively quenched the resonant transfer. The future potential of such studies lies in the ability to alter a particular zero-field excitation transfer process by application of a field (in the Na-Na* resonant transfer case, shutting off the role of the long-range dipole-dipole interaction) and allowing other terms to be studied separately (in the Na-Na* case, higher-order nonresonant terms of the collision potential could be studied). The larger the field, the shorter the collision time can be and still have the excitation transfer dynamics altered. This line of research promises to become important once fields of sufficient strength are available.
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