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E National Facilities INTRODUCTION Condensed-matter studies have traditionally been small-scale phys- ics, performed at laboratories located in the scientist's home institu- tion, be it a university, governmental, or industrial laboratory. How- ever, with the advent of neutron scattering in the 1950s and, more recently, synchrotron radiation, as important probes of solids and liquids, a significant fraction of condensed-matter physics research requires the use of extremely costly facilities that are available only at national laboratories. In addition, special environments (e.g., high magnetic fields) required to understand the condensed states of matter are again beyond the financial resources of individual institutions. For both reasons the national facilities at either national laboratories or particular universities have come to play a special and important role in condensed-matter physics research. The two most substantial items in the national facilities budget are for sources of neutron beams and of synchrotron radiation. Table E. 1 explores how these two techniques are utilized in different and complementary ways in condensed-matter physics studies. 265
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APPENDIX E 267 SYNCHROTRON RADIATION RESEARCH Introduction Synchrotron radiation is electromagnetic radiation emitted from particle accelerators by charged particles (usually electrons) with large energy in the range from hundreds of MeV to 10 GeV or more. With suitable instrumentation one can select a narrow bandpass of mono- chromatic radiation and explore a range from the visible (2-eV photon energy) to the hard x-ray region (50,000 eV or more). The current widespread interest in synchrotron radiation is illustrated by the increase in U.S. users from 50-60 in 1973 to 550-650 in 1983. Synchrotron radiation has a number of characteristics desirable for research, e.g., high brightness,8 wide tunability, strong collimation, linear polarization, stability, and the fact that the radiation occurs in 0.1- to 1-nanosecond pulses. Synchrotron sources provide intense radiation at wavelengths for which laser sources are either unavailable or not yet tunable (photon energies above 50 eV or wavelengths shorter o than 250 A). Synchrotron radiation is essentially incoherent but can be made to be partially coherent by the use of devices known as undula- tors that provide huge enhancements of brightness (by a factor of 1000-10,000), at certain specific wavelengths, over the already bright conventional bending magnet sources of synchrotron radiation. Similar devices, called wigglers, provide correspondingly large enhancements of the intensity of synchrotron radiation at specific wavelengths. Summary of Present Synchrotron Facilities in the United States We briefly describe the current status of the five synchrotron laboratories in the United States with the approximate numbers and characteristics of their user communities along with the evolutionary nature of research currently being carried out. Cornell High Energy Synchrotron Source (CHESSJ CHESS is the parasitic synchrotron laboratory that is part of the particle-physics operation at the 4-8 GeV Cornell Electron Storage * Spectral brightness is defined as the number of photons. within a narrow energy bandpass, emitted per unit area of the source and per unit solid angle. Radiation intensity is the integral of spectral brightness over the source area. Some experiments use only high intensity, while others require high brightness as well.
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268 APPENDIX E Ring (CESR). CHESS has supplied synchrotron radiation for about 2 1/2 years and is funded by the National Science Foundation (NSF). Because of the large circumference of CESR, single bunch operation gives CHESS a particularly useful time structure, which has been used to study x-ray diffraction on a nanosecond time scale. Three primary beam lines supply hard x rays (hm > 3 keV) to six experimental stations for general use. Three stations scan energy continuously in the ranges 3-20, 3-35, and 3-50 keV, respectively, and two other lines provide focused x-ray beams. CHESS has ~75-85 active proposals with one third from Cornell faculty and staff, one third from other universities, and one third from industrial and national laboratories. National Synchrotron Light Source (NSLS) The NSLS at Brookhaven National Laboratory is a Department of Energy (DOE)-funded facility with a 750-MeV electron storage ring for ultraviolet (UV) and soft-x-ray research and a 2.5-GeV storage ring for x-ray research. Each ring is designed for high brightness from conven- tional bending magnets, and each is 100 percent dedicated to photon production. The UV ring has recently completed its first year of operation, whereas the x-ray ring is at an early operational stage and using photons for alignment of instrumentation. There are 16 ports on the UV ring and 28 ports on the x-ray ring; each port can accommodate several experimental stations. Twenty-two experimental stations are in operation or under construction on the UV ring, and 34 stations are under construction on the x-ray ring. About 60-70 percent of the beam lines are being funded and constructed by participating research teams (PRTs) from a variety of university, industrial, and national laboratory groups. The PRTs receive up to 75 percent of the beam time for their investment in equipment and support, with the other 25 percent as well as ~100 percent of time on NSLS user beam lines available to the general scientific community on a proposal basis. At present there are 34 PRTs with a total membership of nearly 200 scientists. A free- electron laser on the UV ring has been installed, and several undulators and wigglers are under development for installation in the x-ray ring in the near future. Stanford Synchrotron Radiation Laboratory (SSRL) SSRL is a DOE-funded facility at the Stanford Linear Accelerator Center that utilizes the 1.5-4 GeV storage ring, SPEAR. Fifty percent of its time is dedicated to photon production, and the remainder is
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APPENDIX E 269 available for parasitic use of synchrotron radiation during particle- physics experiments. At present, there are 18 experimental stations on 7 ports, of which 3 are conventional bending magnet ports and 2 are either 8-pole wigglers or 30-period undulators. Two new beam lines are being developed in joint SSRL and PRT collaborations: one has a 54-pole wiggler, and the other has interchangeable undulators to optimize different portions of the spectral region 10-1000 eV. One of the remaining straight sections is committed to development of an in-vacuum undulator capable of producing 8-keV x rays, and another section is for an additional multipole wiggler to be developed in collaboration with outside PRTs. Almost all of the available time is assigned to the general scientific user community on a proposal basis. At present there are 150-200 active proposals from a user community of approximately 200 scientists. Synchrotron Radiation Center (SRCJ SRC is operated by the graduate school of the University of Wisconsin-Madison and is funded entirely by the National Science Foundation (NSF) as a user facility. At present, SRC consists of two storage rings: Tantalus I, a 240-MeV ring in operation since 1968, and Aladdin, a l-GeV ring in the early stages of operation and commission- ing. Aladdin is a high-brightness source for both UV and soft-x-ray research in the range 6-3000 eV. It has 36 primary ports, each averaging 50 milliradians. At present, 22 of these ports are being equipped with beam lines for synchrotron research, 9 by PRTs and 13 by the SRC either with new monochromators specially designed for high brightness or with instruments currently installed on Tantalus but with upgraded beam-line optics to take advantage of the high brightness capability of Aladdin. Four more PRTs have been proposed but are not yet funded. This represents about 100-150 scientists from all parts of the country. Besides the normal bending-magnet sources, there are four long straight sections, three of which are available for installation of wigglers, undulators, and free-electron lasers. One bending magnet port is dedicated to use as an inverse-Compton-scattering source of gamma-ray photons at energies of ~50 MeV and greater. Synchrotron Ultraviolet Radiation Facility (SURF) SURF II is a 280-MeV storage ring at the National Bureau ot Standards (NBS) and is dedicated to production of synchrotron radia- tion. The small beam cross section results in a high-brightness source
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270 APPENDIX E for photon energies up to 400 eV. There are 11 experimental stations nearly operational, which are used by about 50 scientists mainly from NBS and the Naval Research Laboratory (NRL), together with their collaborators. In addition there are PRTs from the NRL and the University of Maryland. An active area of research at SURF II is atomic and molecular gas-phase spectroscopy, including the spectros- copy of laser-excited states. Research Highlights of the Past Decade Research with synchrotron radiation has evolved tremendously during the last decade or so, as indicated by the fact that only one of the five national facilities existed as a dedicated storage ring in the early 1970s. Almost all of the many ways in which photons have been put to use in condensed-matter physics have been revitalized by synchrotron radiation (SR) studies. This is illustrated in Table E.2, which summa- rizes the many techniques in use in SR studies in the vacuum ultraviolet (VUV) and x-ray spectral ranges. Electronic structure. A new era in experimental studies of solid- state electronic structure began in the 1970s as emphasis shifted from the use of more conventional absorption and resection studies to the use of photoelectron spectroscopy. Angle-resolved photoelectron spectroscopy (ARPES) coupled with SR tunability has been used to determine experimentally the energy-momentum dispersion, E(k), for elements such as Cu and Ni, as well as for more complex materials such as GaAs and CdS. This allows the calculated E(k) relation to be compared directly with experimental data rather than indirectly through calculated optical constants. Magnetism. Typical of the studies possible with SR on magnetic materials are results from photoemission spectroscopy of nickel. The d-band width is experimentally determined to be ~25 percent smaller than the value predicted by the best theoretical band calculations. The measured exchange splitting near the Fermi energy is ~50-60 percent of its calculated values. The temperature dependence of the exchange splitting has been measured through the Curie temperature and dem- onstrates the inadequacy of a purely band model of ferromagnetism. Structural Studies and Phase Transformations. SR studies have contributed to our understanding of phase transitions during the past decade. One example is two-dimensional (2-D) melting and wetting.
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APPENDIX E 271 TABLE E.2 Techniques in Use in Synchrotron Radiation Studies in the Vacuum Ultraviolet (VUV) and X-Ray Spectral Ranges' Measurement Remarks VUV Absorption Reflection Ellipsometry Luminescence Fluorescence Photoelectrons integrated angle-resolved surfaces Photoionization Photodesorption Laser-induced absorption ~ O O 01 · O O OJ O ~ ~ O O O ~ O O · O O O O O O · O ~ O O O O O 0 O O O to O O O O O O ~ O O O Optical constants of materials Exciton dynamics Molecular solids First SR experiments, 1972 First SR experiments, 1975 Surface shifts observed, 1980 X ray EXAFS ~ O O O First SR experiments, 1974 SEXAFS ~ O O ~Routine use requires wiggler XPS O O O O surface structures Raman O O O O O Holography O O O O Fluorescence O O O ~ Topography O ~O O O Scattering O ~O O High-Q resolution Anomalous scattering ~ ~O O Phase problem Microscopy ~ OO O O Lithography O O ~ Angiography O ~O O 32-keV requires wiggler ~ ., Very important; O. moderate importance; O. some importance. Scattering studies at various wavelengths have been used on glassy amorphous materials to probe selectively the radial distribution func- tions of Ge, Se, and As atoms. In addition, the high resolution of SR in a small-angle x-ray scattering (SAXS) geometry has been demon- strated, together with its versatility in studies of phase transitions, particularly in macromolecular systems such as liquid crystals.
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272 APPENDIX E EXAFS and NEXAFS. The first SR studies of extended x-ray absorption fine structure (EXAFS) were performed in 1974 by moni- toring the transmission of x rays through a gas sample. Since then a large number of secondary detection techniques have been developed: fluorescence, Auger, photon desorption, and total yield, whose spectra contain the EXAFS information and can be used for studying liquid and solid electrolytes in greater dilution, surfaces, complex alloys, and amorphous systems. One cannot summarize adequately here the advances due to EXAFS in condensed-matter studies of ionic conduc- tors, metallic compound formation, mixed-valent, layered, amor- phous, and impurity systems. A typical example from the field of catalysis is the EXAFS study of the bimetallic, supported Pt-lr catalyst, which shows different local structure around Pt and Ir atoms during catalytic reactions. Studies of near-edge x-ray absorption fine structure (NEXAFS) began in about 1979-1980 as a separate area of study with SR, since it was recognized that one could extend band theory into this range and achieve an understanding of the sharp, complex peaks at energies of 0-50 eV, below the EXAFS range. Edge structure, together with data on model compounds, has been used to determine the oxidation state and local geometries of molecules on surfaces. Surface Structure. Along with phase transition studies of surfaces and other 2-D systems have been rapidly growing areas of research owing in part to SR techniques. The development of the surface- extended x-ray absorption fine structure (SEXAFS) has been a major accomplishment of SR, providing much more accurate bond lengths than those obtained with older methods. The first results, on metal surfaces, were published in 1978, and by 1981 workers had extended the use of Auger detection to semiconductors and molecular solids and had developed total yield and ion desorption modes of detection as well. The high brightness of a wiggler or undulator beam line has been used to extend SEXAFS studies to a concentration of ~ 1/10 monolay- er. Complementary to the local structure provided by SEXAFS are studies of surface x-ray scattering, which helped to clarify the nature of surface reconstruction on Ge(100) 2 x 1 surfaces and demonstrated a commensurate-incommensurate transition in the melting of lead on the (1 10) face of Cu. Interface Studies. There have been numerous SR studies on the formation of semiconductor-oxide, semiconductor-semiconductor, and
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APPENDIX E 273 semiconductor-metal interfaces, motivated in part by the widespread technological importance of these systems. Some of the notable results are detection of core-level chemical shifts for atoms that are one to three layers from the interface, modification of Schottky-barrier height that is due to cooperative interdiffusion processes, and determination of heterojunction band-gap discontinuities by photoemission. In the past 3 years several groups have added the capability for in situ preparation of interface samples by molecular-beam epitaxy (MBE) which gives layer-by-layer control over the samples. Topography. Compared with those of conventional sources, SR has made spectacular improvements in the field of topography; the SR apparatus is simpler and cheaper. Stress propagation studies have been carried out under dynamic conditions with the use of a video imaging detection scheme. Lithography. Research in x-ray lithography is in progress at a number of industrial laboratories, in which conventional sources are used; nonetheless, there is considerable interest in the use of SR for this purpose because its intensity is 102-103 times greater than that of laboratory sources. Structures as small as 7 rim have been produced, but most of the recent work is directed toward structures with high aspect ratios, e.g., 3-4 ,um widths with 0.02-0.1 Em vertical edge orientations. An example of such a structure is shown in Figure E. 1. The high brightness of SR allows one to use relatively simple and well-defined processing procedures in spite of their low intrinsic sensitivity. Microscopy. X-ray microscopy with SR is still in its infancy, but initial results look promising. Scanning x-ray microscopes with ele- mental selectivity were first demonstrated in 1973-1974 but with only 1-2 ,um resolution. Recent studies using soft x rays (1.5-4.5 nm) have increased this resolution by an order of magnitude. Although many biological applications exist for x-ray microscopy there are important technological applications as well, since the specimens can be studied in high-pressure or liquid environments. Time-Resolved Studies. The high brightness and intensity of SR has great potential for allowing real-time dynamical processes to be followed, but comparatively few such studies have been completed. Most common are fluorescence lifetime studies that use the repetitive pulsed nature of the SR beam. Time-of-flight (TOF) spectrometers are
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274 APPENDIX E ~_4 1 Jam 1 am 43 r..t .... FIGURE E l An example of a high-aspect-ratio microscopic structure formed by etching a photolithographic pattern made by exposure to 10 A synchrotron x rays. Note in particular the diffraction-limited straightness of the vertical walls (~0.1 ,um). (Cour- tesy of IBM Thomas J. Watson Research Center.) now commonly used for the photoelectron spectroscopy of gases and for ion Resorption studies of surfaces. Time-resolved diffraction was first demonstrated on biological samples in 1976. Recently the struc- tural changes of silicon during laser annealing studies were observed with a time resolution of 10 ns, and time-resolved EXAFS studies have probed local structural changes at the 10-~s to 10-ms level. Such experiments usually require single-bunch operation of the storage ring, and special time periods are assigned to time-resolved studies. New advances in detector schemes are likely to make time-dependent studies more frequent and productive in the future.
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APPENDIX E 275 Future Directions in Synchrotron Research In a rapidly expanding field such as synchrotron radiation research it is extremely difficult to predict what new exciting advances will occur. However, many of the recent developments mentioned above have merely demonstrated the feasibility of a new measurement or instru- ment. The past has shown that with each increase in resolution or intensity one has the opportunity for new areas of understanding. EXAFS. With new and/or planned wiggler beam lines the concen- tration limits for EXAFS of 1 part in 103-104 will be pushed to 1 part in 105-106 routinely. This is extremely important for condensed-matter studies since many interesting defect properties occur in the 10-6 concentration range. SEXAFS will become a common structural tool used in conjunction with other surface probes. Scattering. High-resolution elastic scattering will be applied to smaller samples, such as quasi-one-dimensional conductors and other novel materials. In addition, high-resolution 3-10 meV inelastic x-ray scattering will be developed to complement both SR studies of phase transitions and elastic- and inelastic-scattering studies with neutrons. Surface scattering will become a routine tool for surface crystallogra- phy and will be used in concert with other structural probes such as SEXAFS. Nuclear resonance scattering promises x-ray beams with unprecedented spectral purity. Photoelectron Spectroscopy. High resolution in both energy and momentum will be achieved with /`E < 50 meV and Ok < 0.01 Am. This will allow more accurate experimental determination of electronic structure, as well as studies of systems with small Brillouin zones, such as reconstructed semiconductors and novel superlattice materials. Measurements with detection of photoelectron spin will become rou- tine, in spite of the 1o-~4 loss of intensity for spin-resolved compared with conventional measurements. Applied Research. Although many fundamental studies are moti- vated by the technological applications of new materials, SR experi- ments have usually not been coupled strongly to high-technology materials preparation. This is already changing rapidly with advances such as the development of on-line MBE and catalytic reactor systems. Future development of realistic processing capability in lithography and microscopy has the potential of involving SR more directly in
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280 APPENDIX E example, in suitably anisotropic materials, that 1-D and 2-D spin waves continue to propagate freely above the magnetic ordering temperature. More highly localized nonlinear disturbances, known in classical mechanics as solitary waves, have been shown to have quantum- mechanical analogs in certain 1-D magnets. Inelastic-scattering measurements have also probed the nature of spin excitations in amorphous magnets and provided relaxation times for magnetic dis- turbances in both valence fluctuation and spin-glass systems. Critical Phenomena and Phase Transformations. The 1970s may well be remembered among future condensed-matter physicists as the decade of the phase transformation, and neutron-scattering experi- ments on magnetic systems played a leading role in contributing to this perception. Of the experimental quantities of interest (see Chapter 3)- correlation length, order parameter, susceptibility, and heat capacity- ~ neutrons alone provide measurements of the first and, in the case of antiferromagnetism, of the second of these quantities as well. The decade began with the demonstration of large critical magnetic fluctu- ations in the ferromagnetic metals iron and nickel, progressed through careful quantitative neutron studies of critical exponents in antifer- romagnets, the effects of dimensionality in quasi-l-D and 2-D magnetic systems (including experimental verification of Onsager's famous solution of the 2-D Ising model), and the effect of long-ranged dipolar interactions, and continues most actively at present in studies of the effect of random fields and impurities. Important neutron-scattering studies were also performed on structural phase transformations, particularly with regard to the dynamical role of unstable phonon modes as active agents in driving structural transformations. Surfaces. Pioneering studies of the structures of physisorbed lay- ers of simple gases at coverages ranging from less than one monolayer to several layers thickness have been carried out by neutrons on bulk samples of graphite that have been exploded in a controlled way to produce very large numbers of internal, but essentially free, surfaces upon which materials can be adsorbed. Depending on the conditions of temperature and pressure, several sorts of adsorbate structures are observed: commensurate crystalline phases in which the interatomic separations of the adsorbed layer accommodate to the underlying graphite lattice, floating or incommensurate films that are unregistered with the graphite, and 2-D fluids without well-defined long-range order. Molecular gases may be freely rotating or precisely oriented with respect to the substrate. Some of these effects are quite subtle and benefit from the higher-resolution studies that are now possible using
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APPENDIX E 281 synchroton x-ray sources. It is important to remark that delicate questions concerning the nature of long-range order in two dimensions, currently of great fundamental interest, can only be addressed by diffraction measurements. Thus, the continuing value of x rays and neutrons to surface studies is assured despite the spectacular recent successes in direct imaging of surface structure using electron micro- scopes. Neutrons have also begun to contribute to inelastic-scattering studies of physisorbed and chemisorbed surface phases. Polymers. The fact that the 1970s witnessed major progress and growth in application of small-angle neutron scattering to polymer physics is directly traceable to the technique of isotopic contrast manipulation, which in essence works as follows. By favorable circum- stance hydrogen and deuterium differ greatly in their neutron- scattering power. Thus a hydrogen-containing macromolecule, or a given segment of it, can be labeled by selective deuteration, with the result that neutrons sense only the presence of these labeled molecules when mixed in a dilute solution of their chemically similar undeuter- ated counterparts (see Figure E.21. The problem waiting for this tool was the polymer conformation question, i.e., what is the shape assumed by an individual macromolecule in a polymer melt? The answer, settling a long-standing theoretical controversy, is that the molecules assume the shape of a random coil, a behavior paradoxically more ideal than that of polymers in dilute solution, for which the coils are swollen by interaction effects. Similar conformation studies have now been carried out on quenched polymer melts, as well as crystalline and semicrystalline solids and stretched polymers, thereby addressing fundamental questions in polymer rheology. Micro-electron-volt in- elastic neutron scattering has just begun to examine the dynamics of polymer segments at previously unexplored length scales. Although it lies outside the scope of this report, it must be stressed that the use of HID contrast variation has enormously enhanced the power of neutron scattering in structural studies of biologically functional macromole- cules as well. New Materials. Condensed-matter physics is continuously re- newed by the creation of novel materials, and neutron scattering plays an essential role in characterizing the magnetic and dynamical proper- ties of these new materials. A list of recent examples of such materials in which neutron studies figure prominently includes 1-D metals, graphite intercalation compounds, and magnetic multilayer films. An- other case in which the unique capabilities of neutron measurements are readily apparent concerns magnetic superconductors, more pre
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282 APPENDIX E FIGURE E.2 Both tubes contain two Pyrex beads imbedded in glass wool. They become visible in the tube on the left when filled with a solvent with a refractive index matching that of the glass wool (slightly smaller than that of Pyrex). In a similar way individual segments of complex macromolecules are rendered visible to neutrons by adjusting the neutron-scattering power by selective deuteration. . closely the coexistence of magnetic and superconducting long-range order, which was discovered (engineered is a better term) in the late 1970s. For although in these new materials heat-capacity measure- ments indicated the presence of some kind of ordering occurring below the onset of superconductivity, elastic neutron scattering was neces- sary to establish its nature, distinguishing between antiferromagnetism, which can coexist with superconductivity, and ferromagnetism, which suppresses it. The competition between superconductivity and fer- romagnetism is subtle and interesting, as revealed by the appearance in small-angle neutron-scattering experiments of a new type of long- wave-length oscillating magnetic disturbance present at temperatures above those at which ferromagnetism displaces superconductivity (Chapter 8~. Future Directions As is the case in all experimental disciplines, new opportunities are closely coupled with technical advances in instrumentation. In what
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APPENDIX E 283 follows, we identify some of these advances as well as the attendant new physics that they can be expected to address. Epithermal Neutrons Increased epithermal neutron fluxes at spallation sources will make possible further new classes of experiments at unexplored regions of energy and momentum. Among these new experiments are the following: · The study of high-frequency vibrations of a solid, particularly those involving hydrogen atoms. Overtone and combination modes of excitation can be studied to deduce the shape of the vibrational potential energy surface. · The study of magnetic excitations in solids is generally restricted to small momentum values. Kinematic restrictions in the scattering make it necessary to measure with 1-2 eV epithermal neutrons even for rather moderate (~100-meV) magnetic excitation energies. The nature of magnetic spin waves and the continuum of single-particle excitations into which they dissolve is an important example of a long-standing issue that can be addressed. · In the high-momentum transfer limit scattering events in con- densed matter can be considered as taking place from individual atoms. These deep inelastic-scattering events allow a direct measurement of the momentum distribution of light atoms in their ground state. These measurements are particularly important for superfluid liquid 4He, for which there presumably exists a zero-momentum condensate fraction. · The larger accessible range of momentum transfer provides, through Fourier transformation, increased spatial resolution in struc- tural studies of liquids and amorphous solids. Neutron Guides In modern thermal neutron sources, the high flux is of necessity generated in a small (~ 1 m3 or smaller) core or target region. There are obvious practical limitations to the number of instruments that can be tightly grouped around the high-flux region. An innovation that oc- curred too late to be used to optimum advantage in the U.S. high-flux reactors is the neutron guide, which uses neutron mirrors to transport a beam of neutrons large distances from core to instruments with negligible loss. Because neutron mirrors collect over a solid angle proportional to the square of the neutron wavelength, existing guide tubes work best for long-wavelength cold neutrons. Supermirrors, conceptually related to multilayer optical antireflection coatings, prom
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284 APPENDIX E ise to extend the use of neutron guides to shorter wavelengths. These developments allow additional instruments to be added to existing facilities once considered saturated. High-Resolution Spectroscopy Typically, the energy resolution of conventional thermal neutron spectrometers is of the order of 0.1 meV or more. While this is usually adequate to determine the mean energy of excitations in condensed matter, there is much information contained in the widths and lineshapes, particularly in solids at low temperature, which are not accessible at this level. Unconventional instruments using, for exam- ple, neutron spin precession to encode the energy information of the scattering event, and having energy resolution of 1 TV or less, are now in routine use in Europe. They have opened up new areas such as molecular tunneling spectroscopy in molecular crystals and the inves- tigation of low-frequency polymer dynamics. No micro-electron-volt resolution instruments currently exist in the United States. Interferometry The advent of monolithic single-crystal Si interferometers has made it possible to measure neutron wavelengths to unprecedented accu- racy. The technique has thus far been used to measure gravitational and quantum-mechanical properties of the neutron. In the coming years, we may expect to see interferometry turn increasingly to questions of interaction and propagation of neutrons in condensed matter. Growth of the Neutron User Community Two important developments occurred in the 1970s that have begun to restructure the neutron community, attracting appreciable numbers of part-time neutron scatterers from universities and industry: · Powder profile analysis is a technique involving extensive com- puter analysis of powder diffraction patterns to extract incompletely resolved features. This permits the determination of crystal structures four to five times more complex than was previously possible. Since the measurements themselves are rather fast and routine, the technique has become popular among chemists and crystallographers without previous neutron-scattering experience.
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APPENDIX E 285 · Small-angle neutron scattering (SANS) is another technique that is well suited to active outside user participation in the actual data-taking stage. The powerful contrast matching techniques described previously in this study have caused an unparalleled growth of interest in neutron scattering among biologists, polymer scientists, and materials scientists. Principally as a result of these developments and the clarification of DOE policy toward outside users, which occurred in 1979, the U.S. neutron-user community has embarked on a course of rapid expansion in recent years, growing from about 250 per year in 1977 to over 500 per year in 1983. It will be important in the coming decade to continue to nurture these two areas of active user involvement. It is also likely that the spreading use of synchrotron radiation sources will trigger an increased use of neutrons. As more scientists break from the traditional mold, restructuring their research programs around distant synchro- tron facilities, they may simultaneously become more aware of the complementary nature of neutron probes and the strong similarities in methodology and instrumentation that underlie x-ray and neutron- scattering experiments. HIGH-MAGNETIC-FIELDS FACILITIES Description of U.S. Facilities The National Magnet Laboratory (NML) at MIT is the only major user facility for high field research and development in the United States. It has a scientific and technical staff of about 60 and an operating budget of $10 million/year, of which approximately one half is the core support from the NSF for operations, magnet development, and in-house research. The NML facility has a wide variety (24) of steady field magnets with different peak fields, bore size, and homogeneity. The highest field magnet is a hybrid (superconducting solenoid plus conventional mag- net) configuration, which holds the world's record for do fields at 30.4 T (1 tesla = 104 oersteds). In the past year, a 45-T pulsed field facility has become available to users at NML, and engineering studies are under way for the development of 50-70 T pulsed configurations. Research Highlights of the 1970s Much of the in-house effort at NML during the past decade has been dedicated to the development of higher field magnets, through the
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286 APPENDIX E better understanding of superconducting materials, and to their utili- zation in both research and applications. Notable among these efforts have been the measurements of the upper critical field HC2 in the Al5 (e.g., Nb3Sn) and Chevrel phase superconductors, the development of the spin-polarized tunneling technique for probing spin-orbit scattering in high field superconductors and the spin polarization in magnetic materials, high-gradient magnetic-field separation (now used commer- cially to purify clay), the development of magnetoencephalography, and the development of ductile, high-current superconducting wire for high-field magnets. Extensive work has been carried out at NML on magnetic phase diagrams; the observation of the Lifshitz point in MnP was one important accomplishment in this in-house NML program. A major effort in quantum-optics studies in semiconductors (e.g., spin- flop Raman scattering in CdSe) has been in place at NML for more than a decade. Perhaps the most exciting research requiring high fields that has taken place at NML by outside users has been the probing of the electron dynamics of 1-D and 2-D systems. This has led to the discovery of the quantization of the Hall effect and, most recently, of fractional integer filling of the lowest Landau level in GaAs/(Ga,Al)As layers. It is now believed that the ground state of the 2-D electron gas in a high magnetic field is a highly correlated liquid rather than a crystal. Of considerable interest are the magnetotransport experiments on graphite, which show that large (25-T) fields induce a charge-density wave. Other notable work utilizing high magnetic fields has been the studies of the spin-Peierls transition an intrinsic lattice instability in a 1-D, S = 1/2 antiferromagnetic chain coupled to a 3-D lattice; the properties of itinerant ferromagnetic and antiferromagnetic metals; spin fluctua- tion (e.g., UAl2) and valence fluctuation (e.g., YCuAl) systems; and electron spin resonance in the submillimeter region. Opportunities There are a variety of novel and fascinating phenomena that occur in fields that are already available or that will become available in the next decade, with the advent of pulsed fields in the 70-150 T range and beyond. (Nondestructive fields as large as 150 T of microsecond duration have recently been achieved in Japan.) Among the possibili- ties is the observation of superconductivity based on p-wave pairing, which occurs in liquid 3He but has never been found in an electronic
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APPENDIX E 287 system. Strong magnetic fields should also make possible the orientational ordering of large molecules via their diamagnetic suscep- tibilities, measurements of the nonlinear magnetic response of transi- tion and actinide metals, and exploration of the magnetic phase diagrams in materials whose exchange coupling is too large to be affected by the currently available fields. Magnetic fields available in the 1980s should enable us to reach the Paschen-Back limit of strong magnetic fields in hydrogenlike systems in certain semiconductors, such as excitors in InSb. Some of the most interesting developments may be expected in conducting systems of lower dimensionality, such as semiconductor inversion layers (e.g., the common field-effect transistor), and highly anisotropic layered systems such as graphite and certain organic conductors. Already, new phase transitions have been observed in the layered systems, produced by strong magnetic fields. The experimental discovery of the quantized Hall effect serves as a dramatic demonstra- tion that there may remain many exciting phenomena yet to be discovered in the area of condensed-matter systems in high magnetic fields. FACILITIES FOR ELECTRON MICROSCOPY Introduction Electron microscopy is one of the most important multipurpose techniques in the sciences of solid-state and biological materials. Several essential modes of operation are unique to the electron probe. These include microchemistry on a scale of '10 rim by energy loss and x-ray detection; atomic-resolution imaging by lattice fringe methods and by scanning with a probe focused to a few angstroms; and convergent-beam diffraction analysis of structures 10 nm in size. In addition, microscopes can be converted to scattering instruments in which the dispersion relations of crystal excitations are examined through events that combine energy loss and momentum transfer. An important asset is that these experiments can be performed on submicrometer-size samples. Electron microscopy is one of the essen- tial capabilities required to complement new materials development (see Appendix C), particularly in areas of microfabrication. Other modern directions include ultrahigh-vacuum instruments designed for surface spectroscopy (Figure E.3), and high-level computerization of microscopy applied to microchemistry and to diffraction from complex systems, including biomolecular assemblies.
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288 APPENDIX E _ ~ _ _ ~ ~ ~ .= .;B FIGURE E.3 The atoms at the surface of a crystal often reconstruct, assuming a different configuration from that of the bulk. This figure shows an electron microscope image of the profile of the surface of a gold particle showing a reconstructed surface superstructure. The inset shows a simulation corresponding to a missing-row model, which matches the experimental image. [After L. D. Marks and D. J. Smith. Nature 303, 316 (1983).] Description of U.S. Facilities Most electron microscopes in the United States are dedicated to the research of individuals or small groups. Nevertheless, there now exists a wide variety of instruments employed in a user facility mode. Many campuses have electron microscopes operated by individuals but accessible to other on-site users. User operation on a national scale first started in the 1960s with several 1-MeV microscopes located at national laboratories and other public institutions. These high-voltage instruments are specially valuable in biological applications owing to their improved penetration, because they accommodate thick speci- mens and often possess stereo techniques for depth perception. In materials research the emphasis with high-voltage machines has been on environmental effects, on in sitl' deformation measurements, and on radiation damage produced either by the electron beam itself or by an accelerated beam of ions. The considerable expense of the machines made wide accessibility appear desirable and prompted the facility mode of operation. The DOE supports two national user facilities in advanced electron microscopy. The National Center for Electron Microscopy at the Lawrence Berkeley Laboratory includes a 1.5-MeV high-voltage elec
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APPENDIX E 289 tron microscope and an atomic resolution microscope. The Argonne Center for Electron Microscopy includes a 1.2-MeV high-voltage electron microscope to which is coupled a 2-MeV ion accelerator for in situ radiation damage studies. A state-of-the-art analytical electron microscope at 300 to 400 keV voltage will be added in 1985. Both facilities are available to the user community at no charge, unless proprietary work is done. The DOE also supports two other user programs where advanced microanalytical electron microscopy can be performed. These are the Center for Microanalysis of Materials at the University of Illinois and the Shared Research Equipment (SHARE) Program at the Oak Ridge National Laboratory. The only other U.S. facility for high-resolution research is at Arizona State University. It operates with commercial instruments manufactured in Japan, The Netherlands, and the United Kingdom, which have been locally developed and modified for specific types of high-resolution applica- tion. Important work on basic microscope development has been carried out at the University of Chicago, where atomic-resolution scanning transmission machines have been constructed (as prototypes, not facilities). Active efforts in several areas of development and accessory construction are also in progress at other U.S. institutions. Unfortunately, the necessary capabilities are not usually combined in a single machine. Nor are all the techniques learned quickly. For this reason there remains a need for regional centers where a number of complementary machines can be maintained, developed, and used by experts. The grouping of several experts with several machines pro- vokes valuable interactions in a well-equipped environment and is an economically sound organization. Since regional centers of this type can serve teaching and consulting purposes for a wider community, it appears desirable that a number of centers of expertise or facilities of this type be maintained nationwide into the indefinite future. If sited in university communities, centers of this type could help to strengthen U.S. science in the area of electron microscopy both by expert ongoing research programs and through expert training of graduate students to staff microscopy efforts at other institutions nationwide. Advances of the Past Decade in Electron Microscopy A number of significant advances have taken place in the field of electron microscopy over approximately the past decade. These in- clude the following:
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290 APPENDIX E 1. The development of convenient scanning transmission electron microscopes (STEM) that yield information at atomic resolution by rastering a finely focused beam over the sample. 2. The incorporation of energy-dispersive x-ray (EDX) and electron- energy-loss spectroscopies (EELS) in transmission microscopes to make microchemical information available on a 20 A and up lateral scale in EELS, and on a 100 A and up scale in EDX, dependent on specimen characteristics. 3. Development of microdiffraction, including convergent beam diffraction techniques (and suitable electron microscopes), which made available routine crystal-structure identification on particles 100 A or larger in size and definitive symmetry determination on larger particles. 4. Continued development of high-resolution machines and tech- niques to make most interatomic distances in solids resolvable and weak-beam high-resolution defect imaging feasible. 5. The use of electron microscopes and development of vacuum capabilities for high-resolution examination of crystal surface structure and steps' for example. Outlook for the Future One can expect to see important continuation of this progress over the next decade in the following areas among others: Development of the electron microscope as a miniature labora- tory that includes diffraction, scattering, imaging, EDX, EELS, Auger spectroscopy, cathodoluminescence, specimen current yield, and other spectroscopies directly available for analytical purposes. 2. A new generation of flexible intermediate-voltage machines (300-500 keV) with laboratory (rather than institutional) size and cost scale that may satisfy many expectations left unfulfilled by the last generation of high-voltage machines. 3. Microscopes that will be developed with ultrahigh-vacuum and sample-preparation capabilities suitable for routine exploration and analysis of clean surfaces.
Representative terms from entire chapter: