Condensed-Matter Physics (1986) / Chapter Skim
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E National Facilities
E National Facilities
Pages 265-290
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From page 265... ...
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.
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From page 267... ...
, 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.
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From page 268... ...
Three primary beam lines supply hard x rays (hm > 3 keV) to six experimental stations for general use.
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From page 269... ...
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.
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From page 270... ...
An active area of research at SURF II is atomic and molecular gas-phase spectroscopy, including the spectroscopy 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.
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From page 271... ...
geometry has been demonstrated, together with its versatility in studies of phase transitions, particularly in macromolecular systems such as liquid crystals.
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From page 272... ...
were performed in 1974 by monitoring 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.
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From page 273... ...
Scanning x-ray microscopes with elemental 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)
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From page 274... ...
Time-resolved diffraction was first demonstrated on biological samples in 1976. Recently the structural 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.
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From page 275... ...
EXAFS. With new and/or planned wiggler beam lines the concentration limits for EXAFS of 1 part in 103-104 will be pushed to 1 part in 105-106 routinely.
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From page 276... ...
276 o o, ~> o ~ o C~ o oo .= ._ Ct o ._ ._ ._ v Ct 50 o s~ z Ct Ct o ._ Ct z s~ , - .
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From page 277... ...
Over the years reactors have proven to be stable, reliable sources of thermal neutrons, well suited to straightforward diffraction measurement of incident and scattered-neutron energies. Such instruments have been, and probably will continue to be, the instruments of choice for most elastic- and inelastic-scattering measurements on single crystals, traditionally the largest sector of neutronscattering research.
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From page 278... ...
Peak intensities ~100 times larger than the continuous flux of the best reactor sources are possible with moderate average target power densities. · The neutron spectrum can be tailored to produce relatively large numbers of higher-than-thermal-energy neutrons, so-called epithermal neutrons, with energies of about 1 eV.
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From page 279... ...
1976 Production and storage of ultracold neutrons 1976- 1977 N eutron interferometry 1978 Dynamics of liquid The 1979 to date 1980 Magnetic order in superconductors Dynamics of polymers and turbulent flow in liquid crystals 1980 to Spallation neutron sources date Germany, France U.S., Austria U.S., France U.S. France, Norway U.S., Great Britain, Japan phonon interaction in metals, particularly of transition-metal compounds, which stimulated tractable first principles calculations; observation of giant phonon frequency anomalies (Kohn anomalies)
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From page 280... ...
, 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.
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From page 281... ...
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 microscopes. Neutrons have also begun to contribute to inelastic-scattering studies of physisorbed and chemisorbed surface phases.
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From page 282... ...
The competition between superconductivity and ferromagnetism is subtle and interesting, as revealed by the appearance in small-angle neutron-scattering experiments of a new type of longwave-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.
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From page 283... ...
· In the high-momentum transfer limit scattering events in condensed 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.
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From page 284... ...
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 computer 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.
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From page 285... ...
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 synchrotron 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 neutronscattering experiments.
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From page 286... ...
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 fluctuation (e.g., UAl2)
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From page 287... ...
The experimental discovery of the quantized Hall effect serves as a dramatic demonstration 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.
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From page 288... ...
These high-voltage instruments are specially valuable in biological applications owing to their improved penetration, because they accommodate thick specimens 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.
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From page 289... ...
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.
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From page 290... ...
5. The use of electron microscopes and development of vacuum capabilities for high-resolution examination of crystal surface structure and steps' for example.
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