5.0 Scientific and Technological Case

This section summarizes the scientific and technological impact of synchrotron radiation based research. Unlike the Seitz-Eastman of 1984 where the emphasis was on evaluating and projecting the merit of building future facilities, this report focuses more on what has been accomplished in synchrotron science since that report. The evolution of the field over the last decade also indicates the obvious future trends in the areas where extrapolation is possible. Of course, there are areas where a simple extrapolation may not be possible. An example of this is development of 4th generation light sources.

The most straightforward and most important conclusion of this study is that over the past 15 years in the United States synchrotron radiation research has evolved from an esoteric endeavor practiced by a small number of scientists primarily from the fields of solid state physics and surface science to a mainstream activity which provides essential information in the materials and chemical sciences, the life sciences, molecular environmental science, the geosciences, nascent technology and defense-related research among other fields. The research carried out at the four D.O.E. synchrotron sources is both very broad and often exceptionally deep. The breadth of the research is well indicated by NSLS data which show that research results obtained at NSLS have been published in more than 250 journals. The high quality of synchrotron-based research is illustrated by the large number of publications in premier journals such as Science, Nature and Physical Review Letters. This is especially true in the life sciences where in recent years more than 60% of the biological crystal structures published in that field's leading journals were obtained using synchrotron techniques.

Given the remarkable success of synchrotron radiation-based science, it is impossible to do justice to this vast body of research. What will be discussed in this section is a very small fraction of total number of works carried out using synchrotron radiation. We divide this section into the following topical areas: materials research, surface science, polymers, atomic, optical, molecular physics and chemistry, molecular environmental science, geosciences, structural biology, microscopy, and technological impact.

5.1 Materials Research

Synchrotron-based research has had a profound and extraordinarily broad impact on condensed matter physics and materials science. The synchrotron experiments contribute ubiquitously to materials research, ranging from fundamental issues to important practical problems. For example, the users of NSLS have published papers in more than 250 journals, with a significant fraction of these in materials science. The combination of intense, bright sources, tunability and high photon energies has allowed vastly improved resolution with many orders of magnitude increases in signal enabling the study of weak scattering from small samples and surfaces, novel spectroscopies such as magnetic and inelastic scattering, real-time studies and studies using the coherent properties of the beam. These experiments have had the effect of forcing us to rethink our basic understanding of semiconductors, metals, superconductors, alloys, composite materials, liquid crystals, surfaces and interfaces, magnetism, dynamic processes, elementary excitations, electronic structure, and factors controlling phase equilibrium. The impact on science in the last quarter century of these new techniques after the previous 75 years of research with tube-based X-ray sources may be compared to the impact of the electron microscope after hundreds of years of work with the optical microscope. In both cases, whole new worlds have been opened up and in both cases it is difficult to think of an area which has not been significantly impacted as a result. As an investment in science, synchrotron-based materials research has been extraordinarily cost effective and productive for the United States. In this section we will present a selected set of highlights of the accomplishments and potential of synchrotron-based materials research in this country. It is, of course, completely impossible to do

justice to this vast body of work and what we discuss is but a small fraction of the broad range of accomplishments and opportunities.

One of the most important scientific successes spawned by synchrotron sources has been in the area of photoelectron spectroscopy. Synchrotron based photoemission spectroscopy experiments at the Wisconsin Synchrotron Radiation Center (SRC), SSRL and NSLS have profoundly impacted our understanding of a wide range of materials, especially those of semiconductors, their surfaces and interfaces. These experiments have played a critical role in advancing theoretical understanding and calculational capabilities that form the essential scientific foundation for the electronics industry. More recently, synchrotron based experiments at NSLS, SSRL and ALS have contributed significantly to our understanding of C60 and other novel forms of carbon. Issues ranging from crystal structure, phase transitions, electronic structure and superconductivity have been addressed using the power of synchrotron radiation. The experiments on C60 and related materials provide good examples showing how effectively the synchrotron community can respond to problems in material science.

Synchrotron based photoemission experiments have also made fundamental contributions to our understanding of the basic physics of strongly correlated electron systems. These studies have elucidated the electronic structure of high-Tc superconductors and have shown most clearly the d-wave structure in the energy gap in the superconducting state and in the pseudo-gap in the normal state. Angle-resolved photoemission experiments done at SSRL and SRC in particular have been crucial in leading this effort. Fig.5.1.1 reproduces the original observation of the superconducting gap anisotropy. The figure presents photoemission spectra above and below Tc for overdoped Bi2212 at two different locations in k-space. As expected in a d-wave pairing state, while the spectra taken at A clearly show the effects of a gap opening, the spectra taken at B hardly change above and below Te, suggesting that the gap is undetectable within the experimental uncertainty. The experiment shown in Fig.5.1.1 has contributed significantly towards the current consensus of d-wave pairing symmetry in the high-T superconductors. This higher order pairing state, which is different from the s-wave pairing symmetry of conventional superconductors, is a key step towards a comprehensive understanding of the remarkable phenomenon of hightemperature superconductivity. In a similar fashion, the recent observation of a d-wave-like normal state pseudo-gap shows that the superconducting transition is very different from the traditional paradigm of BCS-Eliashberg mean-field theory which describes the transition in conventional superconductors.

Angle-resolved photoemission data from oxide materials with strong antiferromagnetic interactions have also evinced the important role of magnetism in a clear way. Recently, an angleresolved photoemission experiment at SSRL has provided so far the only experimental indication of the separation of spin and charge degrees of freedoms of electrons in one-dimensional solids an important theoretical concept that was first envisaged about thirty years ago. Combined photoemission, spin-polarized photoemission and x-ray absorption experiments at NSLS have contributed significantly to our understanding of the colossal magnetoresistance behavior in La Sr,MnO,. In particular, spin-polarized photoemission experiments provide possible evidence for half-metallic behavior with conduction electrons of a single spin.

X-ray absorption, core level x-ray photoemission spectroscopy (including the pioneering work in Europe using conventional ESCA lab) as well as resonant photoemission spectroscopies have been used to understand the electronic structure of both occupied and empty states of a wide range of materials: metals, semiconductors, insulators, as well as materials exhibiting various magnetic behaviors, metal-insulator transitions, charge-density wave, spin-density wave and superconducting instabilities. Significant efforts have been devoted to strongly correlated materials such as Ce and U intermetallic compounds and transition metal oxides. This is particularly true for the transition metal oxides where a combined experimental and theoretical effort has led to a new

classification of these materials and the introduction and verification of the important concept of charge-transfer insulators. Early x-ray photoemission spectroscopy and resonance photoemission experiments at NSLS and SSRL have provided important information on the orbital character of the conduction electrons as well as important parameters such as the Coulomb interaction U, the charge transfer energy ▲ and the hybridization t. Spin-resolved photoemission, mainly carried out at NSLS in this country, has shown some interesting results from magnetic materials. X-ray magnetic circular dichroism experiments, pioneered at NSLS, and used at SSRL and ALS have also impacted our understanding of the role magnetism on electronic structure.

Another important phenomenon exhibited by certain low dimensional transition metal compounds with nested Fermi surfaces is that of a charge density wave (CDW) instability. Here high resolution synchrotron x-ray diffraction has changed our picture of the charge density wave state. Although it had been well known for quite some time that x-rays were nearly ideal as a probe of the structure of the charge-density wave state, neither conventional laboratory x-ray sources nor even the first generation of synchrotron x-ray sources were capable of realizing that potential. The highest quality samples of a CDW material available to-date are single crystal whiskers of NbSe,. Further, the best NbSe, whiskers are very small with cross sectional dimensions on the order of 2 microns by 10 microns. Early on, this small sample size, coupled with the high q resolution required to resolve these long-range ordered structures, conspired to keep experimental counting rates too low for high quality experiments. However, the high brilliance of modern insertion device beam lines has enabled workers at CHESS to study very small samples at high resolution while still obtaining sufficient count rates to obtain signal to noise ratios between 10 and 10'. These high quality data sets have revealed entirely new physics, which was inaccessible just a few years ago. For example, the tremendous signal to noise ratio revealed that two length scales are required to describe the phase-phase correlation function providing a direct measurement of the amplitude coherence length.

As noted above and as will be discussed in the next section, synchrotron radiation has played an important role in investigations of magnetic materials. One of the most surprising applications that of magnetic x-ray scattering. In the absence of resonant enhancement, the magnetic scattering crossection is typically between 10 and 10% of the charge scattering crossection. In spite of this weak crossection, a number of important investigations have been carried out and undoubtedly this area of research will continue to expand in the future especially if reactor-based neutron scattering facilities in the United States are allowed to diminish. Synchrotron magnetic x-ray scattering is complementary to neutron scattering with several important strengths. First, the small cross section results in extinction-free scattering, so that the order parameter itself and any associated large length scale fluctuations may be reliably determined. This is of particular importance in studies of phase transitions. Second, the small penetration depth of x-rays, typically on the order of 2 μm, may reduce the effect of concentration gradients in alloys and mixed magnetic materials. Third, the high reciprocal space resolution allows large length scales to be probed. Fourth, the relatively poor energy resolution (~10eV) ensures integration over all relevant thermal fluctuations. Fifth, the contributions due to the orbital and spin magnetic moments may be distinguished through polarization analysis.

A recent example has been the studies of the magnetic ordering processes in diluted magnets such as FeZn,,F2 in an applied magnetic field. This represents a model realization of a generic disordered system - the random field Ising model (RFIM). Because of extinction effects, neutron scattering was unable to provide reliable measurements of the order parameter as a function of field and temperature. Figure 5.1.2 shows the results of such measurements in Fe,,Zn F2 carried out at NSLS. These data have led to a new model for the RFIM which in turn has enabled researchers to produce a consistent picture of all existing data - thermodynamic, magnetic, optical