STRONG CORRELATION PHYSICS

A broad range of interesting physical phenomena occur in systems in which the effects of electron interactions extend the physics beyond the domain of simple non-interacting theories of solids. Examples of such systems are two-dimensional electrons, which have quasi-particles with fractional charge (e.g., 1/3 the charge of an electron); and superconductors of various types, in which current is carried by electron pairs which move without dissipation even in disordered materials. These subjects, for which Nobel prizes were given in the last decade, are part of the field of correlated electron systems, which includes heavy-Fermion compounds, Kondo-insulators, mixed valent materials, and high-temperature superconductors.

Although the basic Hamiltonian for the many-electron problem is well-known, in each of these cases one finds dramatic macroscopic phenomena which have not been predicted theoretically, and which often prove to be quite difficult to understand even after their discovery. This is because these phenomena are non-perturbative, arising from the strong interactions of the electrons, while most theoretical physics in all areas is limited to perturbative techniques. (Consider that it took over 50 years to explain the phenomenon of superconductivity.) This illustrates the subtlety and beauty of the many-electron problem as manifest through the study of modern compounds and structures.

Zack Schlesinger has worked on the study of correlated electron systems for the last decade at both Bell Labs, where he was involved with the study of fractional quantization and the two-dimensional quantum Hall effect, and at IBM where he has worked on various exotic compounds including cuprate superconductors, Kondo insulators and manganate materials exhibiting giant magneto- resistance effects. His laboratory at UCSC includes U.V., visible and infrared spectrometers and interferometers, infrared lasers, various low-temperature cryostats, and a 140 kiloGauss superconducting magnet.

Graduate students and post-docs participate in the selection of topics, measurement of relevant samples and the analysis of data in context of related work. This sets up an interactive process in which present results are used to assess the most fruitful direction for future work; independent work is encouraged.

Measuring reflectivity to obtain conductivity as a function of frequency from near d.c. to the near ultraviolet is fundamental and thus useful both for intuitive analysis and comparison to theoretical calculations. Measurements of this type by Schlesinger and his collaborators show strong evidence for a highly unconventional "non-Fermi-liquid" normal state in high-T_c superconductors, as well as anunusual pair excitation spectrum in the superconducting state. Schlesinger has also used this technique to study the phenomena of local-moment compensation in Kondo insulator and mixed-valent systems, which is fundamental to the understanding of magnetic interactions in metals.

Schlesinger's group maintains close contact and collaborations with theoretical physicists and materials scientists at various academic, industrial and government labs, whose respective inputs regarding what is significant and what is possible help shape the course of his research.