U.S.-Federal Republic of Germany Cooperative Research: Microscopic and Phenomenological Theories of Correlated Fermi Systems

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This award supports Professors P. J. Hirschfeld and K. A. Muttalib of the University of Florida to maintain collaborative research links with Dr. Peter Woelfle of the Institute for Condensed Matter Theory of the University of Karlsruhe, Federal Republic of Germany. They plan to investigate superconductivity and magnetism using the single band Hubbard model, which is possibly relevant to materials with high superconducting transition temperatures ("high-Tc materials.") They will apply two new methods that they recently developed: 1) a new functional integral approach, and 2) a conserving slave boson diagrammatic expansion, to study several questions related to the possibility of superconductivity in Hubbard-like systems, the stability of the Fermi liquid state in the presence of strong correlations, and the stability of incommensurate antiferro- magnetic phases. They will also study models of unconventional superconductivity, with an emphasis on the question of how the symmetry of an unconventional order parameter can be determined with external probes, particularly electromagnetic ones. Although the mechanisms responsible for pairing in the high temperature superconductors remain mysterious even after two years of intense research since its discovery, there is accumulating evidence that electronic correlations, rather than electron-phonon interaction play a crucial role. Unfortunately, there is no adequate systematic and controlled method available for dealing with such strongly correlated systems. New methods such as those being used here are essential for the study of all highly correlated Fermi systems, and should be generally applicable to any similar model, including that appropriate for the high-Tc materials. These novel methods originated as a result of extensive collaboration among these scientists when Dr. Woelfle spent several years at the University of Florida. Continuing the interaction should prove to be very productive and mutually beneficial.