High-Pressure Investigations of Correlated-Electron Phenomena

This dissertation includes a discussion of the results of measurements of electrical resistivityfor materials under applied pressure extending over a range in pressures from 0 to 27GPa and temperatures from 1 to 300 K. The primary effect of applying pressure to a solidis to reduce the interatomic distance and to increase the overlap of the electronic orbitals.The secondary effects of applying pressure to a solid include the delocalization of electronsand a broadening of the energy bands. In addition, the application of pressure can inducea variety of transitions, both electronic and structural. Some of the phenomena observedduring the pressure experiments reported in this dissertation include the pressure-inducedenhancement of the superconducting transition temperature in a recently discovered classof bismuth-sulfide layered superconductors, the anomalous “dome-like” behavior in thepressure dependence of the Néel temperature in the first-known synthesis of an itinerantantiferromagnetic metal with non-magnetic constituents TiAu, and the evolution of thepressure-induced first-order transition to antiferromagnetism in the Fe-substituted heavyfermion compound URu2Si2. These and other results including a semiconductor-metaltransition in the normal state of one of the bismuth-sulfide layered superconductingcompounds are a consequence of the electronic and structural transitions that can occur as aresult of the application of pressure. The theoretical context for the experimental resultsincludes a discussion of the effect of pressure on the relevant parameters in condensedmatter theory including the density of states, the exchange interaction in both the localizedand itinerant models of magnetism, and the electron-phonon coupling parameter, amongothers. In particular, the pressure dependence of the magnetic ordering temperature andthe superconducting transition temperature in various materials can be explained in termsof how the few parameters listed above respond to pressure