UC San Diego

This dissertation includes a discussion of the results of measurements of electrical resistivity

for materials under applied pressure extending over a range in pressures from 0 to 27

GPa and temperatures from 1 to 300 K. The primary effect of applying pressure to a solid

is 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 electrons

and a broadening of the energy bands. In addition, the application of pressure can induce

a variety of transitions, both electronic and structural. Some of the phenomena observed

during the pressure experiments reported in this dissertation include the pressure-induced

enhancement of the superconducting transition temperature in a recently discovered class

of bismuth-sulfide layered superconductors, the anomalous “dome-like” behavior in the

pressure dependence of the Néel temperature in the first-known synthesis of an itinerant

antiferromagnetic metal with non-magnetic constituents TiAu, and the evolution of the

pressure-induced first-order transition to antiferromagnetism in the Fe-substituted heavy

fermion compound URu2Si2. These and other results including a semiconductor-metal

transition in the normal state of one of the bismuth-sulfide layered superconducting

compounds are a consequence of the electronic and structural transitions that can occur as a

result of the application of pressure. The theoretical context for the experimental results

includes a discussion of the effect of pressure on the relevant parameters in condensed

matter theory including the density of states, the exchange interaction in both the localized

and itinerant models of magnetism, and the electron-phonon coupling parameter, among

others. In particular, the pressure dependence of the magnetic ordering temperature and

the superconducting transition temperature in various materials can be explained in terms

of how the few parameters listed above respond to pressure.