Uranium (and Cerium) Compounds At High Pressures and Magnetic Fields
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Uranium (and Cerium) Compounds At High Pressures and Magnetic Fields Andrew L. Cornelius, Ravhi S. Kumar, and Brian E. Light Department of Physics University of Nevada, Las Vegas 4505 S Maryland Pkwy, Las Vegas, NV 89154-4002 ABSTRACT Correlated-electron systems are so named due to strong interactions between electrons unlike traditional metals (e.g. copper) that have “free electrons” that interact very weakly. Knowledge of the Fermi surface, density of electron states and band structure are the starting points for a first-principles understanding of the electronic and electronically related macroscopic properties, e.g. equation of state. The use of high pressure and high magnetic fields to alter the electronelectron (hybridization) and electron-lattice interactions give us powerful tools to understand complicated rare earth and actinide correlated-electron systems and allows precise testing of experiment to theory. Correlated-electron systems yield a wide variety of ground states that are a direct result of the hybridization strength including: short and long range magnetic order, spin fluctuating, enhanced Pauli paramagnetism, heavy fermion behavior and superconductivity. We will review some results on U compounds in high magnetic fields and high pressures. By comparing the results to Ce compounds that have significantly more localized f electrons, the effect of direct 5f electron wavefunction overlap in U compounds can be discerned. Consequences on the search for U based heavy fermion superconductors will be discussed. INTRODUCTION Recently, the heavy fermion compound CeRhIn5, which superconducts at the relatively high temperature of 2.1 K at a pressure of ~2 GPa, has been the subject of a great deal of interest. Some magnetically correlated-electron systems are known to superconduct under pressure at a quantum critical point (QCP) where the magnetic ordering temperature goes to 0 K. The way in which a system approaches this quantum critical point, namely the shape of the magnetic ordering temperature as a function of pressure, is due to a competition between magnetic order and the Kondo effect and is clearly seen in many correlated-electron Ce systems including CePdSb and CeRhIn5. Comparing the heat capacity results near the QCP in CeRhIn5 driven by both La dilution and pressure reveals remarkably similar behavior consistent with short range magnetic order. Indications that some U compounds display this behavior have been observed. However, in all know U compounds that display a QCP, the approach to the QCP is much more gradual indicative of itinerant (band) magnetism rather than the more localized behavior seen in Ce compounds. In fact, the degree of 5f electron delocalization in U compounds can be discerned from the pressure dependent magnetic ordering temperature. It has been proposed that two dimensional systems will superconduct at higher temperature than their three dimensional analogs when the cooper pairing mechanism is magnetic in origin [theory]. Pressure dependent structural studies on CeRhIn
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