Competing Ground States and Disorder in Correlated Electron Systems

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

This project involves the application of solid state magnetic resonance techniques to address current questions related to competing ground states in selected correlated electron systems. The approach is based on the examination of several materials where antiferromagnetism competes with a paramagnetic Fermi Liquid phase (and possibly superconductivity) or a Spin-Peierls phase. The materials under study are from two families: heavy fermion and organic superconductors. They are characterized by very different microscopic physics. The goal of the project is to identify common trends in the physical properties of these materials, as well as to explore the physical properties of novel phases. Particular attention is paid to the vicinity of discontinuous and continuous quantum phase transitions, where the qualitative and quantitative effects resulting from varying the amount of disorder is examined. Pressure is the predominate means used to tune the ground state of the systems in question. The project trains two graduate students in the course of measurement design and execution. A course in magnetic resonance for the graduate level is planned for the 2002-2003 academic year. Smaller technical projects, both specific to particular experiments and for general laboratory improvements, are assigned to undergraduates. In a number of materials of current interest, the electrons behave in a correlated manner. The electrical behavior, or state, of the material depends upon which interaction involving the electrons dominates. Some of the materials in this class have properties related to these interactions that are technologically useful, i.e. no electrical resistance at relatively high temperatures or a large response in the electrical resistance to magnetic fields. Typically, the state that is stabilized is controlled by external parameters such as charge doping, high pressure, high magnetic fields, and so on. In this project, a combination of experimental techniques, though primarily solid state magnetic resonance, are used to gain an increased understanding of the competing states in correlated electron systems. The emphasis is on the physical properties of novel phases, and the effects of disorder on these materials. The systems investigated fall into two classes: organic superconductors and heavy fermion materials. Tuning is achieved with pressure and magnetic field; disorder is introduced through chemical substitution. The goal is to use controlled disorder in the efforts to classify transitions, and also to understand the qualitative, and sometimes dramatic, changes in the physical properties induced by the disorder. The project trains two graduate students in the course of measurement design and execution. A course in magnetic resonance for the graduate level is planned for the 2002-2003 academic year. Smaller technical projects, both specific to particular experiments and for general laboratory improvements, are assigned to undergraduates.