Warren E. Pickett
Professor
Ph.D. - State University of New York, Stony Brook, 1975
Theoretical condensed matter physics, materials theory, metals and superconductors, electronic structure and properties, and complex materials
Professor Warren Pickett joined the faculty of the UC Davis Department of Physics in July 1996.
In spite of the myriad materials that have been fabricated and studied, peculiar and intriguing solids continue to be discovered. In many of these systems, the peculiarities are directly related to the spin magnetic moment, or simply "spin," of the electron. One might think that this spin was put on the electron primarily to amuse and occupy the materials physicist; however, the resulting magnetism vastly increases the number and variety of technological applications of solid materials. When an electron is midway between being bound to one atom and being free to move among atoms, novel behaviors arise, such as an extreme "heaviness." And complex materials, such as those with three, four or five different elements in a low symmetry arrangement, can show even richer behavior. The understanding and mathematical description of such complex materials is the focus of my research.
Perhaps the best known of such new materials are the high temperature superconductors based on layers of copper and oxygen, in which an "up" spin electron and a "down" spin electron unite to form a bound superconducting pair. New materials for thermoelectric applications have been grown, starting from theoretical predictions of how to make the electrons more efficient carriers of heat while making the vibrating atoms less efficient heat drains. Unusual magnetic materials, in which the net "up" spins and "down" spins are balanced on the whole but are unbalanced on the microscopic level, provide both new phenomena for study and the likelihood of new applications. My group has been studying such systems. Examples of recent topics follow.
Novel magnetic superconductors. Conventional superconductors abhor magnetic fields. A superconductor sets up spontaneous electric currents to shield it from applied magnetic fields, but a strong field ultimately destroys the superconducting state. A new type of magnet, called a half-metallic antiferromagnet, actually allows a novel type of superconductivity in which only one type of spin, say up, becomes superconducting, so up spins must pair with up spins. Half-metallic ferromagnets are known; they have inequivalent systems of spin-up and spin-down electrons, and one type is metallic while the other is insulating (hence half-metallic). In half-metallic antiferromagnets the up and down spins are balanced on the whole. As a result, there is no intrinsic magnetic field (as in a ferromagnet), and the metallic electrons may pair up to give a new type of superconductor in which the current has a particular spin.
There is one problem: there are no known half-metallic antiferromagnets. We have been applying our computationally based theory of electronic behavior and magnetism to attempt to predict materials that are good candidates for this novel type of material. Compounds with double perovskite structure such as La2MnVO6 and La2MnVO6 appear to be candidates, but more searching, along with the involvement of materials fabricators, is called for.
Exchange interaction constants for magnetic materials. The sophisticated treatment of models that deal with the quantum mechanical behavior of spins – by groups in our department as well as elsewhere – promises to elucidate the underlying mechanism for novel magnetic behavior such as in the exciting "spin gap" and "spin-Peierls" systems. The more interesting compounds contain three or four important interaction constants, and it is impossible to search the complete parameter space to find the constants that provide the best explanation of the magnetic susceptibility or magnetic resonance data. Our group has been calculating these exchange constants from first principles. We find that their values depend strongly on the particular atomic environment, which is often both low symmetry and a combination of ionic and covalent bonded. Knowing the specific values of the exchange constants, physicists can focus the study of spin systems to the regime of direct physical interest, greatly accelerating our understanding of complex materials systems. We expect to extend this approach by combining first principles methods with the explicit inclusion of many-body interactions.
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