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, it vastly increases the number and variety of technological applications of solid materials through the resulting magnetism. When an electron is midway between being bound to one atom or being free to move among atoms, novel behavior arises such as an extreme 'heaviness'. And complexity of the material, such as requiring three, four, or five different elements in a low symmetry arrangement adds to the richness of behavior. The understanding and mathematical description of such complex materials is the focus of my research.

Perhaps the most well known of such new materials are the high temperature superconductors based on layers of copper and oxygen, where an 'up' spin electron and a 'down' spin electron unite to form a bound superconducting pair. New materials for thermoelectric applications have been grown based on theoretical predictions on how to make the electrons more efficient carriers of heat while making the vibrating atoms less efficient drains of heat. 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 as well as the likelihood of new applications. Such systems have been under study in my group. 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 up spin and down spin 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 to give a new type of superconductor for which the current has a particular spin. If examples of such "single spin superconductors" could be found, a wide variety of completely new phenomena can be observed and studied theoretically.

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 in the double perovskite structure such as La2MnVO6 and La2CuVO6 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 dealing 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 using first principles methods. We find that their values are strongly dependent on the particular atomic environment, which often is both low symmetry and a combination of ionic and covalent bonded. Having the specific values of the exchange constants allows physicists to focus their study of spin systems to the regime of direct physical interest, which greatly accelerates our understanding of complex materials systems. We expect to extend this approach to combine first principles methods with the explicit inclusion of many-body interactions.

Extending Theoretical Methods. The problem of describing the behavior of electrons in condensed matter is a difficult one: the electrons interact strongly and there is no natural small parameter to use in perturbation theory. Density-functional based procedures rely on a self-consistent field approach that gives excellent predictions unless inter-electronic correlations become unusual (as it does for many of the systems discussed above). Our group is strongly involved in formulating better approximations, implementing them in numerical studies, and using them to interpret experimental data.

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Honors and Awards
for
Warren E. Pickett

Memberships
  • American Physical Society
  • Materials Research Society
  • Sigma Xi

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    E-mail pickett@physics.ucdavis.edu

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