Moments and Multiplets in Mott Materials

Coordinators: Leon Balents, Matthew P.A. Fisher, Daniil I. Khomskii, George A. Sawatzky, Oleg Tchernyshyov

In materials, atomic orbitals derived from d or f shells are especially tightly bound near the nuclei. This strongly reduces the ability of electrons in those levels to delocalize by hopping from atom to atom, limiting the usefulness of a naive picture of extended band states for materials in which these are the active orbitals. The evolution from band to localized states is known as the Mott transition. The states of the electrons in d or f shells are often degenerate, i.e. they consist of several different discrete quantum spin and/or orbital angular momentum states even when the number of electrons on the ion is held fixed. This leads to an extensive entropy which is removed at low temperature by collective effects involving many (or all) of the electrons in the solid. These collective effects are the focus of the workshop.

What sort of physics do they give rise to? In the simplest cases, the atomic degeneracy is purely due to spin. This leads often to interesting magnetic structures, and in some cases to novel states with strong magnetic fluctuations but no static moments. More often, orbital degeneracy is involved. The directional nature of these states gives rise to a host of complex phenomena. Knowledge of the orbital state is required to understand many important materials properties, including colossal magnetoresistance, ferroelectricity, unconventional superconductivity, and charge ordering.

We plan to bring recent experimental advances, probing such orbital and spin degenerate systems including novel techniques such as resonant x-ray scattering, into perspective for the broader theoretical community. Similarly, advances in our understanding of emergent order and subtle correlations in toy models for frustrated systems will be applied to phenomenological puzzles in these materials. In these ways, the program will bring together theorists from a diverse set of backgrounds, spanning from quantum chemistry to quantum topology, with a steady stream of experimentalists focusing the discussion on particular promising materials.

In association with this program, we intend to host a conference tentatively scheduled for the week of Sept 10-14, 2007. More information will be posted when it becomes available.