In solids, a huge number of electrons interact with one another through Coulomb repulsion, and the correlation effects induced by the interaction can lead to various interesting phenomena. One such effect is unconventional superconductivity ; while the Cooper pairing interaction in superconductors is usually mediated by phonons, the electron correlation effects can also be the origin of the pairing interaction, which can give rise to high Tc and/or unconventional pairing symmetries. We are particularly interested in unconventional superconductivity that is considered to be realized in high Tc materials such as the cuprates and the iron pnictides, and also in organic conductors. Applying many-body theory to model Hamiltonians that take into account the realistic band structure, we provide theoretical understanding of the experimental observations, and also intend to make predictions.
Another subject of interest is the thermoelectric effect. The Seebeck effect transforms heat into electric power, but the efficiency is usually low because large Seebeck effect is often observed in materials with high resistivity. We study thermoelectric materials that give rise to large efficiency through a coexistence of large Seebeck coefficient and low resistivity, which can be realized by a cooperation of peculiar band shapes and electron correlation effects.
Electronic structure calculations are one of the central disciplines of computational physics. These calculations help us to explain the characteristic properties of different materials and to understand the mechanisms behind their functionality. We are interested in the development of new methods of electronic structure calculation. One of our main interests is the development of algorithms for the numerical calculation of many-body electron effects.
Many materials, for example alloys and glasses, are strongly disordered. In such materials the nature of the electronic states is very different from that in crystalline materials. In particular, the phenomena of Anderson localization and the Anderson transition occur, which strongly influences electrical properties at low temperatures. We are performing large scale numerical simulations of Anderson localization with a particular emphasis on the precise analysis of the critical phenomena at the Anderson transition.