Research Activities- ISHIHARA Sumio -

X-Ray and Optical Properties in Correlated Systems

The recent development of the third generation synchrotron radiation source, such as SPring-8 in Japan, opens a large window of the research in the correlated electron systems. One of the newly developed technique is the resonant x-ray scattering; the long range orderings of charge and orbital are observed by the elastic scattering, and a various electronic excitations are measured by the inelastic scattering. Although this is the powerful technique, i.e. site selective and momentum resolved, its scattering processes are not simple. Purpose of this theme is to investigate theoretically the experimental probe using the synchrotron radiation and to establish these method as new probes for the correlated electron systems. We are strongly collaborating with the experimental groups. The obtained results are fed back into the above two themes. In addition to the x-ray, the optical properties in correlated systems, e.g. the 2nd and 3rd nonlinear optical responses, the circular and linear birefringences/absorptions are one of the topics in this theme. Origin of the new and colossal optical responses are elucidated and predicted.

Electron-Lattice and Electron-Electron Correlation

A lot of researches have been done for the high-Tc cuprates from the view point of the electron correlation and the magnetic interactions as a origin of the high-Tc superconductivity. On the other hand, in spite that a lot of dielectric and lattice anomalies have been pointed out experimentally, a little has been done for the combination effects of electron correlation and electron-lattice interaction. In this theme, new types of phenomena caused by the mutual effects between the electron-lattice interaction and electron-electron interaction. For example, [1] Roles of the electron-lattice interaction in novel superconductivity in correlated electron systems, [2] The electronic (anti) ferroelectricity in correlated electron systems, and magnetic-ferroelectrics. A new type of quantum phenomena originating from the two kinds of many body effects are expected to be discovered.

Charge-Spin-Orbital Coupled Phenomena

Various novel phenomena appears in correlated electron systems which are not described well by the one-electron scheme. One of the example is the transition-metal oxides, such as the high-Tc superconductivity in cuprates and the colossal magnetoresistance in perovskite manganites. The d electrons in the transition-metal ions govern the magnetic, electric and optical properties in this compound. In addition to the spin and charge degrees of freedom of electron, the d electron in a solid has the third degree of freedom, i.e. the orbital degree of freedom. These three degrees of freedom are combined and/or separated with each other under the strong electron correlation. As a result, there appear various electronic phases (vacuum) and excitations (particles). At vicinity of the phase boundary, the colossal responses to the external fields are expected. We are studying origin of the novel quantum phenomena and predict new types of the quantum states by using the field theoretical method and the numerical calculation method. The theoretical results will be applied to the design for the new generation materials- correlated electron devices-.

copyright(c)2005 Tohoku University