Research Plans

The replacement of chemical elements and the application of pressure have been widely used as control parameters to metallize the Mott insulating phase peculiar of strongly correlated electron systems. Novel phenomena such as high temperature superconductivity, giant magnetoresistance, and spin triplet superconductivity were observed, so far. The purpose of this research is to clarify the new phenomena emerging in the non-equilibrium physical state determined by steady current flow, in order to establish the effectiveness of DC electric field / current as new control parameters of strongly correlated multi body effects, and to improve the understanding of the mechanism. Our research focuses on the Mott insulator Ca₂RuO₄, where we investigate electronic states realized out of equilibrium, such as the metal—insulator transition that we discovered to be triggered under the application of a direct current. In addition, by searching for similar non-equilibrium / nonlinear phenomena in other Mott insulator oxides, we will clarify the characteristics and generality of these new phenomena. With this research, we aim to become world leaders in research and development of emergent phenomena in non-equilibrium steady states of strongly correlated materials.

Research method

1. We will investigate the mechanism of the electric field-induced insulator--metal transition and of the current-induced magnetism for 4d electron ruthenium oxide Ca₂RuO₄. Concerning the insulator-metal transition, we will clarify the roles of phonon and structural instabilities. Regarding magnetism, we will seek experimental evidence for the application of Okayama's theory by constructing a physical phase diagram where current is the control parameter, and the dependence of the direction of current and magnetic field (anisotropy) is investigated. We will also study current-induced magnetic states in related materials.
2. 3d · 5d We select Mott insulators with a relatively small energy gap among other complex oxides and investigate the effects of DC field and current. We will employ photoelectron spectroscopy under current and simultaneous theoretical construction, collaborative research both in Japan and abroad, research on phonon instability, and research on local dynamics by scanning probe and pump / probe spectroscopy. Finally, we will compare the DC effect subject of this research with the fast relaxation process.

Expected outcome and significance

The ability to control the conductivity and magnetism of a material by turning on and off the steady current is a unique nonequilibrium phenomenon. This opens the possibility of creating an electronic state which could not be induced by conventional control parameters. We expect the emergence of novel electronic phases including unique magnetism and superconductivity, and we expect that this effect could also be measured on similar materials. This interesting direction has a great impact both for device applications and for fundamental physical science, making it possible to induce novel properties in Mott insulators by utilizing a simple non-equilibrium state achieved with the application of a direct current. In addition to changes in chemical composition and the application of pressure that have been used so far, demonstrating the effectiveness of the new control parameter "DC electric field / current" is of great significance to the development of new fields.