Study on electronic states of solids under high pressure by infrared synchrotron radiation

Recent electrical resistivity measurements under high pressure revealed that materials with strongly correlated electrons exhibit many interesting physical properties such as a stabilization of insulating phase, generation of unconventional superconductivity, and so on. However, there seems to be no attempt by optical measurements to study directly the change in the electronic states from metallic phase to insulating one under high pressure in GPa range because of the difficulty in experiments in the far-infrared region. The difficulty is mainly caused by the insufficiency in the brightness of conventional black body sources. SPring-8 supplies a more collimated and brilliant infrared beam to the end station than conventional thermal sources, and then make possible a spectromicroscopic measurement under high pressures above 10 GPa in the far-infrared region. In this paper, we report recent results of the optical study on several kinds of solids which exhibit a cross-over change in the electronic states under high pressure.