Investigation of electronic states on pressure-tuned heavy electron systems in TmTe by infrared spectromicroscopy under high pressure

Recent electric resistivity experiments under high pressure have revealed that materials with strongly correlated electrons exhibit under high pressure many interesting and anomalous physical properties such as a new type of pressure- induced metal-insulator transition (for example, electronic phase transition without any lowering of crystal symmetry), generation of unconventional superconductivity, and so on. However, there are very few attempts by optical measurements to study directly the change in the electronic states under high pressure in several GPa range because of the difficulty in experiment in infrared and THz regions. We performed a spectromicroscopic measurement at room temperature in THz to visible region (25 meV-2.5 eV) under high pressure by using a diamond-anvil-cell(DAC). In this paper, we report recent results of the optical study on TmTe which is a typical material of Tm based monochalcogenide compounds. TmTe is a typical divalent semiconductor material with energy band gap of 0.3 eV at ambient conditions but exhibits an interesting electronic phase transition to metallic state via intermediate valent states without any crystal structure change. We found that the pressure-induced metallic state behaves as a heavy electron system.