Photo-control of charge and orbital states in transition-metal compounds porobed by photoemission spectroscopy

We have studied the charge-orbital states in spinel- type CuIr ₂S ₄ and perovskite-type Pr _0.55(Ca _1-y Sr _y ) _0.45MnO ₃ (PCSMO, y=0.25, 0.40) using X-ray photoemission spectroscopy (XPS) combined with laser illumination. The XPS data of CuIr ₂S ₄ are dramatically changed by the insulator-metal transition (IMT). The line shape of CuIr ₂S ₄ in the insulating phase is consistent with the charge disproportionation of Ir ³⁺:Ir ⁴⁺=1:1. Interestingly, a visible light irradiation at low temperature gives a resistance reduction but does not give any spectral change. In case of PCSMO, the IMT is also clearly observed on the XPS data and the peculiar temperature dependence for y=0.25 suggests the coexisting of the ferromagnetic metallic phase and the charge-orbital ordered insulating phase. The spectra after visible light illumination show the enhancement of the density of states at the Fermi level, accompanied with the drop of the resistance.