Local Lattice Distortion and Photo-Induced Phase Transition in Transition-Metal Compounds with Orbital Degeneracy

Abstract

Transition-metal compounds with spin, charge, and orbital degrees of freedom tend to have frustrated electronic states coupled with local lattice distortions and to show drastic response against external stimuli such as optical excitation. By means of photoemission spectroscopy, we have studied the electronic states of transition-metal compounds with corner-sharing and edge-sharing MX 6 octahedra (M=transition metal, X=O, S, Se, Br) such as prerovskite-type Pr0.55(Ca1−y Sr y )0.45MnO3 and Cs2Au2Br6, spinel-type CuIr2S4, and quasi-one-dimensional Ta2NiSe5. In the perovskite compounds with corner-sharing octahedra, the charge-orbital states are stabilized by Jahn–Teller or breathing-type lattice distortions and can be destroyed by optical excitations. On the other hand, the charge-orbital states in the edge-sharing systems are stabilized by dimer formation and tend to be robust against optical excitations. Based on the photoemission results, we will discuss effects of local lattice distortions on the excitonic states obtained by optical excitations as well as those in ground states.