Theory for photoinduced phase transition from a charge-density-wave state to a Mott-Hubbard insulator in a quasi-one-dimensional Br-bridged Pd compound

Photoinduced phase transition from a charge-density-wave (CDW) state to a Mott-Hubbard (MH) insulator is studied theoretically. According to the very recent experiment [ H. Matsuzaki et al. Phys Rev B 70 035204 (2004)], a quasi-one-dimensional Br-bridged Pd compound, [Pd(chxn)₂][Pd(chxn)₂Br₂]Br₄, can become a MH insulator in its metastable phase that is usually hidden by the stable CDW phase. Since this finding strongly suggests the possibility of a light-driven macroscopic phase change from the latter to the former, we investigate possible relaxation paths realizing such phase changes. It is found that both of two initial photoexcited states, namely, the lowest exciton and a separate electron-hole pair yielded by a high-energy excitation, induce the phase change with no energy barrier. This demonstrates a very “soft” nature of this material, being consistent with recent experimental results.