Photoinduced infrared absorption of quasi-one-dimensional halogen-bridged binuclear transition-metal complexes

We investigate the optical conductivity of photogenerated solitons in quasi-one-dimensional halogen-bridged binuclear transition-metal MMX complexes with particular emphasis on a comparison among the three distinct groups: A_4_[Pt_2_(P_2_O_5_H_2_)4_X]nH_2_O (X=Cl,Br,I; A=Na,K,NH_4,…), Pt_2_(RCS_2_)4_I (R=C_n_H_2n+1) and Ni_2_(CH_3_CS_2_)4_I, which exhibit a mixed-valent ground state with the X sublattice dimerized, that with the M_2 sublattice dimerized and a Mott-Hubbard magnetic ground state, respectively. Soliton-induced absorption spectra for A_4_[Pt_2_(P_2_O_5_H_2_)4_X]nH_2_O should split into two bands, while that for Pt_2(RCS_2_)4_I and Ni_2(CH_3_CS_2_)4_I should consist of a single band. The excitonic effect is significant in Ni_2(CH_3_CS_2_)_4_I.