Subgap optical absorption induced by quantum lattice fluctuations at the Peierls edge in PtCl chain complexes

The temperature dependence of the low-energy tail of the interband optical absorption band at the Peierls edge has been studied in halogen-bridged linear-chain complexes, [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$${)}_4$ and [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{BF}}_4$${)}_4$, en being ethylenediamine. The absorption spectrum and its temperature dependence are explained well in terms of the subgap density of states of the one-dimensional electronic bands induced by the quantum vibration of Cl ions of the breathing mode. Both substances are found to have a dimensionless electron-phonon coupling constant of λ≊1.2, the zero-temperature gap parameter of ${\mathrm{\Delta }}_0$≊1.45 eV, and the transfer integral of ${\mathit{t}}_0$≊0.4 eV. Because of the strong electron-phonon coupling the temperature dependence of the energy gap is dominated by the self-energy effect; even at 0 K the self-energy amounts to 0.07–0.08 eV, being considerably greater than the self-energies in usual semiconductors. The role of the anharmonicity of the chainlike Pt-Cl bonds is discussed in relation to the origin of the large gap parameter. © 1996 The American Physical Society.