The optical properties of the charge-ordering (CO) phase of the one-dimensional (1D) half-filled spinless Holstein model are derived at zero temperature within a well-known variational approach improved including second-order lattice fluctuations. Within the CO phase, the static lattice distortions give rise to the optical interband gap, that broadens as the strength of the electron-phonon (el-ph) interaction increases. The lattice fluctuation effects induce a long subgap tail in the infrared conductivity and a wide band above the gap energy. The first term is due to the multiphonon emission by the charge carriers and the second to the interband transitions accompanied by the multiphonon scattering. The results show a good agreement with experimental spectra of quasi-1D conductors such as ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ and $({\mathrm{TaSe}}_4{)}_2\mathrm{I}.$

• Received 26 August 2002

DOI:https://doi.org/10.1103/PhysRevB.67.214301