dc-conduction mechanism and Peierls gap in organic and inorganic charge-density-wave conductors

Abstract

It has been shown in a recent analysis of the temperature dependence of the dc conductivity of the quasi-one-dimensional conductor (fluoranthene${)}_{2}$${\mathrm{PF}}_{6}$ that in spite of the occurrence of the Peierls transition to a charge-density-wave ground state (formally implicating polarons as excitations), the dc conduction is essentially due to electron-hole transport in bands and acoustical phonon scattering of the carriers. The theory allows for the determination of the temperature dependence of the Peierls gap below and the fluctuating pseudogap above the transition temperature. Our dc-conductivity measurements confirm that a common temperature dependence occurs in organic radical cation salts and in inorganic materials from the groups of the blue bronzes and the transition metal tetrachalcohalogenides. These materials are rather different especially with respect to the nature of the states forming the conduction band and the filling of the latter. Here we reduce the needed information on the band structure to a minimum connected with optical data and extend the theory to the case of a gap small compared to ${\mathit{k}}_{\mathit{B}}$T. The theory is applied to (Fa${)}_{2}$${\mathrm{PF}}_{6}$, ${\mathrm{K}}_{0.30}$${\mathrm{MoO}}_{3}$, and (${\mathrm{TaSe}}_{4}$${)}_{2}$I as representatives of the above-mentioned groups of materials. From the measured conductivity data the temperature dependence of the Peierls gap below and the pseudogap above the transition temperature are determined as well as several conductivity-related quantities. Similarities and differences of the investigated materials are discussed.