Localization in the Peierls gap

The Peierls instability of a one-dimensional metal to a distortion of the lattice that opens a gap in the electronic spectrum at the Fermi energy, E F , has been extensively investigated in a wide range of low-dimensional conductors in the past decade. This field has provided a very rich range of new physics; these include the varied sequences of incommensurate to commensurate superlattice phase transitions and some new transport phenomena. In this paper we concentrate on one aspect of transport in these systems, the way in which the Peierls mechanism, by reducing the density of states at the Fermi level from a metallic level to a low level, can, in the presence of suitable impurity potentials, allow localization of those remaining carriers, so that conduction is by hopping between these states near the Fermi level. We select two systems of materials to illustrate these processes, the layer structure material 1T-TaS 2 and its intercalation complexes with hydrazine, and the conjugated polymers such as polyacetylene. Both these systems can undergo an 'intercalation' type of reaction, TaS 2 with electron donors (hydrazine), the polymers with both donors and acceptors. In both cases there is a charge transfer reaction, and the Coulomb potential set up by the charged intercalant species, which are present between the host material layers or chains, is of strength appropriate to provide carrier localization and conduction by variable-range hopping.