Orientational Phase Transitions in a Quasi-One-Dimensional Conductor

Abstract

A method is presented by calculating orientational phase transitions in quadrupolar strands with itinerant-electrons, which arise from the coupling of the conduction electrons with librons (the small tortional oscillations about the equilibrium orientations of the molecules). The Peierls instability has been studied for a model of one-dimensional conductor in which the rotational motion is described in terms of a pseudo spin S=1 Hamiltonian, where the molecules in the solid interact through quadrupole-quadrupole and dipole-dipole interactions. The electron and libron fields are expressed in the second quantization representation and the Green’s function formalism is used to calculate the excitation spectrum of the coupled system. The derived expressions for both the electron and libron Green’s functions for the coupled system give a simple recursion formula for the Peierls transition temperature T_c, which is driven by the loss of librons from the Peierls condensate through thermal excitation. The recursion formula is solved numerically for a half- filled tight-binding conduction band and the dependence of T_con the values of the different parameters in the problem is studied; the numerical results are graphically pre-sented. An expression for the electrical conductivity is derived, which describes the collective contribution arising from the phase oscillations of the lattice distortion.