Thermodynamic properties of charge-density waves

Abstract

Low-temperature thermodynamic properties of linear-chain compounds exhibiting charge-density waves (CDW) are examined theoretically within a mean-field theory. A result for the spin susceptibility \ensuremath{\chi} is obtained which agrees with the clear-cut available data on ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ for T0.9${T}_{P}$, where ${T}_{P}$ is the Peierls transition temperature. The influence of ordinary impurities on the order parameter \ensuremath{\Delta}, the half-gap ${\ensuremath{\Omega}}_{G}$, and spin susceptibility \ensuremath{\chi} is calculated. Numerical results are obtained for \ensuremath{\Delta} and ${\ensuremath{\Omega}}_{G}$ as a function of the impurity concentration x. Substantial difference is found between the lattice distortion parameter \ensuremath{\Delta} and the half-gap ${\ensuremath{\Omega}}_{G}$ even for relatively small impurity concentration x, which is directly accessible to experimental verification. Beyond a critical concentration ${x}_{c}^{\mathcal{'}}$, the excitation spectrum of CDW condensate does not exhibit a gap. The order parameter also yields the transition temperature as a function of x, in agreement with earlier results of Patton and Sham and with recent experiments on ${\mathrm{TaS}}_{3}$ doped with Nb and Se impurities. Impurities are found to enhance spin susceptibility. However, the susceptibility at zero temperature remains zero for all concentrations, except in the gapless regime.