Low-temperature specific heat and magnetic susceptibility of nonmetallic vanadium bronzes

Abstract

A linear $\gamma T$ term is observed in the low-temperature specific-heat behavior of the nonmetallic vanadiumoxide bronzes of sodium and copper. The low-temperature magnetic susceptibility is equally anomalous and points to a singlet or diamagnetic ground state. It is postulated that the ${\mathrm{V}}^{4+}$ centers in these bronzes form near-neighbor pairs or bipolarons through deformation-induced attraction. The tunneling of these bipolarons between pairs of sites gives rise to the linear term of the specific heat. The progessive breaking up with temperature of the singlet bipolarons provides a natural explanation of both the magnetic susceptibility as well as of the electrical conductivity.