SUPERCONDUCTIVITY I N ALKALI TUNGSTEN BRONZES

Abstract

The anomalous behavior of the superconducting transition temperature in certain alkali tungsten bronzes is shown to be consistent with electron pairing induced by exchange of acoustic plasmons. The superconducting properties of certain alkali tungsten bronzes exhibit some highly unusual features which appear to violate the predictions of the BCS theory. These nonstoichiometric compounds are of the form M W03, where M represents an Alkali atom such as Na, Kb, and hydrogen. As x increases, these compounds become metallic, and the electronic density of states is believed to increase monotonically with the alkali content as indicated by magnetic susceptibility measurements l\f. On the basis of the BCS theory 111 the superconducting transition temperature T should become higher with increasing electronic density of states at the Fermi energy N(0) according to the relation Tc =O.70DexP [-^Jgj-j (1) Where the Debye temperature is roughly 0 400 K in these materials and g is a matrix element of the electron-phonon coupling. For a free electron model /3 N(0) a x , and the predicted T increases with x as shown in Figure 1. If the electron density of states increases faster (say N(0) «x as in Reference /l/),then the transition temperature of course should rise more sharply as a function of alkali content. In sharp contrast to these expectations, the experimental data show an enhancement of T with decreasing x for Rb W03 /3,4/ and Na W03/5/as w x seen in Figure 1. The low values of the electronic specific heat and magnetic susceptibility in these materials /!/ would seem to rule out spin fluctuation effects, and the extrapolated density of t Supported in part by the National Science Foundation Grant No DMR 77-13167 electron states is quite small, resembling the values of Na, Cu, and other non-superconducting metals. The purpose of the present work is to correlate the anomalous T -dependence in these materials with an electron pairing mechanism achieved by virtual exchange of acoustic plasmons.