Soft mode behavior of the high-superconducting transition temperature Sn-and pb-ternary molybdenum sulfides

Abstract

The heat capacities and magnetic susceptibilities of SnMo5S6 and PbMo5.1S6 were measured between 2 and 400 K. Superconducting transitions occur at Tc-values of ∼11.4 K and ∼12.3 K for the Sn-and Pb-ternaries, respectively. The ratio of the electronic heat capacity to susceptibility indicates a lower limit for λ, the electron-phonon mass enhancement, of unity for both materials. The effective Debye temperature of each material varies with temperature much more strongly than that for a simple metal, and suggests, as does the crystal structure, that the molybdenum-sulfur units can, to at least a low-order approximation, be regarded as molecular units, which are weakly bound to each other and to the Sn or Pb. Average internal-and external-mode frequencies are obtained using a simple harmonic model phonon spectrum to fit the heat-capacity data.