Electron-phonon interactions in the superconducting Chevrel phase compounds Mo6Se8-xSx

The specific heat of nine single crystals of the Chevrel phase compounds Mo6Se8-xSx (0≤x<=4.0) has been investigated in the temperature range 1.5--9.0 K in magnetic fields of 0 and 6 T. Using the BCS theory with strong-coupling corrections, the thermodynamic properties and information on the electron-phonon interactions have been obtained. The superconducting transition temperatures Tc of these compounds decrease with increasing S concentration x, and the electron-phonon interactions of this system change from strong to weak. In spite of the change in Tc, the bare electronic density of states is nearly constant for all samples. The value of Tc depends on the average phonon frequency through the electron-phonon coupling constant. The anomalous lattice specific heat has been observed for low concentration samples. This is explained by assuming low-lying soft-phonon modes, which were reported by inelastic neutron experiments for Mo6Se8, in addition to a molecular crystal model. The relationship between the electron-phonon interactions and the low-lying soft-phonon modes is discussed