Isotope Effect of the Superconducting Transition Temperature in Binary Chevrel-PhaseMo6Se8

Isotope-effect measurements on the binary Chevrel superconductor ${\mathrm{Mo}}_6$ ${\mathrm{Se}}_8$ are reported with either Mo or Se replaced by their isotopes. Within our uncertainty the isotope-effect exponent of $T_c\propto M^{-\beta }$ is equal in both cases (${\beta }_{\mathrm{Mo}}=0.27\mathrm{}\pm 0.04$, ${\beta }_{\mathrm{Se}}=0.27\mathrm{}\pm 0.05$). We concluded that only modes to which the six Mo and the eight Se atoms contribute about equally are determining $T_c$: These are acoustic translational modes of the ${\mathrm{Mo}}_6$ ${\mathrm{Se}}_8$ cluster and its internal optical modes, whereas torsional modes of the cluster must be of minor importance.