Multi-Gap Superconductivity in Chevrel Phases

Sub-Kelvin scanning tunnelling spectroscopy (STS) in the quasi-3D Chevrel Phases PbMo$_6$S$_8$ and SnMo$_6$S$_8$ reveals two distinct superconducting gaps at $\sim$~3meV and $\sim$~1meV. The relative spectral contribution from each gap changes with the angle between tip and sample, implying a rather anisotropic Fermi surface. Complementary to our local probe studies, specific heat measurements confirm the strong coupling (2$\Delta$/$k_B$$T_c$~$\sim$~4.9) seen by STS in each material and provide further evidence for multi-gap superconductivity. Hexagonal vortex lattices have been imaged by STS for each compound, with vortex core spectroscopy indicating a pseudogap within the cores. No pseudogap is visible in normal-state spectra, suggesting that the gapped vortex cores may be a novel consequence of two-band superconductivity.