Node-like excitations in superconducting PbMo₆S₈ probed by scanning tunneling spectroscopy

We present a scanning tunneling spectroscopy study on the Chevrel phase PbMo₆S₈, an extreme type-II superconductor with a coherence length only slightly larger than in high-T_c cuprates. Tunneling spectra measured on atomically flat terraces are spatially homogeneous and show well-defined coherence peaks. The low-energy spectral weight, the zero bias conductance, and the temperature dependence of the gap are incompatible with a conventional isotropic s-wave interpretation, revealing the presence of low-energy excitations in the superconducting state. We show that our data are consistent with the presence of nodes in the superconducting gap.