Node-like excitations in superconducting PbMo$_6$S$_8$ probed by scanning tunneling spectroscopy

We present the first scanning tunneling spectroscopy study on the Chevrel phase PbMo$_6$S$_8$, 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.