Tunneling into Dirty Superconductors Near their Upper Critical Fields

We describe experiments on tunneling into $\mathrm{Sn}\mathrm{In}$ and $\mathrm{Pb}\mathrm{Bi}$ alloys near their upper critical fields. In this regime, a theory by de Gennes predicts gapless superconductivity with a density of excited states whose energy scale is field-independent. We have experimentally investigated this density of states in detail, both in the superconducting sheath regime near $H_{c3\mathrm{}}$ (field parallel to the surface) and in the vortex regime near $H_{c2\mathrm{}}$ (field normal to the surface). We find excellent agreement between theory and experiment for the shape of the tunneling characteristic and reasonable agreement for its normalization.