Extreme high-field superconductivity in thin Re films

We report the high-field superconducting properties of thin, disordered Re films via magnetotransport and tunneling density of states measurements. Films with thicknesses in the range of 9 to 3 nm had normal-state sheet resistances of $\ensuremath{\sim}0.2\phantom{\rule{4pt}{0ex}}\mathrm{k}\mathrm{\ensuremath{\Omega}}$ to $\ensuremath{\sim}1\phantom{\rule{4pt}{0ex}}\mathrm{k}\mathrm{\ensuremath{\Omega}}$ and corresponding transition temperatures in the range of 6 to 3 K. Tunneling spectra were consistent with those of a moderate coupling BCS superconductor. Notwithstanding these unremarkable superconducting properties, the films exhibited an extraordinarily high upper critical field. We estimate their zero temperature ${H}_{c2}$ to be more than twice the Pauli limit. Indeed, in 6-nm samples the estimated reduced critical field ${H}_{c2}/{T}_{c}\ensuremath{\sim}5.6\phantom{\rule{0.28em}{0ex}}\mathrm{T}/\mathrm{K}$ is among the highest reported for any elemental superconductor. Although the sheet resistances of the films were well below the quantum resistance ${R}_{Q}=h/4{e}^{2}$, their ${H}_{c2}$'s approached the theoretical upper limit of a strongly disordered superconductor for which ${k}_{F}\ensuremath{\ell}\ensuremath{\sim}1$.