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 ∼0.2kω to ∼1kω 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 Hc2 to be more than twice the Pauli limit. Indeed, in 6-nm samples the estimated reduced critical field Hc2/Tc∼5.6T/K is among the highest reported for any elemental superconductor. Although the sheet resistances of the films were well below the quantum resistance RQ=h/4e2, their Hc2\u27s approached the theoretical upper limit of a strongly disordered superconductor for which kFℓ∼1