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 similar to 0.2 k Omega to similar to 1 k 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 similar to 5.6 T/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/4e(2), their H-c2 's approached the theoretical upper limit of a strongly disordered superconductor for which k(F)l similar to 1.