Superfluid stiffness in strongly disordered superconducting films

In BCS-superconductors, the spectral gap, E_g, the pairing amplitude, ∆, and the mean-field critical temperature T_c0 are essentially identical energy scales. This is no longer the case in the presence of sufficiently strong disorder, where the superconductor-insulator transition (SIT) is approached. Moreover, in BCS-theory the superfluid density stiffness, J_S, is fully determined by ∆ and the normal state resistance T_N. and also this relation no longer holds in the presence of strong disorder, so that J_S becomes a scale of its own right. Recent experiments have determined J_S(T ) in ultrathin NbN films by measuring kinetic inductance and found a sharp Berezinski-Kosterlitz-Thouless(BKT) transition [2]. Our latest data cover J_S(T ) over a wide range of disorder strength. We find that the BKT-transition remains sharp right up to the SIT and measure the characteristic scales E_g, J_S, T_c0 and TBKT independently of two orders of magnitude in R_N. We present complementary numerical calculations of the superfluid stiffness, obtained from the Boguliubov-deGennes (BdG) theory of disordered samples in a very broad range of disorder strengths. Building upon Ref. [1], we reach unprecedented system sizes large enough to capture the effect of mesoscopic wavefunction fluctuations on J_S(0). A detailed comparison of our computational results with the measurements will be presented

REFERENCES

[1] Stosiek et al. PhysRevB . 101 . 144503.

[2] Weitzel et al. Phys. Rev. Lett. 131 . 186002