Fluctuations, dissipation, and nonuniversal superfluid jumps in two-dimensional superconductors

TL;DR

This study investigates the complex AC conductivity of 2D amorphous superconducting InO_x films, revealing nonuniversal superfluid jumps, vortex dynamics deviations from traditional KTB theory, and universal dissipation phenomena in disordered superconductors.

Contribution

It provides the first detailed finite-frequency analysis of superfluid stiffness and dissipation in 2D superconductors, highlighting deviations from universal KTB predictions and intrinsic nonuniversality.

Findings

Superfluid stiffness exceeds the universal KTB value at T_KTB.

Large vortex dielectric constant suggests limitations of low-fugacity KTB theory.

Universal dissipation observed at low temperatures in disordered films.

Abstract

We report a comprehensive study of the complex AC conductivity of thin effectively 2D amorphous superconducting InO_x films at zero applied field. Below a temperature scale T_c0 where the superconducting order parameter amplitude becomes well defined, there is a temperature where both the generalized superfluid stiffness acquires a frequency dependence and the DC mangetoresistance becomes linear in field. We associate this with a transition of the Kosterlitz-Thouless-Berezinskii (KTB) type. At our measurement frequencies the superfluid stiffness at T_KTB is found to be larger than the universal value. Although this may be understood with a vortex dielectric constant of epsilon_v ~ 1.9 within the usual KTB theory, this is a relatively large value and indicates that such a system may be out of the domain of applicability of the low-fugacity (low vortex density) KTB treatment. This opens up the possibility that at least some of the discrepancy from a non-universal magnitude is intrinsic. Our finite frequency measurements allow us access to a number of other phenomena concerning the charge dynamics in superconducting thin films, including an enhanced conductivity near the amplitude fluctuation temperature T_c0 and a finite dissipation at low temperature which appears to be a universal aspect of highly disordered superconducting films.