Superconducting phase transitions in indium/indium-oxide thin-film composites

Low-frequency techniques are used to examine the predictions of the equilibrium Kosterlitz-Thouless theory of the vortex-pair unbinding transition in indium/indium-oxide composite superconducting thin films. The renormalized superfluid density obtained from independent measurements of the kinetic inductance and the exponent of the current-dependent resistance are in agreement. At the transition temperature $T_c$, the critical value of the superfluid density agrees with theory for a finite measurement length. Experimental sensitivity is such that the resistance near $T_c$, measured to be about 9 orders of magnitude below the normal resistance, is explained by the motion of a single thermally excited free vortex in a superfluid background renormalized by bound-vortex pairs. The corresponding critical magnetic field for flux entry is also measured. The resistance of the thermally excited free-vortex plasma and the correlation length above the transition temperature obey the qualitative prediction of the theory. Nonuniversal constants in the renormalization-group theory are obtained from the experiment and are found to be sample dependent.