The nonlinear $I-V$ characteristics below the superconducting transition of high-sheet-resistance Hg-Xe films have been analyzed with use of a model in which the nonlinearity is a consequence of current-induced vortex-antivortex unbinding. The characteristics can be fitted by $V\sim I^{a\left(T\right)\mathrm{}}$, where $a\left(T\right)\mathrm{}$ is a measure of the a real superelectron density $n_s\mathrm{}\left(T\right)\mathrm{}$. The universal relations predicted for the Kosterlitz-Thouless-Berezinskii transition in two dimensional superconducting films are obeyed.

• Received 26 May 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.534