Two-dimensional vortex fluctuations in the nonlinear current-voltage characteristics for high-temperature superconductors

The nonlinear current-voltage (I-V) characteristics are discussed in terms of thermally activated vortex-antivortex pairs in the ${\mathrm{CuO}}_2$ planes. The interaction energy of the two vortices in a pair grows linearly with their separation due to the interplane coupling. This linear interaction gives rise to a critical current which decreases linearly with the temperature. The I-V characteristic has the form V∼I(I-${\mathit{I}}_{\mathit{c}}$ ${)}^{\mathrm{\alpha }\left(\mathit{T}\right)\mathrm{-}1}$. Comparisons with experiments on Y-Ba-Cu-O are discussed. We suggest that the I-V data in the case of a magnetic field may be associated with a thermal vortex-unbinding transition.