Nonlinearity of the current-voltage characteristics for YBa2Cu3O7–x single crystals and the Berezinskii-Kosterlitz-Thouless transition

The nonlinearity of the current-voltage characteristics for YBa₂Cu₃O_{7 ? x} single crystals is investigated in the vicinity of the superconducting transition temperature T _c , and the mechanisms responsible for the nonlinearity of the current-voltage characteristics are considered. The current-voltage characteristics and the amplitudes of the higher voltage harmonics for the YBa₂Cu₃O_{7 ? x} single crystals are directly measured using modulation Fourier analysis. It is shown that the positions of the maxima in the amplitudes of the higher voltage harmonics coincide with the Berezinskii-Kosterlitz-Thouless transition temperature T _BKT. This coincidence can provide a means for determining the transition temperature T _BKT. A number of parameters characterizing the YBa₂Cu₃O_{7 ? x} single crystals in the superconducting state are estimated using the experimental results and the model of a Berezinskii-Kosterlitz-Thouless transition. The parameters thus estimated are in good agreement with the data available in the literature.