Nonlinear Meissner effect in a high-temperature superconductor

The lowest nonlinear correction to the penetration depth, i.e., the nonlinear Meissner effect, is calculated and compared to data from high-quality YBa2Cu3O7-delta (YBCO) films. The calculation is based on the Green-function formulation of superconductivity, and the data consist of the intermodulation power as function of temperature and circulating power. At a low power level, the calculated temperature dependence compares very well with the data, including the divergence as T-2 at very low temperatures. The calculated power dependence of the nonlinear penetration depth follows the data semiquantitatively and is enhanced due to the d-wave symmetry of the order parameter. These results support the assertion that the origin of nonlinearity in high-quality YBCO films is intrinsic. The analysis also implies that the nonlinear corrections to the penetration depth depend primarily on the total current carried by the strip and thus are insensitive to the edges. The comparison of the present approach with an alternative approach, based on quasiparticle backflow, is discussed.