Reanalysis of the magnetic field dependence of the penetration depth: Observation of the nonlinear Meissner effect

We examine recent high-precision experimental data on the magnetic field, H, dependence of the penetration depth lambda (H) in YBa2Cu3O7-delta for several field directions in the a-b plane. In a theoretical analysis that incorporates the effects of orthorhombic symmetry, we show that the data at sufficiently high magnetic fields and low temperatures are in quantitative agreement with the theoretical predictions of the nonlinear Meissner effect.