Nonlinear Meissner effect in double layered high-T-c cuprates investigated by measurement of the penetration depth

The in-plane London penetration depth lambda(ab) was investigated as a function of magnetic field H in YBa2Cu3Oy, Bi2Sr2CaCu2Oy and Tl2Ba2CaCu2Oy. lambda(ab) changes linearly with H at low temperatures, with an almost temperature independent gradient. Below a crossover field H', the H-linear lambda changes into an H-2 behavior. The crossover from H to H-2, the magnitude and temperature dependence of H', and the temperature dependence of the gradient of the H-2 term around zero field are all consistent with the nonlinear Meissner effect in unconventional superconductors. The results of these studies suggest that the order parameter of the double layered cuprates has nodes in k space.