Magnetic Field Dependence of λ in YBa₂Cu₃O_6.95: Results as a Function of Temperature and Field Orientation

We present measurements of the magnetic field dependence of the penetration depth λ(H) for an untwinned YBa₂Cu₃O_6.95 single crystal for temperatures from 1.2 to 7 K in dc fields up to 177 G and directions 0°, ±45°, and 90° with respect to the crystal b axis. The edges of the platelet sample have been polished round to limit extrinsic effects. At 1.2 K, Δλ(H) in the a and b directions could be interpreted as agreeing with the Yip and Sauls prediction for the nonlinear Meissner effect in a d-wave superconductor. However, the systematics versus temperature and orientation, a key aspect of the theory, do not agree, and we conclude that the nonlinear Meissner effect is suppressed by a factor of 10 or larger.