Magnetic Field Dependence ofλinYBa2Cu3O6.95: Results as a Function of Temperature and Field Orientation

We present measurements of the magnetic field dependence of the penetration depth $\lambda \left(H\right)$ for an untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}$ single crystal for temperatures from 1.2 to 7 K in dc fields up to 177 G and directions $0°$, $\pm 45°$, 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, $\Delta \lambda \left(H\right)$ 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.