Absence of nonlinear Meissner effect inYBa2Cu3O6.95

We present measurements of the field and temperature dependence of the penetration depth $\left(\lambda \right)$ in high purity, untwinned single crystals of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}$ in all three crystallographic directions. The temperature dependence of $\lambda$ is linear down to low temperatures, showing that our crystals are extremely clean. Both the magnitude and temperature dependence of the field dependent correction to $\lambda ,$ however, are considerably different from that predicted from the theory of the nonlinear Meissner effect for a d-wave superconductor (Yip-Sauls theory). Our results suggest that the Yip-Sauls effect is either absent or is unobservably small in the Meissner state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}.$