Absence of nonlinear Meissner effect in YBa2Cu3O6.95

We present measurements of the field and temperature dependence of the pene tration depth (lambda) in high purity, untwinned single crystals of YBa2Cu3 O6.95 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 fiel d dependent correction to lambda, however, are considerably different from that predicted from the theory of the nonlinear Meissner effect for a d-wav e superconductor (Yip-Sauls theory). Our results suggest that the Yip-Sauls effect is either absent or is unobservably small in the Meissner state of YBa2Cu3O6.95.