PENETRATION DEPTH MEASUREMENTS OF 3D XY CRITICAL-BEHAVIOR IN YBA2CU3O6.95 CRYSTALS

We report measurements of the electromagnetic penetration depth A(T) i n nominally pure crystals of YBa2Cu3O6.95, for temperatures close to t he critical temperature T-c. Over the range 0.001 < (T-c - T)/T-c < 0. 1, we find that lambda(T) proportional to (1 - T/T-c)(-y) with y appro ximate to 0.33, consistent with the critical behavior of the three dim ensional XY model. The measured critical behavior is not affected by t he presence of small amounts of Zn impurities, in agreement with the H arris criterion.