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

We report measurements of the electromagnetic penetration depth A(T) in nominally pure crystals of YBa2Cu3O6.95, for temperatures close to the 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 approximate to 0.33, consistent with the critical behavior of the three dimensional XY model. The measured critical behavior is not affected by the presence of small amounts of Zn impurities, in agreement with the Harris criterion.