In-Plane Anisotropy Of The Resistivity In Yba2cu3o7-Delta Single Crystals With Various Doping Conditions

We have measured the in-plane anisotropy of the resistivity rho(theta) for YBa2Cu3O7-delta (YBCO) single crystals with various doping conditions in the Josephson vortex (JV) state (H parallel to ab). We found that a novel pinning specified by the dip structure in the rho(theta) depends nonmonotonically on the carrier doping. In our results, this pinning is strong in the heavily overdoped and slightly underdoped YBCO and weak in the optimally doped YBCO. Moreover, a first-order phase transition appears only in the optimally doped region. Thus, we believe that this pinning dominates the JVs phase transition. (c) 2007 Elsevier B.V. All rights reserved.