Magnet or esistance and Intrinsic Pinning of Vortices in the Untwinned YBa_2Cu_3O_(7-δ) Single Crystals with Optimal Oxygen Content and a Small Deviation from the Oxygen Stoichiometry

摘要

We study the temperature dependence of the in-plane magnetoresistance p_(ab)(T) in untwinned YBa_2Cu_3O_(7-δ) single crystals with the optimal oxygen content and with a small deficit of oxygen atoms at different angles between the external magnetic field 15 kOe and the ab-planes α. We found that at high temperatures in the pseudogap region external magnetic field does not affect the p_(ab)(T) but it broadens transitional region T_c - T_(co) from 0.3 K at zero field and α=0 to approximately 6 K at α = 60° in the field. In case of optimal doping the function p_(ab)(T) display a 3D to 2D dimensional crossover when temperature decreases from T_c to T_(co) and scaling near the T_(co) which we relate to the flux-flow and vortex-lattice melting. In the underdoped sample the vortices are effectively pinned by the random oxygen vacancies and the function p_(ab)(T) in transitional region has smooth tails without any traces of crossover and scaling.