Evolution and analysis of the vortex solid to vortex liquid melting line in Y_(1-x)Pr_xBa_2Cu_3O_(6.97) and YBa_2Cu_3O_(6.5) to 45 tesla

摘要

By extending magnetotransport measurements to fields up to 45 T, we have been able to examine the melting line of Y_(1-x)Pr_xBa_2Cu_3O_(6.97) (x=0-0.4) thin film samples and that of an oxygen deficient YBa_2Cu_3O_(6.5) single crystal over an extended temperature range of 0.03T_c ≤T ≤ T_c, larger than heretofore reported. These measurements provide an opportunity to examine the evolution of the behavior of vortices along nearly the entire melting line of this important high-T_c superconducting system. Within the context of the quantum-thermal fluctuation model of Blatter and Ivlev [Phys. Rev. Lett. 70, 2621 (1993)], we empirically arrive at a modified form of their expression for the vortex-lattice melting line by considering that the mechanism responsible for the vortex flux line relaxation time is not simply the scattering time of quasiparticles within the vortex core. This form of the melting line is demonstrated to fit over the entire melting line of each sample, implying no crossover in dynamics from three dimensions to two dimensions. This result implies that the physical mechanism responsible for the manner and conditions under which the melting of the vortex-solid ensemble takes place can be described smoothly over the entire temperature-field range. It is proposed that the relaxation time of a single vortex flux line, displaced by quantum-thermal fluctuations, is related to the critical behavior associated with the scaling properties of the fluctuation conductivity.