Coherent Josephson nanostructures and the dissipation of the persistent current in the a−b planes of YBa2Cu3O7−δ thin films

Abstract

We investigated the dissipation of the persistent current and associated vortex dynamics in YBa2Cu3O7−δ (YBCO) thin films of high Jc, in the remanent critical magnetization regime. Ring-shaped samples were used to measure both the relaxation of the persistent current from the critical level, and the magnitude of the critical current as a function of temperature over a range of 10–90 K. The energy barrier Ueff(J) was calculated using the analysis of the relaxation data proposed by Maley et al. [Phys. Rev. B 42, 2639 (1990)]. We found the correlation between the temperature dependence of the critical current Jc(T) and the dependence of Ueff(J) on the current density. When Jc(T) is dominated by the Ginzburg-Landau (GL)-like temperature dependence [Jc(T)∝(Tc−T)3/2], the empirical formula for the energy barrier is Ueff(J)=a⋅Jc(T)⋅exp(−(3π/2)J/Jc0). In the Ambegaokar-Baratoff (AB) regime of Jc(T), Ueff(J)=Jc(T)[a1exp(−(3π/2)J/Jc0)+a2exp(−(9π/2)J/Jc0)]. These relationships were found to describe an intrinsic property of all thin films studied, independent of the growth conditions, substrates, film thickness, Tc, and the magnitude of the critical current density Jc. The factors −(3π/2)J/Jc0 and −(9π/2)J/Jc0 in the exponents of Ueff(J) represent tilted washboard potential Uj(φ)/Ej=−cosφ−(J/Jc0)φ for an overdamped (resistively shunted) Josephson tunnel junction, locked at phases of 3π/2 and 9π/2. These phases correspond to the maximum resistive dissipation in a junction. The experimental data imply that the superconductor behaves like a single overdamped Josephson junction (or an extremely coherent array of resisitively-shunted Josephson junctions). The dissipation of the persistent current occurs due to the collective motion of vortices through the Josephson nanostructures in YBCO. The nanostructures have been revealed in the high-resolution electron microscope studies of Etheridge [Philos. Mag. A 73, 643 (1996)] and in the studies of the AB to GL crossover effects in the temperature dependence of the critical current [Darhmaoui and Jung, Phys. Rev. B 53, 14 621 (1996)]. The results favor discrete models of the physical properties of high-temperature superconductors, developed by Stroud, Emery, and Kivelson. We postulate that the vortex pinning in YBCO thin films originate from the variation of the Josephson coupling energy within the Josephson nanostructures.

Source-reported materials — not catalogue approval

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