Transport and microwave properties of yttrium barium(2) copper(3) oxygen(7-x) superconducting films.

摘要

High quality YBCO c-axis oriented films were made by a laser ablation technique. Josephson weak links were fabricated on a modified microbridge by using bicrystal YBCO films. Shapiro steps were observed on modified microbridge weak links.; Vortex ring nucleation by thermal activation and quantum tunneling processes in high {dollar}Tsb{lcub}c{rcub}{dollar} superconductors is discussed. The nonlinear resistance scales as {dollar}esp{lcub}-(Jsbnu/J)spnu{rcub}{dollar}. Thermal activation yields a critical index of {dollar}nu{dollar} = 1, while quantum tunneling (without normal current dissipation) yields {dollar}nu{dollar} = 2. I-V characteristics were measured on both disordered (low {dollar}Jsb{lcub}c{rcub}{dollar}), ordered (high {dollar}Jsb{lcub}c{rcub}{dollar}) YBCO films and microbridges below {dollar}Tsb{lcub}c{rcub}{dollar}. The data were examined in terms of both thermal activation and quantum tunneling models. For a disordered film, the {dollar}nu{dollar} = 1 scaling law is in reasonable agreement with the data, while {dollar}nu{dollar} = 2 scaling is required to explain the results of an ordered film at low temperatures. For the microbridge, the {dollar}nu{dollar} = 2 scaling extended even to high temperatures.; Intrinsic critical current densities in high {dollar}Tsb{lcub}c{rcub}{dollar} superconducting films are discussed. The enhancement of the quantum electrodynamic tunneling process due to normal current dissipation is proposed to explain the measured {dollar}Jsb{lcub}c{rcub}{dollar} results for YBCO films. The depairing current density calculated from vortex ring model is in the order of 10{dollar}sp9{dollar} {dollar}A/cmsp2{dollar} and an upper limit of {dollar}Jsb{lcub}c{rcub}{dollar} is in the order of 10{dollar}sp7{dollar} {dollar}A/cmsp2{dollar}. Supercurrent was observed to flow across a bridge constriction with a {dollar}Jsb{lcub}c{rcub}{dollar} up to 1.3 {dollar}times{dollar} 10{dollar}sp9{dollar} {dollar}A/cmsp2{dollar}, the highest critical current density yet reported. The limitation of {dollar}Jsb{lcub}c{rcub}{dollar} appears to be due to vortex ring creation. We found that studying {dollar}Jsb{lcub}c{rcub}{dollar} of bridge constrictions was a way to approach the depairing current limit.; A novel microwave self-resonant (MSR) technique was developed to measure surface resistance, {dollar}Rsb{lcub}s{rcub}{dollar}, of YBCO superconducting films directly and surface reactance, {dollar}Xsb{lcub}s{rcub}{dollar}, indirectly. {dollar}Rsb{lcub}s{rcub}{dollar} at 21 GHz decreased by about three orders of magnitude as the temperature decreased from 90K to 80K. The smallest surface resistance was 2.7 {dollar}times{dollar} 10{dollar}sp{lcub}-4{rcub}Omega{dollar} at 21 GHz and at 15K, and the surface reactance was 0.031{dollar}Omega{dollar}. Using the MSR technique, we measured the London penetration depth and coherence length, and found that both {dollar}lambda{dollar} and {dollar}xi{dollar} were anisotropic, their values depended on the direction of the microwave electric field relative to the c-axis. We found that {dollar}lambdasbVert{dollar}(0) (penetration depth in a-b plane at zero temperature) was about 1840A and {dollar}lambdasbperp{dollar}(0) about 3640A, while {dollar}xisbVert{dollar}(0) {dollar}sim{dollar} 25A and {dollar}xisbperp{dollar}(0) {dollar}sim{dollar} 11A. The Ginzburg-Landau parameters were {dollar}kappasbVert{dollar} in the range of 110-165 and {dollar}kappasbperp{dollar} in the range of 62-74, yield an anisotropic factor {dollar}gamma{dollar} about 2.