TRANSPORT AND MAGNETIC-PROPERTIES OF YBA2CU3O7-DELTA-BRY SINGLE-CRYSTALS

Deoxygenated nonsuperconducting YBa2Cu3O6.2 (YBCO) single crystals doped with Br are studied. The resulting crystals become superconducting with T(c) approximately 92 K, DELTAT(c) approximately 1.0 K as determined by four-point-probe resistivity, measurements. The resistivity in our best sample decreases linearly with temperature. The large ratio in the resistivity at 300 K of the brominated to the pristine YBCO single crystals (approximately 35:1) suggests that bromination greatly increases the scattering rate. The upper critical field H(c2)(T) parallel and perpendicular to the ab plane is determined resistively. H(c2)(0) and the corresponding coherence lengths xi(ab)(0) and xi(c)(0) are estimated. A comparison with the fully oxygenated YBCO single crystals shows that xi(ab)(0) remains approximately the same, whereas xi(c)(0) decreases by a factor of approximately 3, suggesting that Br never enters the CUO2 Planes. The pinning energy for vortex motion in the ab plane decreases after bromination and this decrease can be attributed to the increased anisotropy. For vortex motion along the c axis, however, the pinning energy decreases or increases depending on temperature and field. Compared with the fully oxygenated YBCO single crystals, the critical current density is suppressed by bromination and is strongly dependent on the applied magnetic field. The reduced lower critical field H(c1) in the brominated YBCO single crystals indicates a reduction in the carrier density (by a factor of between 2 and 3), which is consistent with the observed reduction in the plasma frequency.