Transport and magnetic properties of YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]]Br[sub [ital y]] single crystals

Deoxygenated nonsuperconducting YBa[sub 2]Cu[sub 3]O[sub 6.2] (YBCO) single crystals have been doped with Br. The resulting crystals become superconducting with [ital T][sub [ital c]][similar to]92 K, [Delta][ital T][sub [ital c]][similar to]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 ([similar to]35:1) suggests that bromination greatly increases the scattering rate. The upper critical field [ital H][sub [ital c]2]([ital T]) parallel and perpendicular to the [ital ab] plane is determined resistively. [ital H][sub [ital c]2](0) and the corresponding coherence lengths [xi][sub [ital a][ital b]](0) and [xi][sub [ital c]](0) are estimated. A comparison with the fully oxygenated YBCO single crystals shows that [xi][sub [ital a][ital b]](0) remains approximately the same, whereas [xi][sub [ital c]](0) decreases by a factor of [similar to]3, suggesting that Br never enters the CuO[sub 2] planes. The pinning energy for vortex motion in the [ital ab] plane decreases after bromination and this decrease can be attributed to the increased anisotropy. For vortex motion along the [ital c] axis, however, the pinning energy decreases or increases depending on temperature and field. Compared withmore » 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 [ital H][sub [ital c]1] 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.« less