Transport and magnetic properties of YBa 2 Cu 3 O 7 − δ Br y single crystals

Deoxygenated nonsuperconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.2}$ (YBCO) single crystals have been doped with Br. The resulting crystals become superconducting with ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}92 K, \ensuremath{\Delta}${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}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 (\ensuremath{\sim}35:1) suggests that bromination greatly increases the scattering rate. The upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) parallel and perpendicular to the ab plane is determined resistively. ${\mathit{H}}_{\mathit{c}2}$(0) and the corresponding coherence lengths ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0) and ${\ensuremath{\xi}}_{\mathit{c}}$(0) are estimated. A comparison with the fully oxygenated YBCO single crystals shows that ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0) remains approximately the same, whereas ${\ensuremath{\xi}}_{\mathit{c}}$(0) decreases by a factor of \ensuremath{\sim}3, suggesting that Br never enters the ${\mathrm{CuO}}_{2}$ 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 ${\mathit{H}}_{\mathit{c}1}$ 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.