Anisotropic superconducting properties ofBa2YCu3O7−y

Abstract

The upper critical magnetic field $H_{c2\mathrm{}}$ of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_{7-y}$ single crystals is estimated from the resistive transition curves as a function of temperature and field direction. $H_{c2\mathrm{}}$ exhibits a large anisotropy between the $c$ axis and the $a-b$ plane. The $H_{c2\mathrm{}}$ data have linear temperature dependence, giving well-defined $\frac{d{H}_{c2\mathrm{}}}{\mathrm{dT}}$ values: -0.56 T/K for the $c$ axis and -3.3 T/K for the $a-b$ plane. The angular dependence of $H_{c2\mathrm{}}$ is in good agreement with the effective-mass model when the ratio $\frac{M_{\perp }}{M_{\parallel }}=60\mathrm{}$, which is comparable to resistivity anisotropy in the normal state. On the other hand, no signs of anisotropy in the $a-b$ plane can be recognized. This indicates that supercurrent flows dominantly in the Cu-O sheets, and that the Cu-O chains with oxygen deficiency are less conductive.