We have measured the critical magnetic fields $H_{c2\mathrm{}\parallel }$ (parallel to $a-b$ plane) and $H_{c2\mathrm{}\perp }$ (perpendicular to $a-b$ plane) of single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ as a function of temperature in fields up to 19 T. The slope - $\frac{d{H}_{c2\mathrm{}}}{\mathrm{dT}}$ is greater by about a factor of 4 when the field is applied parallel to the $a-b$ plane than when it is applied parallel to the $c$ axis.

• Received 21 September 1987

DOI:https://doi.org/10.1103/PhysRevB.37.619