Anisotropy of the Upper Critical Magnetic Field in Single Crystal YBa₂Cu₃O7+y

The upper critical magnetic field H c2 was measured on single crystals of the recently-discovered superconducting YBa 2 Cu 3 O 7+ y oxide. The H c2 along the basal plane is estimated to be larger than that along the c -axis by a factor of 5. A small anisotropy of H c2 in the basal plane is also found. It is not clear at present whether this small anisotropy is intrinsic or due to crystal imperfectness such as micro-twinnings.