Upper critical field in anisotropic superconductors.

Anisotropic superconductivity has been studied in a variety of one-dimensional, ' intercalated, s and artificially prepared" " systems. Much of the experimental work in these systems is related to measurements of the temperature and angular dependence of the upper critical fields H, 2. In many cases, the experiments determine the critical field at which surface superconductivity nucleates, i.e., H, 3. ' The relationship between H, 3 and H, 2 is well established both theoretically and experimentally in isotropic superconductors. " In anisotropic superconductors„ very little work has been done either experimentally or theoretically regarding the angular and temperature dependence of H, 3. ' 's We present here the experimental temperature dependence of H, 3 for an anisotropic superconductor. The results show that the temperature dependence of H, 3 is (T, —T)'i2 unlike in homogeneous superconductors where it is linear. Moreover, at low tempera ture, the ratio H, 3/H, 2-1.7 ~ 0.1, as found and predicted in dirty isotropic superconductors at all temperatures. ' '