Anisotropic Upper Critical Fields of Disordered Nbo.53Tio.47-Ge Multilayers

Studies are reported of the upper critical fields of Nbo.53Tio.47-Ge multilayers consisting of thick Ge layers and varying-thickness Nbo.s3Tio.47 layers. Both the angular dependence and the temperature dependence of the upper critical fields indicate a dimensional crossover at a Nbo.s3Tio.47 layer thickness near 200 ~ . All the 2D samples display a cusplike upper critical field angular dependence with a sharper cusp for thinner Nbo.s3Tio.47 layers. The parallel upper critical fields are tentatively fitted with an expression combining the 2D field dependence of Rickayzen, the paramagnetic limiting behavior of Maki, and the disorder-related Coulomb interaction effects of Maekawa and Fukuyama. The perpendicular fields are fitted with the Maekawa, Ebisawa, and Fukuyama theory; better agreement is obtained for thinner Nbo.53Tio.47 sublayers when the paramagnetic limiting effect is included.