CRITICAL FIELDS IN SUPERCONDUCTING SANDWI CHES SIS

A study has been made of the temperature and angular dependences of th e critical magnetic field in both the V/Si/V sandwiches consisting of two identical metal layers separated by a semiconductor and the asymme tric sandwiches of various types with different parameters of the two superconducting films. A phenomenon of the dimensional crossover has b een observed for all thin-film sandwiches in parallel magnetic field. This phenomenon is supposed to be associated with a transition between two different two-dimensional states. An existence of the crossover i ndicating the change of the localization of the superconducting nucleu s is confirmed by the measurements of angular dependences of the criti cal field. Distinctions between the crossover mechanisms for symmetric and asymmetric sandwiches have been discussed as well as the peculiar ities caused by the formation of one-dimensional mixed state in one of the superconducting layers.