Upper critical field of V-Ag multilayered superconductors

We report investigations of the upper critical field (H_c2⊥ and Hc2?) of V-Ag proximity-coupled superconducting multilayer structures, prepared by ultrahigh-vacuum electron-beam evaporation. By means of x-ray diffraction analysis, successful formation of an artificial superstructure is confirmed. From resistivity measurement parallel to the layers, the electron mean free paths in the V and Ag layers are determined. With reference to these transport properties, characterization of Hc2 has been made. For three-dimensional multilayer structures, Hc2 is analyzed in terms of the temperature dependence of H_c2⊥, the zero-temperature coherence length, and the anisotropy Hc2?/H_c2⊥. Our results are in quantitative agreement with the relevant theories. For quasi-two-dimensional multilayer structures, a peculiar temperature dependence has been revealed in both H_c2⊥ and Hc2?. H_c2⊥ exhibits positive curvature near Tc, and it is continuously enhanced with the increase of multilayer period, as predicted theoretically. In contrast with the monotonic variation in H_c2⊥, a drastic upturn at a certain temperature (dimensional crossover) and an upward inflection at a higher temperature have been observed in Hc2?. These two types of anomalies in Hc2? are discussed in the context of the commensurability between the multilayer structure and the vortex lattice.