Transport properties in weak and strong localization regime of Nb_0.53Ti_0.47/Ge superlattices

Multilayer Nb_0.53Ti_0.47/Ge superlattices with varying Nb_0.53Ti_0.47 and Ge layer thickness were sputter deposited on sapphire substrates at 200°C. The physical properties of these superlattices depend on the Nb_0.53Ti_0.47 layer thickness which is in turn related to the degree of disorder of the metal layer. The spectrum of samples can be subdivided roughly into three regions according to their different transport properties—a superconducting region, a weak localization region and a strong localization region. The sheet resistance, R_□ of films in the weak localization region is in the range of 300ω<R_□<5kω and they exhibit a logarithmic temperature dependence, R_□(T)lnT at low temperatures. One film in the strong localization region, with R_□= 220kω, showed an exponential temperature dependence of the resistance, R_□ exp 1/√T. Negative (positive) magnetoresistance is observed for strongly (weakly) localized samples at low field. The high field magnetoresistance showed a linear dependence in lnH, except for the parallel magnetoresistance of the strongly localized film; the latter had approximately an H² dependence. The magnetoresistance is anisotropic for both the weakly and strongly localized films with the perpendicular magnetoresistance always larger than the parallel values. The weak localization to strong localization transition is consistent with the theoretically predicted mobility edge near R_□≂ħ/0.12e²≅30kω.