On the peculiarities of critical fields for inhomogeneous superconducting films
Abstract
Experimental studies of thin vanadium films (d = 250−400 Å) have revealed anomalies in the temperature dependence of the upper critical field Hc∥(T) when H is parallel to the sample plane. At a certain temperature T0, a sharp kink is observed between two linear segments in the Hc‖2(T) dependence. The anomaly in the Hc∥(T) dependence of vanadium thin films is explained by assuming that the vanadium film is split into two parallel layers with different parameters (critical temperature Tc, coherence length ξ, thickness d), At temperatures above and below T0, the dependence Hc∥(T) is mainly determined by the characteristics of just one layer. The kink in the dependence Hc‖2(T) is due to a jump-like displacement of the superconducting nucleus from one layer to the other at temperature T0. Anomalies in the dependence Hc∥(T) are also observed in sandwiches consisting of two identical films separated by a thin (∼30 Å) insulating layer. In this case, however, the transition from one segment to the other is smooth. In the case of identical layers, a peculiar crossover is observed: if for T→Tc the critical field Hc∥(T) is equal to its value for a layer of double thickness, then Hc∥(T) asymptotically approaches the critical field for one layer upon a decrease in temperature. The calculations made with the help of this model provide a satisfactory explanation for the experimental results.