Resistive-state anomaly in superconducting nanostructures

Abstract

The resistive anomaly, an increase of sample resistance above the normal state value at the top of the superconducting transition, has been studied experimentally in various aluminum nanostructures. The effect is not absolutely reproducible being dependent on the cooling history and on the particular arrangement of voltage and current probes for multiterminal samples. If the anomaly is clearly observed, its magnitude can be suppressed by a strong bias current and/or magnetic field. It is shown that the main factor determining the form of the $R(T)$ transition in lift-off fabricated nanostructures is the inevitable film inhomogeneity, being a ``fingerprint'' of each particular sample. The origin of the anomaly is attributed to a superposition of two processes. First, the formation of deformed (not perpendicular to the wire axis) $N/S$ boundaries, and, second, the existence of a nonequilibrium region inside the superconducting domain close to this $N/S$ interface, characterized by a finite value of the electric field and the corresponding effective resistance. Only a combination of the above effects can give a reasonable quantitative agreement with experimental data.