Imaging the Current Density in Superconducting Thin Films Using Nonequilibrium Phonons

Abstract

There is considerable interest in the superconducting properties of thin films because certain granular films exhibit a 2D phase transition at the KosterlitzThouless transition temperature T _KT [1]. The conventional method of studying such systems is to make detailed measurements of the current-voltage characteristics of the films close to T _KT and compare the results with theory [2]. It is usually assumed that the film is narrower than the in plane magnetic penetration depth,₁, which is expected to be of the order of millimetres at T _KT. However, there are few if any measurements of λ_⊥ which enable the validity of this assumption to be checked. One way to find λ_⊥ is to probe the variation in current density, J, across the sample. It is expected to be largest within a region about λ_⊥ wide at the film edges. In this paper we describe a technique in which a movable beam of nonequilibrium phonons is used to probe the current distribution in thin tin films.