Universal point contact resistance between thin-film superconductors

TL;DR

This paper investigates the universal behavior of point contact resistance between thin-film superconductors, revealing a nearly-activated temperature dependence that vanishes at zero temperature due to superconducting fluctuations.

Contribution

It introduces a minimal model and a quantum phase-slip formulation to describe the contact resistance, highlighting its universal temperature dependence and weak destruction of the Josephson effect.

Findings

Resistance vanishes rapidly with temperature, following a universal nearly-activated form.

At low temperatures, the activation barrier is mainly determined by superfluid stiffness.

The resistance behavior is consistent across different regimes and contact qualities.

Abstract

A system comprising two superconducting thin films connected by a point contact is considered. The contact resistance is calculated as a function of temperature and film geometry, and is found to vanish rapidly with temperature, according to a universal, nearly activated form, becoming strictly zero only at zero temperature. At the lowest temperatures, the activation barrier is set primarily by the superfluid stiffness in the films, and displays only a weak (i.e., logarithmic) temperature dependence. The Josephson effect is thus destroyed, albeit only weakly, as a consequence of the power-law-correlated superconducting fluctuations present in the films below the Berezinskii-Kosterlitz-Thouless transition temperature. The behavior of the resistance is discussed, both in various limiting regimes and as it crosses over between these regimes. Details are presented of a minimal model of the films and the contact, and of the calculation of the resistance. A formulation in terms of quantum phase-slip events is employed, which is natural and effective in the limit of a good contact. However, it is also shown to be effective even when the contact is poor and is, indeed, indispensable, as the system always behaves as if it were in the good-contact limit at low enough temperature. A simple mechanical analogy is introduced to provide some heuristic understanding of the nearly-activated temperature dependence of the resistance. Prospects for experimental tests of the predicted behavior are discussed, and numerical estimates relevant to anticipated experimental settings are provided.