A THz time-domain susceptibility for superconductors including strong-current effects

TL;DR

This paper introduces a simple approximate time-domain susceptibility model for superconductors in the THz range, capturing strong-current effects and enabling better simulation of their electromagnetic response.

Contribution

It provides the first simple, causality-compliant time-domain susceptibility expression for superconductors, including nonlinear current-dependent effects.

Findings

Model accurately predicts superconductor transmission and reflection in THz frequencies.

Incorporates nonlinear response by allowing Delta to vary with current.

Demonstrates applicability in finite-difference time-domain simulations.

Abstract

Finite-difference time-domain methods are increasingly being used to develop, model and analyze the response of materials, including engineered metamaterials that may contain superconductors. Though simple and useful expressions for the time-domain susceptibility exist for basic metals and dielectrics, the time-domain response for a superconductor has not been developed, mainly because the frequency-dependent expressions themselves are rather complex. In this paper we present a simple approximate expression for the time-domain susceptibility of a superconductor for the hbar/2Delta time scale (where Delta is the BCS energy gap) that fulfills causality requirements, and demonstrate its ability to model the transmission and reflection of a fully-gapped superconductor in the THz region. By allowing Delta to be a function of current, we also show how this model function can be used to describe nonlinear microwave response in superconductors.