Models for Enhanced Absorption in Inhomogeneous Superconductors

TL;DR

This paper investigates how inhomogeneities in superfluid density in superconductors lead to additional low-frequency absorption, providing a theoretical framework that explains experimental observations related to quenched disorder and charge density waves.

Contribution

It introduces a classical model to quantify absorption due to superfluid density inhomogeneities, linking fluctuations to measurable optical properties in superconductors.

Findings

Inhomogeneities cause Lorentzian-shaped absorption at low frequencies.

Absorption strength is proportional to the variance of superfluid density fluctuations.

Model calculations match experimental observations of enhanced absorption.

Abstract

We discuss the low-frequency absorption arising from quenched inhomogeneity in the superfluid density rho_s of a model superconductor. Such inhomogeneities may arise in a high-T_c superconductor from a wide variety of sources, including quenched random disorder and static charge density waves such as stripes. Using standard classical methods for treating randomly inhomogeneous media, we show that both mechanisms produce additional absorption at finite frequencies. For a two-fluid model with weak mean-square fluctuations <(d rho_s)^2 > in rho_s and a frequency-independent quasiparticle conductivity, the extra absorption has oscillator strength proportional to the quantity <(d rho_s)^2>/rho_s, as observed in some experiments. Similar behavior is found in a two-fluid model with anticorrelated fluctuations in the superfluid and normal fluid densities. The extra absorption typically occurs as a Lorentzian centered at zero frequency. We present simple model calculations for this extra absorption under conditions of both weak and strong fluctuations. The relation between our results and other model calculations is briefly discussed.