Surface impedance of superconductors with weak magnetic impurities

TL;DR

This paper studies how weak magnetic impurities alter the surface impedance of superconductors, focusing on dissipation, excitation spectrum modifications, and potential quasiparticle trapping effects.

Contribution

It provides a detailed analysis of the impact of weak magnetic impurities on superconductor impedance, highlighting changes near the BCS gap and low-frequency absorption behavior.

Findings

Weak magnetic impurities significantly modify the excitation spectrum.

Impurities introduce discrete energy states that can trap quasiparticles.

Impurities can reduce low-frequency absorption by quasiparticles.

Abstract

Magnetic impurities affect the spectrum of excitations of a superconductor and thus influence its impedance. We concentrate on the dissipative part of the surface impedance. We investigate its dependence on frequency, the density and strength of magnetic impurities, and the density and temperature of quasiparticles. Even a small concentration of weak magnetic impurities significantly modifies the excitation spectrum in the vicinity of the BCS gap. Therefore, we give special attention to the absorption threshold behavior at zero temperature and to the low-frequency absorption by quasiparticles. The discrete energy states introduced at low density of magnetic impurities may serve as traps for nonequilibrium quasiparticles, reducing the absorption in some range of low radiation frequencies.