Analysis of impurity-induced circular currents for the chiral superconductor Sr_{2}RuO_{4}

TL;DR

This paper investigates how non-magnetic impurities induce circular currents in the chiral superconductor Sr₂RuO₄, revealing bound and continuum quasiparticle states that generate observable magnetic fields.

Contribution

It provides an analytic, non-self-consistent analysis of impurity-induced currents in a chiral superconductor, linking microscopic quasiparticle states to macroscopic magnetic phenomena.

Findings

Impurity scattering transfers angular momentum to quasiparticles.

Continuum states dominate the current at low temperatures.

A counterflow in the condensate compensates the impurity current.

Abstract

The microscopic mechanism of circular currents induced in the vicinity of a non-magnetic impurity is analyzed for the (time-reversal symmetry breaking) chiral superconducting state ${\bf d}({\bf k}) = \hat{{\bf z}} (k_x \pm i k_y)$ which is very likely realized in Sr_2RuO_4. From the analytic, not self-consistent solution of the Bogolyubov-de Gennes equations we find two types of quasiparticle states, a bound state and the continuum state. Through impurity scattering the condensate transfers angular momentum to the quasiparticle states which generate a circular current. At low temperature the continuum part of the quasiparticle spectrum gives the main contribution. The non-selfconsistent solution yields a state of finite angular momentum. The comparison with the corresponding Ginzburg-Landau theory reveals the existence of a compensating counterflow created by the superconducting condensate. The currents and magnetic fields appearing around the impurity have possibly been observed by muon spin relaxation measurements.