Spectral Properties of Quasiparticle Excitations Induced by Magnetic Moments in Superconductors

TL;DR

This paper investigates how localized magnetic moments affect the spectral properties of quasiparticle excitations in superconductors, revealing a quantum transition in s-wave superconductors and stability in d-wave cases.

Contribution

It provides a detailed analysis of impurity-induced quasiparticle states and compares non-self-consistent and self-consistent theoretical approaches in superconductors.

Findings

Localized quasiparticle excitation occurs above a critical magnetic moment in s-wave superconductors.

Quantum transition involves a change in the ground state's spin quantum number from 0 to 1/2.

D-wave superconductors' ground state remains stable regardless of magnetic moment when particle-hole symmetry exists.

Abstract

The consequences of localized, classical magnetic moments in superconductors are explored and their effect on the spectral properties of the intragap bound states is studied. Above a critical moment, a localized quasiparticle excitation in an s-wave superconductor is spontaneously created near a magnetic impurity, inducing a zero-temperature quantum transition. In this transition, the spin quantum number of the ground state changes from zero to 1/2, while the total charge remains the same. In contrast, the spin-unpolarized ground state of a d-wave superconductor is found to be stable for any value of the magnetic moment when the normal-state energy spectrum possesses particle-hole symmetry. The effect of impurity scattering on the quasiparticle states is interpreted in the spirit of relevant symmetries of the clean superconductor. The results obtained by the non-self-consistent (T matrix) and the self-consistent mean-field approximations are compared and qualitative agreement between the two schemes is found in the regime where the coherence length is longer than the Fermi length.