Antiferromagnetic correlations and impurity broadening of NMR linewidths in cuprate superconductors

TL;DR

This paper models how antiferromagnetic correlations and impurity effects influence NMR linewidths in cuprate superconductors, explaining experimental observations through impurity-induced magnetism and quasiparticle interference.

Contribution

It introduces a combined model accounting for impurity-induced paramagnetism and quasiparticle interference to explain NMR linewidth broadening in cuprates.

Findings

Local magnetism around impurities broadens NMR lines

Quasiparticle interference smears impurity satellite peaks

The model semi-quantitatively matches experimental NMR line shapes

Abstract

We study a model of a d-wave superconductor with strong potential scatterers in the presence of antiferromagnetic correlations and apply it to experimental nuclear magnetic resonance (NMR) results on Zn impurities in the superconducting state of YBCO. We then focus on the contribution of impurity-induced paramagnetic moments, with Hubbard correlations in the host system accounted for in Hartree approximation. We show that local magnetism around individual impurities broadens the line, but quasiparticle interference between impurity states plays an important role in smearing out impurity satellite peaks. The model, together with estimates of vortex lattice effects, provides a semi-quantitative description of the impurity concentration dependence of the NMR line shape in the superconducting state, and gives a qualitative description of the temperature dependence of the line asymmetry. We argue that impurity-induced paramagnetism and resonant local density of states effects are both necessary to explain existing experiments.