Impurity-induced spin polarization and NMR line broadening in underdoped cuprates

TL;DR

This paper develops a theory explaining how impurities like Ni and Zn induce local magnetic moments in underdoped cuprates, affecting NMR line broadening and revealing impurity-specific magnetic behaviors related to the spin gap.

Contribution

It introduces a model describing impurity-induced magnetic moments and their impact on NMR signals, linking impurity effects to the spin gap and local antiferromagnetic correlations in cuprates.

Findings

Ni impurities are partially screened, forming effective S=1/2 moments.

Zn impurities induce local S=1/2 moments on Cu sites.

The model explains temperature-dependent NMR line broadening phenomena.

Abstract

We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments on Cu sites in the proximity of the impurity, a process which is intimately related to the spin gap phenomenon in cuprates. Below a characteristic Kondo temperature, the Ni spin is partially screened by the Cu moments, resulting in an effective impurity spin S=1/2. We further analyze the Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a polarization of the effective impurity moments and derive expressions for the corresponding ^{17}O NMR line broadening. The peculiar aspects of recent experimental NMR data can be traced back to different spatial characteristics of Ni and Zn moments as well as to an inherent temperature dependence of local antiferromagnetic correlations.