Enhancement of Antiferromagnetic Correlations Induced by Nonmagnetic Impurities: Origin and Predictions for NMR Experiments

TL;DR

This paper demonstrates that nonmagnetic impurities in various antiferromagnetic spin models enhance local antiferromagnetic correlations, leading to observable broadening in NMR signals across multiple compounds and geometries.

Contribution

It provides a unified explanation for impurity-induced antiferromagnetic enhancement based on short-range resonating-valence-bond correlations, supported by computational studies across different models.

Findings

Rapid destruction of spin gap by vacancies

Enhanced local antiferromagnetic correlations near vacancies

Predicted broadening of low-temperature NMR signals

Abstract

Spin models that have been proposed to describe dimerized chains, ladders, two dimensional antiferromagnets, and other compounds are here studied when some spins are replaced by spinless vacancies, such as it occurs by $Zn$ doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence induces enhanced antiferromagnetic correlations near those vacancies. The study is performed with computational techniques which includes Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group methods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models and cluster geometries, a common simple explanation for its presence may exist. It is argued that the resonating-valence-bond character of the spin correlations at short distances of a large variety of models is responsible for the presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takes place regardless of the long distance properties of the ground state, and it is caused by a ``pruning'' of the available spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low temperature NMR signal for the compounds analyzed here. This broadening should be experimentally observable in the structurally dimerized chain systems $Cu(NO_3)_2\cdot2.5H_2O$, $CuWO_4$, $(VO)_2P_2O_7$, and $Sr_{14}Cu_{24}O_{41}$, in ladder materials such as $Sr Cu_2 O_3$, in the spin-Peierls systems $CuGeO_3$ and $NaV_2 O_5$, and in several others since it is a universal effect common to a wide variety of models and compounds.