Impurity effects at finite temperature in the two-dimensional S=1/2 Heisenberg antiferromagnet

TL;DR

This paper investigates how various impurities affect the magnetic properties of a two-dimensional quantum S=1/2 Heisenberg antiferromagnet, revealing classical-like Curie behavior and divergence due to fluctuations.

Contribution

It provides a detailed quantum Monte Carlo analysis of impurity effects, including a simple effective model to distinguish impurity types and explain observed behaviors.

Findings

Impurities with S_i > 0 show Curie susceptibility contributions.

Logarithmic divergence in susceptibility due to transverse fluctuations.

Effective few-spin model reproduces key simulation results.

Abstract

We discuss effects of various impurities on the magnetic susceptibility and the specific heat of the quantum S=1/2 Heisenberg antiferromagnet on a two-dimensional square lattice. For impurities with spin S_i > 0 (here S_i=1/2 in the case of a vacancy or an added spin, and S_i=1 for a spin coupled ferromagnetically to its neighbors), our quantum Monte Carlo simulations confirm a classical-like Curie susceptibility contribution S_i^2/4T, which originates from an alignment of the impurity spin with the local N\'eel order. In addition, we find a logarithmically divergent contribution, which we attribute to fluctuations transverse to the local N\'eel vector. We also study frustrated and nonfrustrated bond impurities with S_i=0. For a simple intuitive picture of the impurity problem, we discuss an effective few-spin model that can distinguish between the different impurities and reproduces the leading-order simulation data over a wide temperature range.