Impurities in triangular lattice spin 1/2 antiferromagnet

TL;DR

This study investigates how nonmagnetic impurities affect local magnetic properties in a frustrated triangular spin-1/2 antiferromagnet, aiming to explain experimental NMR line broadening in a quantum spin liquid material.

Contribution

It combines high-temperature series expansion and mean field calculations to analyze impurity effects, revealing the need for higher disorder levels to match experimental observations.

Findings

Local susceptibility decays rapidly near impurities at high temperatures.

Low impurity density cannot account for observed line broadening.

High impurity density explains broadening down to 5K, but not below.

Abstract

We study effects of nonmagnetic impurities in a spin-1/2 frustrated triangular antiferromagnet with the aim of understanding the observed broadening of $^{13}$C NMR lines in the organic spin liquid material $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$. For high temperatures down to $J/3$, we calculate local susceptibility near a nonmagnetic impurity and near a grain boundary for the nearest neighbor Heisenberg model in high temperature series expansion. We find that the local susceptibility decays to the uniform one in few lattice spacings, and for a low density of impurities we would not be able to explain the line broadening present in the experiments already at elevated temperatures. At low temperatures, we assume a gapless spin liquid with a Fermi surface of spinons. We calculate the local susceptibility in the mean field and also go beyond the mean field by Gutzwiller projection. The zero temperature local susceptibility decays as a power law and oscillates at $2 k_F$. As in the high temperature analysis we find that a low density of impurities is not able to explain the observed broadening of the lines. We are thus led to conclude that there is more disorder in the system. We find that a large density of point-like disorder gives broadening that is consistent with the experiment down to about 5K, but that below this temperature additional mechanism is likely needed.