Determination of effective microscopic models for the frustrated antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ by density functional methods

TL;DR

This study uses density functional theory to analyze the electronic and magnetic properties of frustrated triangular-lattice antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$, focusing on exchange couplings and structural effects.

Contribution

It provides a detailed analysis of how structural optimization influences exchange couplings in these materials, highlighting the importance of spin polarization in accurate modeling.

Findings

Exchange couplings depend on subtle structural details.

Good agreement with experiments requires spin-polarized structural optimization.

Interlayer and longer-range couplings significantly affect magnetic properties.

Abstract

We investigate the electronic and magnetic properties of the frustrated triangular-lattice antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ in the framework of density functional theory. Analysis of the exchange couplings J and J' using the available X-ray structural data corroborates the values obtained from experimental results for Cs$_2$CuBr$_4$ but not for Cs$_2$CuCl$_4$. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs$_2$CuCl$_4$ employing different exchange-correlation functionals. We find that the exchange couplings depend on rather subtle details of the structural optimization and that only when the insulating state (mediated through spin polarization) is present in the structural optimization, we do have good agreement between the calculated and the experimentally determined exchange couplings. Finally, we discuss the effect of interlayer couplings as well as longer-ranged couplings in both systems.