Determination of effective microscopic models for the frustrated antiferromagnets Cs₂CuCl₄ and Cs₂CuBr ₄ by density functional methods

We investigate the electronic and magnetic properties of the frustrated triangular-lattice antiferromagnets Cs₂CuCl₄ and Cs ₂CuBr₄ 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 Cs₂CuBr₄ but not for Cs₂CuCl ₄. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs₂CuCl₄ 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.