Realization of a LargeJ2Quasi-2D Spin-Half Heisenberg System:Li2VOSiO4

Abstract

Exchange couplings are calculated for $\mathrm{Li}{}_2{\mathrm{VOSiO}}_4$ using the local-density approximation (LDA). While the sum of in-plane couplings $J_1{+J}_2=9.5\mathrm{}\pm 1.5\mathrm{K}$ and the interplane coupling $J_{\perp }\sim 0.2$– $0.3\mathrm{K}$ agree with recent experimental data, the ratio $J_2{/J}_1\sim 12$ exceeds the reported value by an order of magnitude. Using geometrical considerations, high temperature expansions and perturbative mean field theory, we show that the LDA-derived exchange constants lead to a remarkably accurate description of the properties of these materials including specific heat, susceptibility, Néel temperature, and NMR spectra.