Regensburg 2002 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 5: Niedrigdimensionale Systeme, Magnetotransport I

TT 5.5: Vortrag

Montag, 11. März 2002, 15:45–16:00, H20

Realization of a large J₂ quasi-2D spin-half Heisenberg system: Li₂VOSiO₄ — •H. Rosner¹, R.R.P. Singh¹, W.H. Zheng², J. Oitmaa², S.-L. Drechsler³, and W.E. Pickett¹ — ¹University of California, Davis, CA, 95616, USA — ²University of New South Wales, Sydney NSW 2052, Australia — ³IFW Dresden e.V.,Postfach 270116, D-01171 Dresden

Frustrated square-lattice spin-half Heisenberg antiferromagnets with nearest neighbour exchange J₁ and second neighbour (diagonal) exchange J₂ have received considerable attention recently. The large J₂ limit of the model is a classic example of quantum order by disorder, leading to collinear magnetic order in a columnar pattern. Apparently, an experimental realisation for this model was reported recently by Melzi et al. (PRL 85, 1318 (2000)) in the compound Li₂VOSiO₄. We present here a caclculation of the exchange couplings for this frustrated quantum Heisenberg antiferromagnet compound. From a two band tight-binding model fitted to the LDA band structure and mapped onto a Heisenberg model, the in-plane and inter-plane exchange integrals were obtained. The sum of the in-plane exchange couplings J₁ + J₂ = 9.5 ± 1.5 K and the inter-plane coupling J_⊥ ∼ 0.2–0.3 K agree well with recent experimental data. Our ratio J2/J1 ∼ 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, Neél temperature and NMR spectra.