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TT: Tiefe Temperaturen

TT 7: Niedrigdimensionale Systeme

TT 7.8: Vortrag

Dienstag, 25. März 2003, 11:30–11:45, HSZ03

Large J₂ Heisenberg Systems: Li₂VOSiO₄ and Li₂VOGeO₄ — •H. Rosner¹, R.R.P. Singh², W.H. Zheng³, J. Oitmaa³, S.L. Drechsler⁴, and W.E. Pickett² — ¹MPI CPFS Dresden — ²UC Davis, USA — ³UNSW, Sydney, Australia — ⁴IFW Dresden

Frustrated square-lattice spin-half Heisenberg antiferromagnets with nearest neighbor exchange J₁ and second neighbor (diagonal) exchange J₂ have received considerable attention recently. Exchange couplings are calculated for the model systems Li₂VOSiO₄ (Li₂VOGeO₄) using a two band tight-binding model fitted to the LDA band structure and mapped to a Heisenberg model. The LDA results with J₁=0.75K (1.7K), J₂=8.8K (8.1K) and J_⊥=0.25K (0.19K), respectively, place both systems deeply in the large J₂ regime of the model. While the sum of in-plane couplings J₁+J₂ and the inter-plane coupling J_⊥ agree with recent experimental data, the ratio J₂/J₁ ∼ 12 (5) exceeds the reported value by an order of magnitude. We develop high temperature expansions (HTE) for the susceptibility and specific heat of the J₁−J₂ Heisenberg model. Combined with a perturbative mean-field theory, we obtain accurate results for the susceptibility of the large-J₂/J₁ Heisenberg model at all temperatures. For the specific heat, the HTE show good convergence down to the peak temperature. Using the HTE results we show that these materials are well described by a large-J₂/J₁ Heisenberg model in agreement with the LDA predictions. Furthermore, the measured Néel temperature is consistent with our LDA derived J_⊥ values. Further experiments which would be particularly suited to an accurate determination of the J₂/J₁ ratio for these systems are discussed.