Regensburg 2007 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 20: Correlated Electrons - Poster Session
TT 20.34: Poster
Mittwoch, 28. März 2007, 14:00–17:45, Poster A
Magnetic exchange interactions in 1D and 2D cuprates — Ulrike Nitzsche1, Stefan-Ludwig Drechsler1, and •Helge Rosner2 — 1IFW Dresden — 2MPI CPfS Dresden
In many copper oxygen networks, especially for Cu-O bond angles close to 90∘, ferromagnetic (FM) interactions play an important role in addition to the well known superexchange J ∼ t2/U. We present a systematic study of the electronic structure and the exchange integrals J for different types of 1D and 2D spin 1/2 cuprates: edge shared (Li2CuO2) and corner shared (A2CuO3 [A=Sr,Ca]) single-chains, double-chains (SrCuO2), and planar (CaCuO2, Sr2CuO2Cl2, R2CuO4 [R=La,Nd,Sm]) arrangements. Based on full potential LDA and LDA+U band structure calculations and subsequent tight-binding (TB) models we estimate sign and magnitude of the most relevant J’s. We compare the results of total energy calculations with those of TB models from one-band and multi-band approaches. The FM contributions can be estimated from the difference between the TB and the total energy results. We investigate the effect of the inter-plaquette geometry (edge shared vs. corner shared CuO2 plaquettes) and the intra-plaquette geometry under pressure on the coupling strength. In excellent agreement with various experiments, our calculations result in: (i) dominant FM nearest neighbor (NN) coupling J1 ∼ -17 meV in Li2CuO2, (ii) strongly enhanced AFM NN J1 ∼ 180 meV for the 1D corner shared chains compared to (iii) J1 ∼ 110...150 meV for the 2D compounds where we observe a sequence of J’s (with respect to the cations) in agreement with the experimental data.