Dresden 2006 – scientific programme

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TT: Tiefe Temperaturen

TT 19: Correlated Electrons: Low-dimensional Materials

TT 19.3: Talk

Tuesday, March 28, 2006, 14:30–14:45, HSZ 301

Electronic structure of the incommensurate composite crystal Sr₁₄Cu₂₄O₄₁ — •Cosima Schuster and Udo Schwingenschlögl — Institut für Physik, Universität Augsburg, 86135 Augsburg

The incommensurate composite systems (Ca,Sr)₁₄Cu₂₄O₄₁ are based on two different structural units, CuO₂ ladders and CuO chains, where the Cu ions have the average valence 2.25. In the Sr compound nearly all Cu²⁺ ions (with S=1/2) are found on the ladders, and nearly all Cu³⁺ ions are located on the chains. The latter form Zhang-Rice singlets, socalled holes. Substitution of Sr by Ca leads to a transfer of holes from the chains to the ladders. The spin order on the chains likewise depends on the doping and ranges from dimers to antiferromagnetic order.

Our calculations are based on density functional theory and take into account the details of the crystal structure [1] by means of an unit cell including 10 chain and 7 ladder units. The LDA results show that the chains and ladders can be treated independently. We present systematic investigations of the local density of states at the chain sites by studying a reduced unit cell without CuO₂ ladders. The crystal structures of the Ca and Sr-rich compound differ in a symmetric or asymmetric alignment of the CuO chains. We find that two bands contribute to the d_xz-states near E_F in the asymmetric case, whereas they are degenerate in the symmetric case. A tight binding fit shows that the orbital overlap between nearest and next-nearest neighbours is of the same order of magnitude. Details of the magnetic ordering and the role of electronic correlations result from spin-polarized and LDA+U calculations.

[1] Y. Gotoh et al., Phys. Rev. B 68, 224108 (2003)