Berlin 2008 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 13: Correlated Electrons: Low-dimensional Systems - Materials 1
TT 13.12: Vortrag
Dienstag, 26. Februar 2008, 12:30–12:45, H 2053
Charge excitations and local magnetism: Li2CuO2 — •Stefan-Ludwig Drechsler1, Jirři Málek1,2, Martin Knupfer1, Ulrike Nitzsche1, Helge Rosner3, and Helmut Eschrig1 — 1IFW-Dresden, D-01171 Dresden — 2Institute of Physics, ASCR, Prague — 3Max-Planck-Inst. f. Chem. Phys. Fester Stoffe
Electron energy loss spectroscopy and optical conductivity data of the frustrated edge-shared chain cuprate Li2CuO2 are reanalyzed within exact diagonalizations of multiband Hubbard models for CuO2 chains taking into account Cu 3dxy, 4s and/or 4px,y and O 2px,y orbitals. We show that Zhang-Rice (ZR) singlet charge excitations which are generic for most cuprates are strongly suppressed in favor of ZR triplets as a consequence of substantial ferromagnetic correlations at T=0. A significant suppression also at T=300 K is obtained approximately. The frequency region above 6eV is dominated by O 2p to Cu 4py, 4s and 4px transitions. The cluster mapping of these multiband Hubbard models on the Heisenberg J1-J2 model is in accord with a total energy analysis of various magnetic structures within LSDA+U FPLO calculations. According to all these theoretical results the observed ferromagnetic ordering of Li2CuO2 is caused already by a strong nearest-neighbor (CuO4 plaquette) ferromagnetic in-chain interaction J1≈ -200 K compared with the next-nearest neighbor exchange J2≈ 35 K and it is only a bit further stabilized by a specific frustrating weak antiferromagnetic interchain exchange in contrast with previous studies [1].
[1] Y. Mizuno et al., Phys. Rev. B 57, 5326 (1998).