Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 13: Correlated Electrons: Low-dimensional Systems - Materials 1

TT 13.12: Talk

Tuesday, February 26, 2008, 12:30–12:45, H 2053

Charge excitations and local magnetism: Li₂CuO₂ — •Stefan-Ludwig Drechsler¹, Jirři Málek^1,2, 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 3d_xy, 4s and/or 4p_x,y and O 2p_x,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 4p_y, 4s and 4p_x 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).