Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2

Y. Matiks, A. N. Yaresko, K. Myung-Whun, A. Maljuk, P. Horsch, B. Keimer, and A. V. Boris
Phys. Rev. B 84, 245116 – Published 13 December 2011

Abstract

We report the results of a comprehensive spectroscopic ellipsometry study of NaCu2O2, a compound composed of chains of edge-sharing Cu2+O4 plaquettes and planes of Cu1+ ions in a O-Cu1+-O dumbbell configuration, in the spectral range 0.756.5 eV at temperatures 7300 K. The spectra of the dielectric function for light polarized parallel to the Cu1+ planes reveal a strong in-plane anisotropy of the interband excitations. Strong and sharp absorption bands peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along (perpendicular) to the Cu2+O2 chains. They are superimposed on flat and featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on density-functional calculations, the anomalous absorption peaks can be assigned to transitions between bands formed by Cu1+ 3dxz(dyz) and Cu2+ 3dxy orbitals, strongly hybridized with O p states. The major contribution to the background response comes from transitions between Cu1+ 3dz2 and 4px(py) bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the Cu2+O2 chains develops a weak two-peak structure centered at 2.1 and 2.65 eV upon cooling below 100 K, along with the appearance of spin correlations along the Cu2+O2 chains. These features bear a striking resemblance to those observed in the single-valent Cu2+O2 chain compound LiCuVO4, which were identified as an exciton doublet associated with transitions to the upper Hubbard band that emerges as a consequence of the long-range Coulomb interaction between electrons on neighboring Cu2+ sites along the chains. An analysis of the spectral weights of these features yields the parameters characterizing the on-site and long-range Coulomb interactions.

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  • Received 23 August 2011

DOI:https://doi.org/10.1103/PhysRevB.84.245116

©2011 American Physical Society

Authors & Affiliations

Y. Matiks1,*, A. N. Yaresko1, K. Myung-Whun1,2, A. Maljuk1,3, P. Horsch1, B. Keimer1, and A. V. Boris1,†

  • 1Max Planck Institute for Solid State Research, Heisenbergstr. 1, DE-70569 Stuttgart, Germany
  • 2Department of Physics and IPIT, Chonbuk National University, Jeonju 561-756, Korea
  • 3Leibniz Institute for Solid State and Materials Research IFW Dresden, Helmholtzstr. 20, DE-01171 Dresden, Germany

  • *Y.Matiks@fkf.mpg.de
  • A.Boris@fkf.mpg.de

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Vol. 84, Iss. 24 — 15 December 2011

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