Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 26: Metal-Insulator Transition

TT 26.4: Talk

Thursday, March 29, 2007, 14:45–15:00, H19

Valence, spin and orbital states of the quasi-one-dimensional Ca₃CoRhO₆: band structure calculations and x-ray absorption spectroscopy study — •Hua Wu, Zhiwei Hu, Tobias Burnus, Daniel Khomskii, and Liu Hao Tjeng — II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln

Quasi-one-dimensional cobaltate Ca₃Co₂O₆ [1,2] and its isostructural Ca₃CoRhO₆ have drawn attention recently due to the peculiar crystal structure and the fascinating magnetic properties. The valence (and orbital) state issue and the origin of the intra-chain Ising ferromagnetism remain unresolved for Ca₃CoRhO₆. Taking the insulating state of this material as a key finding, we applied the density-functional theory plus Hubbard U (DFT+U) method to study its electronic structure. We find that Ca₃CoRhO₆ has the high-spin Co²⁺ and low-spin Rh⁴⁺, and that it is the spin-orbit coupling which lifts the Co²⁺ orbital degeneracy, thereby enabling the DFT+U to generate the band gap. We predict that the Co²⁺ ion has a huge orbital magnetic moment of 1.7µ_B, which accounts for the Ising magnetism. We also propose a model to explain the intra-chain ferromagnetism. Those results are confirmed by our x-ray absorption spectroscopy and x-ray magnetic circular dichroism study.

[1] Hua Wu, M. W. Haverkort, Z. Hu, D. I. Khomskii, and L. H. Tjeng, Phys. Rev. Lett. 95, 186401 (2005).

[2] T. Burnus, Z. Hu, M. W. Haverkort, J. C. Cezar, D. Flahaut, V. Hardy, A. Maignan, N. B. Brookes, A. Tanaka, H. H. Hsieh, H.-J. Lin, C. T. Chen, and L. H. Tjeng, Phys. Rev. B 74, 245111 (2006).