Dresden 2006 – scientific programme

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MA: Magnetismus

MA 5: Spin-Structures and Magnetic Phase Transitions I

MA 5.8: Talk

Monday, March 27, 2006, 12:00–12:15, HSZ 403

Orbital ordering and spin gap in ruthenate La₄Ru₂O₁₀ — •Hua Wu¹, T. Burnus¹, Z. Hu¹, J.D. Denlinger², L.-Y. Jang³, H.H. Hsieh⁴, P.G. Khalifah⁵, F. Wang⁶, J.W. Allen⁶, K.S. Liang³, D.I. Khomskii¹, and L.H. Tjeng¹ — ¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany — ²Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA — ³National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30076, Taiwan — ⁴Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan — ⁵Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA — ⁶Randall Laboratory of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA

It was discovered [P. Khalifah et al., Science 297, 2237 (2002)] that La₄Ru₂O₁₀ undergoes a rare 4d-orbital ordering transition below 160 K and acquires a spin gap. We study this interesting orbital-ordered spin-gap state both by x-ray absorption spectroscopy measurements and by LDA+U band calculations. Our results show that the Ru⁴⁺ ions remain in the normal spin=1 state. A distinct orbital ordering is identified, which leads to a significant anisotropy of antiferromagnetic exchange couplings. As a result, the spin gap is opened due to formation of the Ru⁴⁺-Ru⁴⁺ spin-singlet dimers but not to the originally assumed spin-state transition. Thus, La₄Ru₂O₁₀ appears to be a novel orbital-ordering-assisted spin-ladder material.