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TT: Tiefe Temperaturen

TT 31: Correlated Electrons: Metal Insulator Transition - Part 2

TT 31.7: Talk

Thursday, March 30, 2006, 15:30–15:45, HSZ 301

Polarization- and temperature-dependent Ru-L_2,3 XAS study of Ca₂RuO₄ — •T. Burnus¹, Z. Hu¹, T. T. Tran², T. Mizokawa², H. H. Hsieh³, L.-Y. Jang⁴, S. Nakatsuji⁵, Y. Maeno^5,6, M. Braden¹, K. S. Liang⁴, and L. H. Tjeng¹ — ¹II. Phys. Inst., Univ. zu Köln, Zülpicher Str. 77, 50937 Köln — ²Dep. of Physics, Univ. of Tokyo, Bunkyo-ku, Tokyo 113, Japan — ³Chung Cheng Inst. of Tech., National Defense Univ., Taoyuan 335, Taiwan — ⁴Nat. Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30077, Taiwan — ⁵Dep. of Physics, Kyoto Univ., Kyoto 606-8501, Japan — ⁶Intern. Innov. Center, Kyoto Univ., Kyoto 606-8501, Japan

In the quasi-two-dimensional Ca_1−xSr_xRuO₄, the strong correlations due to narrow electron bands and the active orbital degree of freedom yield a wide range of interesting properties. Ca₂RuO₄ has a transition at 350 K from a paramagnetic metal to a paramagnetic insulator and, cooling further down, it turns at 110 K into an antiferromagnetic insulator. We carried out polarization- and temperature-dependent X-ray absorption spectroscopy (XAS) at the Ru-L_2,3 edges in order to determine the occupation of the Ru 4d orbitals and their changes crossing the phase transitions; until now the occupation has been only obtained from O-K XAS, which contrary to the Ru-L_2,3 XAS only indirectly contains this information via the hybridization of the oxygen 2p with the ruthenium 4d orbitals. We observed the hysteretic change of the orbital occupation crossing the phase transitions. Furthermore, we confirmed the recently found additional phase transition around 260 K (Zegkkinglou et al. PRL, 2005).