•L.H. Tjeng^1,2, T. Mizokawa^2,3, G.A. Sawatzky², G. Ghiringhelli⁴, O. Tjernberg⁴, N. B. Brookes⁴, H.-J. Lin⁵, C.T. Chen⁵, S. Schuppler⁶, H. Fukazawa⁷, S. Nakatsuji⁷ und Y. Maeno^7,8
2Solid State Physics Laboratory, University of Groningen, The Netherlands
³Department of Complexity Science, University of Tokyo, Japan
⁴ESRF, Grenoble, France
⁵SRRC, Hsinchu, Taiwan
⁶FZK, Karlsruhe
⁷University of Kyoto, Japan
⁸CREST-Saitama, Japan
¹II. Physikalisches Institut, Universität zu Köln

The nature of the phase transitions in the Ca_2-xSr_xRuO₄ system has been studied using O 1s x-ray absorption spectroscopy. For the Mott insulator CaRuO₄, we have observed that the orbital population dramatically change with temperature. Together with our finding that a substantial orbital angular momentum is present in the Ru 4d band, we argue that the cooperation between the strong spin-orbit coupling and the small distortion of the RuO₆ octahedra causes the interesting changeover of the spin and orbital anisotropy as a function of temperature.

For the x = 0.09 system, we have also observed a dramatic change in the orbital occupation across the metal-insulator transition. This occupation change is discontinuous and shows a hysteresis behavior. The possible relationship between orbital occupation, magnetic interactions and effective Coulomb interactions and bandwiths will be discussed.