Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 641–645 Orbital polarization of LaSrMnO 4 studied by soft X-ray linear dichroism W.B. Wu a,b , D.J. Huang a,b,∗ , G.Y. Guo b,c , H.-J. Lin b , T.Y. Hou b , C.F. Chang b , C.T. Chen b , A. Fujimori d , T. Kimura e,1 , H.B. Huang f , A. Tanaka f , T. Jo f a Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan, ROC b National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC c Department of Physics, National Taiwan University, Taipei 106, Taiwan, ROC d Department of Complexity Science and Department of Physics, University of Tokyo, Tokyo 113-0033, Japan e Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan f Department of Quantum Matters, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Available online 1 April 2004 Abstract With polarization-dependent soft X-ray absorption, we determined the orbital character of occupied and the lowest unoccupied e g orbitals of prototype single-layered perovskite LaSrMnO 4 . Our measurements indicate that LaSrMnO 4 exhibits strong orbital polarization in the e g states of Mn 3+ sites, leading us to conclude that LaSrMnO 4 has a “ferro-orbital” ordering of 3z 2 -r 2 . We found that linear dichroism in soft X-ray absorption is an effective experimental method to identify the nature of orbital ordering in transition-metal oxides such as manganites. © 2004 Elsevier B.V. All rights reserved. Keywords: Polarization; Prototype; Perovskite 1. Introduction Orbital ordering is one of the important subjects in cur- rent research of transition metal compounds [1]. In some compounds, there is a real space ordering of the charge car- rier in certain orbitals, i.e., orbital ordering, resulting from superexchange interaction, lattice distortion, and Coulomb correlation of electrons. Orbital ordering manifests itself in the site dependent orientation of the quadrupole moment as a result of the spatial distribution of the outermost valence d electrons. Orbital ordering gives rise to the anisotropy of the electron-transfer interaction. As the magnetic and transport properties are closely correlated to the orbital and charge de- grees of freedom, there have been many efforts to clarify the microscopic mechanisms giving rise to such exotic phenom- ena. For example, a fundamental issue in the physics of the manganites concerns the mechanism of the charge-orbital ordering and charge stripe formation. The mechanism of charge-orbital ordering is being extensively discussed [2–9]. ∗ Corresponding author. E-mail address: djhuang@nsrrc.org.tw (D.J. Huang). 1 Present address: Los Alamos National Laboratory, MST-MISL, Mail Stop K774, Los, Alamos, NM 87545, USA Measurements of resonant X-ray scattering (RXS) at the Mn 1s the threshold have been argued for the evidence of 3d orbital ordering of manganites [10,11]. In one of the inter- pretations of RXS measurements, a Mn 1s electron is first excited to the unoccupied Mn 4p state by absorbing the in- cident photon and then a photon is emitted resulting from the 4p → 1s de-excitation. Consequently, the ordering of 4p orbitals, e.g., the alternately arrangement of 4p x and 4p y orbitals of Mn in LaMnO 3 , is directly observed. Through intra-atomic 3d-4p multipole interaction, ordering of Mn 3d orbitals is indirectly observed. However, such an interpre- tation of RXS at Mn K-edge is controversial. Orbital or- dering in transition-metal oxides is typically accompanied by Jahn–Teller lattice distortion. Calculations based on a local-density approximation including on-site Coulomb in- teractions (LDA + U) [12,13] and multiple scattering the- ory [14] indicate that RXS measurements pertain mainly to Jahn–Teller distortion, instead of directly observing 3d or- bital ordering. To identify the orbital character of 3d states in orbital-ordered manganites is an important subject. Polarization-dependent soft X-ray absorption (XAS), particularly linear dichroism, provides us with a pow- erful means to identify spin and orbital occupation of transition-metal oxides. For example, linear dichroism (LD) in XAS measurements on V 2 O 3 enabled important 0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.02.072