Synchrotron Radiation Science Div.2, Institute of Materials Structure Science^* Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, USA^** National Synchrotron Light Source, Brookhaven National Laboratory, USA^*** Ames Lab., Iowa State Univ., USA and Argonne National Laboratory, USA^**** Ames Lab., Iowa State Univ., USA^***** Argonne National Laboratory, USA^******
○Yusuke Wakabayashi^* Mary H. Upton^** Stephane Grenier^** John P. Hill^** Christie S. Nelson^*** Jong-Woo Kim^**** Philip J. Ryan^**** Alan I. Goldman^***** Hong Zheng^****** John F. Mitchell^******

Strongly correlated electronic systems exhibit a diverse range of electronic behavior, including the phenomena of orbital ordering --that is a preferential occupancy and orientational alignment of the occupied electronic orbitals. The orbital ordering has been studied a great deal in the manganites. So far, most of this work has been in the form of bulk studies, with very little work performed addressing the role of the surface in orbital order. Very recently, this question has started to attract attention [1-3].
In this study, surface x-ray diffraction measurements on a (001) cleaved surface of the orbitally ordered layered manganite La_0.5Sr_1.5MnO₄ were performed at X22C and X21 at the NSLS and at 6ID at the APS. We have succeeded in observing crystal truncation rod (CTR) scattering from the orbital ordering for the first time. It allows us to probe the "orbital surface" and its relationship to the crystallographic surface directly. Our striking finding is that the chemical and orbital surfaces are different. Transverse profiles of CTR intensity for chemical surface were as sharp as the instrumental resolution, indicating that the cleaved surface was extremely flat. In contrast, the transverse profile of the CTR intensity from the orbital ordering has two components: One was as sharp as the bulk orbital ordering superlattice reflection, while the other was significantly broader. This implies directly that the surface of the orbital ordering is rough in contrast to the atomically smooth chemical surface.
References:
[1] A. Ohtomo et al., Nature 419 (2002), 378.
[2] S. Okamoto and A. J. Millis, Nature 428 (2004), 630.
[3] J.W. Freeland et al., Nature Materials. 4 (2005), 62.