RESONANT X-RAY SCATTERING AS A PROBE OF ORBITAL AND CHARGE ORDERING.
Resonant x-ray scattering is a powerful experimental technique for probing orbital and charge ordering. It involves tuning the incident photon energy to an absorption edge of the relevant ion and observing scattering at previously ''forbidden'' Bragg peaks, and it allows high-resolution, quantitative studies of orbital and charge order--even from small samples. Further, resonant x-ray scattering from orbitally ordered systems exhibits polarization- and azimuthal-dependent properties that provide additional information about the details of the orbital order that is difficult, or impossible, to obtain with any other technique. In the manganites, the sensitivity to charge and orbital ordering is enhanced when the incident photon energy is tuned near the Mn K absorption edge (6.539 keV), which is the lowest energy at which a 1s electron can be excited into an unoccupied state. In this process, the core electron is promoted to an intermediate excited state, which decays with the emission of a photon. The sensitivity to charge ordering is believed to be due to the small difference in K absorption edges of the Mn{sup 3+} and Mn{sup 4+} sites. For orbital ordering, the sensitivity arises from a splitting--or difference in the weight of the density of states [239]--of the orbitals occupied by the excited electron in the intermediate state. In the absence of such a splitting, there is no resonant enhancement of the scattering intensity. In principle, other absorption edges in which the intermediate state is anisotropic could be utilized, but the strong dipole transition to the Mn 4p levels--and their convenient energies for x-ray diffraction--make the K edge well-suited to studies of manganites. The Mn 4p levels are affected by the symmetry of the orbital ordering, which makes the technique sensitive to the orbital degree of freedom. Therefore resonant x-ray scattering can be used to obtain important quantitative information concerning the details of this electronic order. Two mechanisms for splitting of the 4p states have been proposed: a Coulomb coupling of the Mn 3d and 4p states--either directly, or indirectly through the hybridization with 0 2p states [240]--and the motion of oxygen ions due to the Jahn-Teller interaction. These two mechanisms have opposite signs in terms of the direction of the splitting, but both are consistent with experimental results to date. Which mechanism is dominant therefore remains an open question; however, in either case, the resonant scattering reflects the symmetry of the orbital ordering through the perturbation of the local electronic states at the Mn{sup 3+} sites, and the peak positions and widths determined in the x-ray experiments measure the orbital periodicity and correlation lengths, respectively. In addition, as is described below, detailed analysis of the resonant scattering provides information about the occupation of the ordered orbitals--a determination that cannot be made through the use of nonresonant scattering.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- AC02-98CH10886
- OSTI ID:
- 797113
- Report Number(s):
- BNL-69165; R&D Project: PO011; KC020201; TRN: US0201801
- Resource Relation:
- Other Information: BOOK TITLE:MANGANITES AND OTHER COMPOUNDS, CHAPTER 9, PP.179-183, EDITOR ELBIO DAGOTTO, SPRING VERLAG, N.Y.; PBD: 13 May 2002
- Country of Publication:
- United States
- Language:
- English
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