Orbital Occupancy Order inV2O3: Resonant X-Ray Scattering Results

Abstract

Resonant x-ray scattering experiments at the vanadium K edge demonstrate the existence of orbital ordering in V 2 O 3 . Bragg peaks due to the long-range order of 3d orbitals occupancy are observed when the photon energy is tuned to the threshold of the vanadium 3d bands. The azimuthal dependence of the resonant intensities confirms that the resonance arises from the ordering of the vanadium orbital occupancy. The observed orbital structure accounts for the complex magnetic structure of V 2 O 3 . The measured magnetic and orbital responses have the same critical temperature T N .[S0031-9007 (99)09287-X] PACS numbers: 78.70.Ck, 71.30. + h, 75.50.EeTwenty years ago, Castellani et al. [1] proposed that long-range order in the occupancy of the vanadium 3d orbitals was responsible for the complex magnetic properties of V 2 O 3 . Upon doping with Cr and/or under the application of hydrostatic pressure [2,3] V 2 O 3 exhibits both insulating and metallic phases with peculiar magnetic correlations [4][5][6]. It was suggested [1] that the spatial ordering of the occupancy of degenerate electronic orbitals accounts for the anisotropic exchange integrals found in the antiferromagnetic insulator phase (AFI) [5]. Furthermore, fluctuations in the orbital occupancy have been invoked to explain the evolution of the magnetic correlations in various phases of the V 2 O 3 system [6]. It appears that orbital occupancy plays a central role in the physics of V 2 O 3 , but no direct proof for orbital order could be produced experimentally since the original proposal in the late 1970s.In this Letter we present resonant x-ray scattering (RXS) experiments at the K edge of vanadium that demonstrate unambiguously the existence of orbital order in V 2 O 3 and provide information on the type of ordering. RXS is sensitive to the occupancy of electronic orbitals because it probes the symmetry of vacant electronic states through resonant multipole electric transitions; the variation of the orbital resonant scattering cross section with the direction of the incident polarization (azimuthal angle F) reflects the spatial symmetry of ordered orbitals. Furthermore, RXS may be tuned to probe selectively the electronic shells where orbital order takes place. In the case of V 2 O 3 , theoretical calculations [7] have shown that the resonance at the vanadium K edge provides observable cross sections arising from the order of the 3d vanadium states.RXS experiments were performed at the ID20 magnetic scattering undulator beam line at the European Synchrotron Radiation Facility [8]. A double crystal, Si(111), monochromator located between two focusing mirrors defined a narrow energy band around the vanadium K edge (FHWM 0.8 eV) with a high degree of linear s polarization. The x-ray beam was diffracted by the sample onto a pyrolitic graphite crystal analyzer [(004) reflection] to separate the s and p components of the scattered radiation. The sample was mounted with beeswax in a closed cycle refrigerator which could be rotated about the scattering vector to p...

Spin-1 effective Hamiltonian with three degenerate orbitals: An application to the case of V2O3

“…In their paper Mila et al 19 claim that the ferro-orbital ordering they propose, while different from that invoked by Paolasini et al 13 , is also consistent with the experimental findings. However we feel that their argument was rather scant and we try to present here a more in depth analysis of the implications of X-ray resonant scattering data, deferring a more complete treatment to a future work.…”

“…We notice that, considering the f n as scalars, we recover the crystallographic extinction rules (the reflections with h + k + l odd (b = −1) are lattice forbidden) and the selection rules for non resonant magnetic scattering (for b = −1, those with h even are magnetically allowed, those with h odd are forbidden). The classification by Paolasini et al 13 of the lattice forbidden reflections into purely orbital and purely magnetic was actually based on the non resonant magnetic scattering and might therefore be only approximate.…”

“…It all started when Fabrizio et al 12 suggested that the existence of an OO of the kind suggested by CNR 7 could be revealed by means of x-ray resonant diffraction at the Vanadium K-edge. Following this suggestion Paolasini et al 13 carried out such measurements. On the basis of a classification of forbidden reflections into pure magnetic and orbital, they interpreted the (111) monoclinic reflection as evidence of an orbital ordering in V 2 O 3 , in keeping with the prediction of the CNR model.…”

Resonant x-ray diffraction of the magnetoresistant perovskitePr0.6Ca0.4MnOGrenier1, Hill2, Gibbs3 et al. 2004Phys. Rev. B1025711View full textAdd to dashboardCite

“…The experiments were performed utilizing resonant x-ray diffraction which involve measuring the intensity of a reflection as a function of the incident photon energy [26,27,28,29,30]. By tuning the incident energy to the Mn absorption K-edge, 1s electrons are promoted to an intermediate unoc- Figure 3: The diffraction geometry.…”

Orbitally driven spin pairing in the three-dimensional nonmagnetic Mott insulatorBaVS3:Evidence from single-crystal studies

“…To cite a well-known example: the interplay of magnetic and orbital long range ordering, and strong coupling to the lattice, account for the metal-insulator transitions of the V 2 O 3 system [2,3]. In contrast, the metal-insulator transition of the S = 1/2, 3d 1 electron system BaVS 3 is not associated either with magnetic long range order, or with any detectable static spin pairing.…”