Spin-ordering mediated orbital hybridization in CoO at high pressures

Yang Ding, Javier Fernandez-Rodriguez, Jungho Kim, Fangfei Li, Diego Casa, Mary Upton, Thomas Gog, Ho-kwang Mao, Michel van Veenendaal
2012 Physical Review B  
2 We demonstrate a strong dependence of high-resolution 1s3p (or K β ) resonant X-ray emission spectroscopy (RXES) on local spin correlations at high pressure. We show that the pre-edge region in K β RXES of CoO can be separated into a local quadrupolar contribution and non-local dipolar intensity, which arises from the mixing of cobalt 4p states with neighboring cobalt 3d orbitals. For pressures of 0-12 GPa, we observe a two-fold increase in non-local contributions with respect to local
more » ... ect to local contributions. For pressures greater than 12 GPa, the ratio remains nearly constant. The observed pressure dependence and strong intensity changes cannot be explained by a conventional picture of bond length shortening and is ascribed to increased intersite 4p-3d mixing due to changes in magnetic ordering. These results are supported by theoretical calculations. 4/10/2012 3 I. INTRODUCTION In the Mott-insulators, the charge fluctuations are strongly suppressed due to the onsite Coulomb interactions, which results the 3d electrons to show both localized and delocalized behaviors 1 . The competition between localization and delocalization, i.e., the interplay between electron correlations and intersite orbital hybridization in low-lying electronic structures, accounts for many elementary excitations that determine the transport, magnetic, and electronic properties of 3d transition metal compounds. 2, 3, 4 Understanding such duality of the 3d electrons is central to the physics of 3d transition metal compounds. Core-level x-ray spectroscopic methods are the extensively applied means for studying the dynamics of 3d electrons. However, despite the advances made in theory and experiments 5-15 over the last two decades, it still remains a challenge to apply them to study the non-local excitations, which could be associated with the delocalized behaviors of 3d electrons (such as intersite orbital hybridization and spin-ordering). That is because it is difficult to conclusively identify the nonlocal excitations without directly observing how they form and evolve in the core-level x-ray spectra. To further explore this subject, we integrated the recently emerging high-resolution RXES technique 4,16 with high-pressure environment to study non-local transitions arising from intersite hybridization in CoO. CoO is a prototype Mott insulator that displays very little intersite hybridization under ambient conditions 13 . Our motivation is that the non-local transitions are sensitive to changes in nearest-neighbor correlation functions 17 , which can be tuned by an external pressure. Therefore, the nonlocal transitions should show the pressure dependent changes, while the local excitations will not show the such changes. This could enable us to directly identify the non-local excitations from the local excitation in the RXES spectra. In this paper, we report high-resolution 1s3p RXES measurements on CoO at pressures of up to 28 GPa. We focus on the pre-edge features and demonstrate that they can be separated into local quadrupolar and non-local dipolar intensities. The non-local contribution occurs due 4/10/2012 4 to the mixing of 4p states with neighboring cobalt 3d orbitals. Although this mixing is weak, due to strong dipolar matrix elements, the intensity is comparable to that of quadrupolar transitions. Moreover, because non-local features only exist by virtue of the coupling to neighboring sites, they are sensitive to changes in nearest-neighbor correlation functions. In our study, the relative intensities of non-local and local RXES increased by a factor of two as the pressure increased from 0-12 GPa, after which it became nearly constant. This pressure dependence is impossible to attribute to bond length shortening (which is equal to 1.9% at 12 GPa) but is in good agreement with an increase in non-local intensity due to changes in nearest-neighbor spin correlations. II. EXPERIMENTS High-pressure experiments were performed with Mao-type panoramic high-pressure cells 18 . CoO polycrystalline samples (99.99% purity), which were purchased from Sigma-Aldrich, were loaded with ruby chips (as a pressure standard) into an approximately 150-μm diameter hole in a beryllium metal gasket. The pressures were determined by ruby fluorescence. RXES measurements were performed at the 30-ID and 9-ID beamlines of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL) (a detailed description of these instruments can be found elsewhere 19 ). Incident X-rays from two APS undulators were monochromatized by high-heat load monochromators and high-resolution monochromators, which were then focused at the sample position. The incident energy was calibrated with a standard Co foil at the K-edge absorption inflection point at 7709 eV. X-ray fluorescence emitted from the sample was analyzed with a Ge(444) spherically diced analyzer on a 1-m Rowlandcircle spectrometer. The sample scattering plane was horizontal. The overall energy resolution was estimated from the FWHM of the elastic peak, and a value of 0.16 eV was obtained at the emission energy of Co K β13 (7650 eV). For the high-pressure RXES measurements, the diamond anvil cell was set to allow both incident and emitted X-rays to penetrate the Be gasket window. RXES spectra were 4/10/2012
doi:10.1103/physrevb.86.094107 fatcat:yhztiwfkt5bnvia2o7qg2c3ltm