Continuum Charge Excitations in High-Valence Transition-Metal Oxides Revealed by Resonant Inelastic X-Ray Scattering

Abstract

We present a theoretical investigation of the origin of Raman-like and fluorescencelike (FL) features of resonant inelastic x-ray scattering (RIXS) spectra. Using a combination of local-density approximation + dynamical mean-field theory and a configuration interaction solver for Anderson impurity model, we calculate the L-edge RIXS and x-ray absorption spectra of high-valence transition-metal oxides LaCuO3 and NaCuO2. We analyze in detail the behavior of the FL feature and show how it is connected to the details of electronic and crystal structure. On the studied compounds we demonstrate how material details determine whether the electron-hole continuum can be excited in the L-edge RIXS process. arXiv:1803.03820v4 [cond-mat.str-el]

LDA+DMFTapproach to resonant inelastic x-ray scattering in correlated materials

“…This difference originates from the hybridization of the excited Co ion with the continuum of conduction states above E F , that differs in the two lattice geometries as mentioned above. The presence/absence of the FL feature in the corner/edge sharing structure resembles the behavior of the FL feature isoelectronic high-valence cuprates (LaCuO 3 and NaCuO 2 ), theoretically predicted recently [23]. The FL feature in SrCoO 3 is more intense compared to that in LaCoO 3 despite com-parable hybridization intensities above E F between the two, see Figs.…”

“…This results in only a minor intensity modulation of the FL feature, suggesting that the FL L 3 -RIXS feature of NiO reflects projected EHP continuum with an extra d electron sitting on the excited Ni site (local UH state) and a hole propagating in the LH or O 2p bands. This observation would be common in a large-gap system and qualitatively differs from the behavior of the FL feature in high-valence TMO with a small gap [23].…”

“…Here, |n , |m , and |f represent the initial, intermediate, and final states with energies E n , E m , and E f , respectively. The individual contributions from the initial states are averaged over, weighted with the Boltzmann factors [23,25]. Γ is the inverse lifetime of the corehole in the intermediate state, set to 300 meV throughout the present study.…”

“…The RIXS spectra are then calculated with the configuration-interaction impurity solver [25]. This approach not only allows modelling continuum electronhole excitations, but eliminates most of the empirical parameters of the traditional cluster model [23,25].…”

“…Therefore it provides a good description of the incident photon energy ω in -dependence of the RIXS spectra, which contains information on electron localization in the intermediate states of RIXS. A complex situation arises when formation of core-valence excitons compete with continuum excitations in intermediate states at a given ω in [23,26]. This is manifested, for example in high-valence nickelates [27] and titanium heterostructures [26], by coexistence of Raman-like (RL) and fluorescence-like (FL) features near the x-ray absorption edge.…”