Resonant inelastic x-ray scattering study of the electronic structure of Cu_{2}O (original) (raw)
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On-site interband excitations in resonant inelastic x-ray scattering from Cu2O
Physical Review B, 2008
The electronic structure of cuprite ͑Cu 2 O͒ has been studied by high-resolution x-ray photoemission ͑XPS͒, x-ray absorption ͑XAS͒, and resonant x-ray emission spectroscopies ͑XES͒ supported by band structure calculations using a hybrid exchange approximation to density functional theory. A pronounced loss feature at about 4.5 eV due to on-site interband excitation has been identified in resonant inelastic x-ray scattering from Cu 2 O close to the L 3 ͑Cu 2p 3/2 ͒ core threshold. Although Cu 2 O nominally has a filled upper valence band of Cu 3d states and an empty conduction band of Cu 4s states, the band structure calculations show that there is substantial 3d character in the conduction band and that the inelastic loss is dominated by on-site 3d to 3d excitation conforming to the selection rule ⌬l = 0 rather than 3d to 4s transitions with ⌬l = −2. However, unlike in previous work, these transitions do not arise from ligand field splitting of the Cu 3d states but rather from on-site 3d-4s hybridization which introduces 3d character into the conduction band. Comparison between XPS, XES, and XAS data shows that Cu L 3 XAS is dominated by a core exciton lying 0.65 eV below the bottom of the conduction band and that inelastic scattering is only observed for photon energies below that required to excite the core electron into the conduction band.
Hidden electronic state of CuO revealed by resonant inelastic x-ray scattering
Physical Review B, 2002
High-resolution resonant inelastic x-ray scattering ~RIXS! spectra from CuO in the 1s2p emission region were measured. A surprisingly complicated set of spectra was observed. Detailed analysis of these spectra reveals the existence of an excited state, which is hidden in the Cu-K x-ray absorption spectrum of CuO. Furthermore, the energy of this excited state corresponds well to a Cu 4pz-like state predicted by a real-space multiple-scattering technique, but has not been observed experimentally up to now. This result demonstrates the potential use of RIXS to complement x-ray absorption spectroscopy.
Physica B-condensed Matter, 2009
We have used X-ray absorption (XA) and resonant inelastic X-ray scattering (RIXS) spectroscopies to study a series of copper compounds, namely Cu 2 O, CuO, CuðOHÞ 2 , CuCl 2 , Cu 2 S, CuSO 4 , malachite ðCu 2 ðCO 3 Þ 2 ðOHÞ 2 Þ and atacamite ðCuCl 2 Á 3CuðOHÞ 2 Þ. Cu 2p XA spectra provide information about oxidation states. Divalent copper gives a single narrow line due to excitations into the empty 3d state, whereas monovalent copper gives a broad band at higher energy due to transitions to 4s states. Chemical shifts of the main line in the Cu 2þ XA spectra of different compounds are observed but in some cases they are too small to make a clear distinction between the species. It is shown that RIXS at the Cu 2p edge has a great potential to distinguish between the species due to large differences in spectral shapes for the same energy of the incident photon beam. First evidence for the possibility of detecting chemical composition of copper compounds is presented and discussed in details.
Resonant inelastic X-ray scattering of Sr2CuO3
Journal of Electron Spectroscopy and Related Phenomena, 2005
We measured resonant inelastic X-ray scattering spectra of one-dimensional Mott insulator Sr 2 CuO 3 near the Cu 1s absorption edge (hν ∼ 8.999 keV) for the purpose of observing the interband transitions between the occupied bands and the unoccupied bands. The obtained RIXS spectra have shown dispersions depending on the wave number difference k from k = 2.25π to 3.75π along the single Cu-O chain. Each spectrum shows two main features, one is at around 5.5 eV and the other at around 3.1 eV for k = 3.0π. The 5.5 eV feature does not show noticeable k dependence and can be interpreted as a charge transfer excitation from the Cu 3d-O 2p antibonding state to the upper Hubbard band (UHB). The feature observed near 3.1 eV for k = 3.0π shows prominent k dependence. The energy loss is the largest near k = 3.0π and 5.0π and the smallest near k = 2.0π and 4π. This structure is interpreted as the excitation from the Zhang-Rice singlet to the UHB.
Journal of Electron Spectroscopy and Related Phenomena, 2004
Being a second order optical phenomenon, resonant inelastic X-ray scattering (RIXS) spectroscopy can be used to disentangle bound-state excitations from continuous absorption spectrum more effectively than ordinary first order absorption spectroscopy. When the excitation energy is lower than the K absorption threshold by about 10 eV or more, RIXS spectra observed approach a mirror image of 1s-core-hole lifetime-broadening-removed (LBR) XANES spectra, although the scattering intensity is prohibitively weak because of the unsatisfactory resonant condition employed. Approaching closer to the resonance, RIXS intensity increases by several orders of magnitude and concurrently observed is a surprisingly complicated set of spectra which heavily depends on the excitation energies. From each RIXS spectrum obtained under close to the resonant conditions, 1s-core-hole LBR-XANES or 1s- and 2p-core-hole LBR-XANES can be deduced by analytical method or numerical calculations. The RIXS-XANES method has been applied to CuO to reveal an existence of hidden electronic states near the absorption threshold.
Study of the electronic properties of Cu2O thin films by X-ray absorption spectroscopy
Optik, 2018
We have investigated the thickness effect of Cu2O thin films on the electronic structure deposited by the successive ionic layer adsorption and reaction (SILAR) method. Crystal, optical and electronic properties of the Cu2O thin films were studied by X-ray diffraction (XRD) and X-ray absorption near edge spectroscopy (XANES). According to the crystal structure analysis, films were determined to be mainly in cubic Cu2O structures. The XANES study have shown that Cu L2,3 absorption edges are influenced by the chemical state of the Cu atoms strongly and a sole ionic picture is not enough to describe the Ledge spectra of Cu2O. It has been observed that Cu L2,3-edge spectra of the samples present typical electronic features of both monovalent Cu (I) and Cu (II) divalent states. The grains have an average size of 2.5 nm and XRD measurements revealed that (111) plane is the preferential orientation. Optical studies have shown that the optical absorption edge shifted to higher energies as the film thickness increases. It was found that the optical band gap was significantly influenced 2 by the film thickness. Our results exhibited that the increment of the optical band gap of Cu2O thin films associated with a significant decrease of Cu-Cu interaction as a result of the increase in the film thickness.
Electronic and crystal structure of Cu2−xS: Full-potential electronic structure calculations
Physical Review B, 2007
Electronic structure calculations are presented for Cu 2−x S using the full-potential linearized muffin-tin orbital method. In the simple cubic antifluorite structure, Cu 2 S is found to be semimetallic both in the local density approximation ͑LDA͒ and using the quasiparticle self-consistent GW ͑QSGW͒ method. This is because the Cu d bands comprising the valence band maximum are degenerate at the ⌫ point and the fact that the Cu s band, which can be considered to be the lowest conduction band, lies slightly below it at ⌫. Small deviations from the ideal antifluorite positions for the Cu atoms, however, open a small gap between the Cu d valence and Cu s-like conduction bands because the symmetry breaking allows the Cu s and Cu d bands to hybridize. Supercell models are constructed for cubic and hexagonal chalcocite Cu 2 S as well as cubic digenite Cu 1.8 S by means of a weighted random number structure generating algorithm. This approach generates models with Wyckoff site occupancies adjusted to those obtained from experimental x-ray diffraction results and with the constraint that atoms should stay within reasonable distance from each other. The band structures of these models as well as of the low-chalcocite monoclinic structure ͑Cu 96 S 48 ͒ are calculated in LDA with an additional Cu s shift obtained from the QSGW-LDA difference for the antifluorite structure. Even with this correction, smaller band gaps of about 0.4-0.6 eV ͑increasing from cubic to hexagonal to monoclinic͒ than experimentally observed ͑1.1-1.2 eV͒ are obtained for the Cu 2 S composition. Decreasing the Cu content of Cu 2−x S in the range 0.06Ͻ x Ͻ 0.2 is found to essentially dope the p-type material by placing the Fermi level 0.2-0.3 eV below the valence band maximum but also to increase the gap between highest partially filled valence band and lowest conduction bands to about 0.7-1.0 eV. This results from a reduced Cu d-band width. Thus, the optical band gap or onset of optical absorption increases in part but not exclusively due to the Moss-Burstein effect. The total energies of the structures are found to increase from monoclinic to hexagonal to cubic to antifluorite. This is consistent with the fact that the simple antifluorite structure is not observed and that the systems change from monoclinic to hexagonal to cubic with increasing temperature, under the assumption that the electronic energy of the system dominates the free energy. We find that Cu 2 S is unstable toward the formation of Cu vacancies even in thermodynamic equilibrium with bulk Cu metal. The experimental data on the band gaps and optical absorption are discussed. We find no evidence for indirect band gaps in the hexagonal materials and argue that the experimental results are consistent with this in spite of previous reports to the contrary. The presence of a second onset of absorption located about 0.5 eV higher than the minimum band gap observed in experiment is explained by a rise in conduction band density of states at this energy in our calculations. The calculated increase in gap with decreasing Cu concentration is in agreement with experimental observations.
Doping effects on the electronic structure of CuO2 planes
Physica B: Condensed Matter, 1997
We have exploited angle-resolved photoelectron spectroscopy to study changes in the quasiparticle excitations of CuO2 planes as a function of the carrier density, in optimally doped and underdoped BiaSrzCaCuOs+x and in SrzCuOzCI 2. We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Fermi surface away from optimal doping. We have also found that the carrier density has a nontrivial effect on the symmetry properties of these states.
1999
Band structure calculations have been used to identify the different bands contributing to the polarisation-dependent photoemission spectra of the undoped model cuprate Sr2CuO2Cl2 at the high-symmetry points of the CuO2 plane Γ, (π/a, 0) and (π/a, π/a) and along the high-symmetry directions Γ − (π/a, π/a) and Γ − (π/a, 0). Results from calculations within the local density approximation (LDA) have been compared with calculations taking into account the strong electron correlations by LDA+U, with the result that the experimental order of energy levels at the high-symmetry points is better described by the LDA+U calculation than by the simple LDA. All the main peaks in the photoemission spectra at the high symmetry points could be assigned to different Cu 3d and O 2p orbitals which we have classified according to their point symmetries. The dispersions along the high-symmetry directions were compared with an 11-band tightbinding model which was fitted both to the LDA+U band structure calculation and the angle-resolved photoemission data. The mean field treatment successfully describes the oxygen derived bands but shows discrepancies for the copper ones.
Physical Review Letters, 2004
We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L 3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estimated with a simple crystal field model. The RIXS at the L 3 edge demonstrates here a great potential, thanks to the resonance strength and to the large 2p spin-orbit splitting.