Photoemission study of CoO (original) (raw)
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Doping effects on the electronic and structural properties of CoO2: An LSDA+U study
Physical Review B, 2004
A systematic local spin density approximation ͑LSDA͒ + U study of doping effects on the electronic and structural properties of single layer CoO 2 is presented. Undoped CoO 2 is a charge transfer insulator within LSDA+ U and a metal with a high density of states (DOS) at the Fermi level within LSDA. ͑CoO 2 ͒ 1.0− , on the other hand, is a band insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no charge orderings are present. Due to the strong interaction between the doped electron and other correlated Co d electrons, the calculated electronic structure of ͑CoO 2 ͒ x− depends sensitively on the doping level x. Zone center optical phonon energies are calculated under the frozen phonon approximation and are in good agreement with measured values. Softening of the E g phonon at doping x ϳ 0.25 seems to indicate a strong electron-phonon coupling in this system. Possible intermediate spin states of Co ions, Na ordering, as well as magnetic and charge orderings in this system are also discussed.
Physical Review B - PHYS REV B, 2008
The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to 28000cm-1 and for temperatures 8<T<325K . A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at TN=289K has been observed. At low temperatures the splitting amounts to 15.0cm-1 . In addition, we studied the splitting of the cubic crystal-field ground state of the Co2+ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below TN , magnetic-dipole transitions between the exchange-split levels are identified, and the energy-level scheme can be well described with a spin-orbit coupling lambda=151.1cm-1 , an exchange constant J=17.5cm-1 , and a tetragonal crystal-field parameter D=-47.8cm-1 . Already in the paramagnetic state electric-quadrupole transitions between the spin-orbit split level have been observed. At high frequencies, two electronic levels of the crystal-field-split d m...
Electronic structure investigation of CoO by means of soft x-ray scattering
2002
The electronic structures of transition-metal compounds have been extensively investigated due to the discovery of exotic properties such as high-Tc superconductivity, giant magnetoresistance, and insulating behavior. For a theoretical description in ordinary band theory, an understanding of the electronic structure of transition-metal oxides represents a fundamental problem since the metal ions show a more or less atomiclike behavior.
Theoretical study of the crystal field excitations in CoO
Chemical Physics, 1998
A theoretical investigation of the crystal field excitations in CoO is presented. Special attention is given to the excitation energy of the 4 A state. In recent experimental and theoretical studies an excitation energy around 3.1 eV was reported. 2g This is in disagreement with the 2.1 eV deduced from optical spectroscopy data. After analyzing electron correlation effects, spin-orbit interactions and the material model to represent the CoO crystal, we can confirm the interpretation of the optical data, not only for the 4 A state but also for all other low-lying crystal field excitations. Electron correlation effects are 2g found to have a significant differential effect on the excitation energies, ranging from q0.3 to y0.6 eV. Spin-orbit interactions are less important, affecting the excitation energies by at most 0.05 eV. Finally, we discuss the effect of the Pauli repulsion between the cluster ions and the first shell of ions around the cluster. This affects the excitation energies by a small, but significant, amount.
Physical Review B, 2021
Cobalt oxides with multiple local CoO coordination environments such as octahedral, tetrahedral, and tetragonal networks display versatile electronic and magnetic properties which have attracted great interest in many fields. Understanding the ground-state properties and determining the fundamental band gap remain challenges in cobalt-based compounds and thin films, which have been investigated here using synchrotron-based Co L 23-edge x-ray absorption measurements followed by configuration interaction cluster calculations. We focus on the detailed Co L 23-edge absorption spectral variations in different octahedral crystal fields as well as in the less investigated tetrahedral and tetragonal systems, taking into account Co ions with different valence states. From a quantitative comparison between the simulated spectrum and an accurately measured absorption spectrum of a specified compound, the crystal field value can be extracted from the Co L 23-edge absorption spectrum, which is complementary to the results obtained in optical measurements and other calculations. Furthermore, Co L 23-edge x-ray linear dichroism shows the same spectral evolutions as a result of either local CoO 6 cluster with tetragonal symmetry or the magnetic exchange field, whereas both coexist in most antiferromagnetic cobalt oxide thin films. Detailed temperature and polarization-dependent Co L 23-edge absorption measurements have been proposed to distinguish both contributions, which show different spectral variations due to the specified modifications of the ground and final states at different temperatures. Our results offer theoretical guidance for understanding the multiplet structure of Co L 23-edge absorption spectrum, obtaining the precise crystal field value for cobalt oxides with versatile coordinations, and explaining the underlying mechanism of x-ray linear dichroism, as well as understanding the fundamental physical properties and their potential applicability of cobalt oxides and their thin films.
Physical Review B, 2008
We present synchrotron-excited oxygen x-ray K-emission spectroscopy ͑O K␣ XES͒ and oxygen x-ray absorption spectroscopy ͑O 1s XAS͒ spectra of transition-metal ͑TM͒ oxides MnO, CoO, and NiO. The comparison of oxygen K-emission and absorption spectra to valence band photoemission and bremsstrahlung isochromat spectra measurements shows that O 1s XAS is not strongly influenced by the core hole effect, whereas the TM 2p XAS significantly shifts to a lower energy. New and effective methods for determining the band gap and anion-to-cation cation charge-transfer energies of the oxides from the measured spectra are presented and applied, and the combination of O XAS and XES is shown to agree well with the results of numerical electronic structure methods applied to strongly correlated oxides. For MnO, the charge-transfer energy is found to be 6.6 eV and the band gap is 4.1 eV; for CoO, the values are 6.1 and 2.6 eV and for NiO, the values are 5.4 and 4.0 eV.
Electronic structure near the Fermi level in the Ca Co layered cobalt oxide
Journal of Electron Spectroscopy and Related Phenomena, 2005
Synchrotron radiation angle resolved photoemission spectroscopy measurements were performed on Ca 3 Co 4 O 9 , which is one of promising candidates for the thermoelectrical material because of its possession of the large thermoelectric power, metallic electrical conduction, and low thermal conductivity. Three dispersing bands were observed in the energy range of E = 0-0.9 eV below the Fermi level, and one of these bands constructs a hole-like Fermi surface (FS) centered at point. These bands were successfully assigned as those from two-dimensionally spanned CoO 2 layers by observing their periodicity in the reciprocal space. It was also found that the FS possesses a hexagonal shape with its edge center aligned on the direction of the primitive reciprocal lattice vector K. By using tight-binding fit on the band that crosses E F , we successfully reproduced the characteristic features in the density of states that is responsible for the large thermoelectric power and the metallic electrical conduction.
Journal of Physics and Chemistry of Solids, 2011
We have investigated three-dimensional electronic structure for Na x CoO 2 (x ¼ 0.77 and 0.65) by highresolution angle-resolved photoemission spectroscopy to study the origin of antiferromagnetic (AF) transition of highly doped Na x CoO 2 ðx 4 0:75Þ. The a 1g large hole-like Fermi surface (FS) in x ¼ 0.77 shows distinct three-dimensionality along the k z direction, and a three-dimensional small electron pocket appears around G point, indicating strong inter-layer electronic correlation. On the other hand, x ¼ 0.65 sample does not show three-dimensional behavior. This result indicates that transition of FS as a function of band filling is closely related to the occurrence of the magnetic transition in highly doped Na x CoO 2 .