Revisiting the valence-band and core-level photoemission spectra of NiO

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.     

Photoemission and inverse photoemission spectroscopy of NiO

Photoemission (UPS) and inverse photoemission (BIS) spectra of NiO grown as a thin film on metallic Ni are reported. The data are discussed with respect to the electronic structure of the occupied and empty states in NiO.     

Acceptors in Li doped NiO

The electronic structure of the Li doped large gap semiconductor (insulator) NiO is investigated by photoemission and inverse photoemission spectroscopy. Spectral weight is produced by the Li doping within the gap at 1.4 eV above the top of the valence band. It is interpreted as due to an acceptor level associated with a LiO complex.     

NiO: correlated band structure of a charge-transfer insulator

The band structure of the prototypical charge-transfer insulator NiO is computed by using a combination of an ab initio band structure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni d and O p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This brings an important progress in a long-standing problem of solid-state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories.     

Bandstructure and atomic energy levels in Ce-Metal,NiO and NiS

The results obtained by photoemission and inverse photoemission spectroscopy (PES and IPES) for compounds in which electron correlations are important such as Ce, NiO and NiS are compared to the Mott-Hubbard model. It can be shown that some interesting inconsistencies exist that emphasize the importance of hybridization. In the metallic phase of NiS the correlation energyU is large as compared to the 3d-bandwidth. Therefore the sulfur-nickel electron exchange seems to be an important mechanism in the conductivity of the compound.     

Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnet

Polarization dependent x-ray photoemission electron microscopy was used to investigate the spin structure near the surface of an antiferromagnetic NiO(001) single crystal in response to the deposition of a thin ferromagnetic Co film. For the cleaved NiO surface we observe only a subset of bulklike antiferromagnetic domains which is attributed to minimization of dipolar energies. Upon Co deposition a spin reorientation near the NiO interface occurs, with the antiferromagnetic spins rotating in plane, parallel to the spins of the Co layer. Our results demonstrate that the spin configuration in an antiferromagnet near its interface with a ferromagnet may significantly deviate from that in the bulk antiferromagnet.     

Photoemission on NiO

The interpretation of the photoemission spectra of NiO is considered with respect to the dispersion of thed-bands. The dispersion of thed-bands can not be measured due to the formation of an exitonic state. The electronic nature of the states which are responsible for the optical gap in NiO is discussed.     

Fine structures in the X-ray photoemission spectra of MnO,FeO, CoO,and NiO single crystals

The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced.     

Electronic excitations in NiO

A many-body theory for a multi-orbital tight-binding model is developed by taking account of local three-body correlations. The theory is applied to the antiferromagnetic NiO. The multiple structures of three-particle states are fully taken into account. It is shown that the self-energy correction leads to the correct value of the band gap, the orbital character of the valence band top which is consistent with the charge-transfer insulator, and the presence of the satellite intensities. The quasiparticle band is obtained in good agreement with the angular-resolved photoemission data of NiO.     

Pseudogap of metallic layered nickelate R(2-x)Sr(x)NiO4 (R = Nd, Eu) crystals measured using angle-resolved photoemission spectroscopy

We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates, R(2-x)Sr(x)NiO4 (R = Nd, Eu), isostructural to La(2-x)Sr(x)CuO4. Angle-resolved photoemission spectroscopy on the barely metallic Eu(0.9)Sr(1.1)NiO4 (R = Eu, x = 1.1) has revealed a large hole surface of x2-y2 character with a high-energy pseudogap of the same symmetry and comparable magnitude with those of underdoped (x<0.1) cuprates, although the antiferromagnetic interactions are 1 order of magnitude smaller. This finding strongly indicates that the momentum-dependent pseudogap feature in the layered nickelate arises from the real-space charge correlation.     

On the interpretation of valence band photoemission spectra of NiO

The valence band photoemission spectrum and the L₃VV Auger spectrum of NiO are compared. The satellite found in the valence band of NiO and other Ni compounds is interpreted as an unscreened 3d⁷ final state, whereas the main d-emission is a 3d⁷ final state screened by a d electron in an exitonic state.     

Magnetic moments at the surface of antiferromagnetic NiO(100)

The orientation of magnetic moments at the (100) surface of antiferromagnetic NiO single crystals is studied by x-ray linear magnetic dichroism in photoemission microscopy. T domains are observed terminating at the surface, with domain boundaries running mostly along in-plane [10] directions. From the detailed polarization dependence we find that the magnetic surface structure of a cleaved crystal is bulk terminated. This is in contrast to sputtered surfaces, where magnetic moments lie within the surface plane, forming a magnetically relaxed structure. These findings are of importance for understanding the exchange bias phenomenon.     

Si 2p and O 1s photoemission from oxidized Si(0 0 1) surfaces depending on translational kinetic energy of incident O2 molecules

The influence of translational kinetic energy of incident O₂ molecules for the passive oxidation of the partially oxidized Si(0 0 1) surface has been studied by photoemission spectroscopy. The incident energy of O₂ molecules was controlled up to 3 eV by a supersonic molecular beam technique. Two incident energy thresholds (1.0 and 2.6 eV) were found out in accordance with the first-principle calculations. Si 2p and O 1s photoemission spectra measured at representative incident energies showed the incident energy induced oxidation at the backbonds of the dimer and the second layer (subsurface) Si atoms. Moreover, the difference of oxygen chemical bonds was found out to be as the low and the high binding energy components in the O 1s photoemission spectra. They were assigned to bridge sites oxygen and dangling bond sites oxygen, respectively.     

The electronic structures of epitaxial CrSi2 film prepared on Si(111) substrate

The valence band (VB) electronic structures of CrSi₂ were studied by synchrotron radiation photoemission. Overall features of the VB photoemission spectra measured at room temperature (RT) and 20 K by using synchrotron radiation (photon energy, hν=20–120 eV) were similar. Two characteristic emissions were observed corresponding to the bonding and the nonbonding Cr-d partial density of states (PDOS) in the CrSi₂. The onset of the VB photoemission measured at 20 K was located at about 0.32 eV below Fermi level, due to the energy band gap of CrSi₂ more than 0.32 eV.     

Localized electronic excitations in nickel oxide

Valence-electron ionization and optical excitation energies of NiO have been calculated by the SCF?Xα cluster method and transition-state procedure. Ni 3d-like spin orbitals are localized above the O 2p band, leading to well defined peaks in the photoemission spectra. Ligand-to-metal charge-transfer excitations are responsible for strong optical absorption between 4 and 7eV.     

Dynamical properties of the one-dimensional band insulator (NbSe4)3I

Optical and photoemission experiments reveal unexpected spectral signatures of one-dimensional band insulators. In the model compound (NbSe (4))3I the optical conductivity decays as a power law sigma(1)(omega) approximately omega(-4.25) above a sharp gap edge. Photoemission observes both the valence and a shadow band, produced by a commensurate superstructure. We identify an optical and photoemission band gap consistent with other measurements but much smaller than the energy scale defined by the dispersion of the band peak in the photoemission spectra.     

Quantum mechanical effects in internal photoemission THz detectors

The variation in spectral shape around the threshold frequencies between model and experimental responsivity spectra in heterojunction interfacial workfunction internal photoemission (HEIWIP) infrared detectors was investigated. This is attributed to the loss of photoexcited carriers, within the escape cone, prior to photoemission. The energy dependent transmission of excited carriers is incorporated to the existing photoemission model to show that emission around the threshold frequency is reduced considerably by quantum mechanical reflection of photoexcited carriers as observed in experimental results. In the new model, the photoemission of carriers become bias dependent.     

Chemisorption and reaction of NH3 on Ni(111)

The chemisorption of ammonia on Ni(111) has been investigated using LEED, thermal desorption, and angle-resolved photoemission. For exposures at 200 K, thermal desorption shows a coverage-dependent binding energy associated with dipole-dipole interactions. A (2 × 2) LEED pattern occurs at 2–4 L exposure. Time dependence of the LEED pattern and changes in the thermal desorption induced by the LEED beam indicate that the (2 × 2) pattern is due to a stable intermediate decomposition species. Using synchrotron radiation photoemission all three valence orbitals of ammonia have been observed for the first time. The energies of the ammonia-induced features in the photoemission (?22.0, ?11.0 and ?6.7 eV below the Fermi energy) and the observed symmatries positively identify the absorbed species as molecular ammonia. Additional structure observed in the photoemission spectra after electron bombardment is associated with the stable adsorbed intermediate.     

The structure and properties of the stepped surfaces of MgO and NiO

The static lattice computer simulation method has been used to study the structure and properties of high index faces of MgO and NiO. The (10_n) series of faces can be considered as stepped (001) surfaces and have been studied for both materials. In addition, the (403) and (302) faces which can be considered as stepped (101) surfaces have been studied for NiO. The calculated energies for steps on the (001) face are 3.72×10^?10 and 3.62×10^?10 J m^?1 for MgO and NiO respectively. The NiO energy also requires correction for the crystal field splitting. The energy of steps on the (101) surface is at least an order of magnitude lower. The interaction between the steps is repulsive but of short range. The large variation of surface energy with angle indicates that torque terms cannot be neglected in the analysis of thermal grooving experiments. The step structure is modified by substantial ionic displacements leading to an obtuse step angle. The structure is qualitatively similar for both NiO and MgO. The large distortions are likely to modify the step properties from those deduced by consideration of only the ideal geometry.