Band Electronic Properties Research Articles

Structure and electronic properties of Fe nanostructures on MgO(0 0 1)

We present and discuss X-ray absorption spectroscopy and resonant photoemission measurements on Fe nanostructures self-assembled on MgO(0 0 1). For Fe coverages below 1 ML equivalent we measured an increase of the Fe L 23 branching ratio and changes in the splitting, widths and relative intensities of the different final states in the L 3M 23M 23 resonant Auger peak. Scanning tunnelling microscopy indicates that the average lateral dimensions of the self-aggregated structures decrease with decreasing Fe amount, from 12 nm at 15 ML nominal Fe amount to 5 nm at 2 ML Fe. This observation allows to interpret the observed changes in the 3d band electronic properties in terms of the evolution of the Fe local atomic coordination from a bulk-like situation to a configuration where low dimensionality effects are significant.

Electronic and structural properties of the (001) SrZrO 3 surface

The structural and electronic properties of SrZrO 3 selected surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The relaxation effects for two symmetric and asymmetric terminations are analyzed. The electronic and energy band properties are discussed on the basis of band structure as well density of states. There is a more significant rumpling in the SrO as compared to the ZrO 2 terminated surfaces. The calculated indirect gap is 4.856, 4.562, 4.637 eV for bulk, ZrO 2 and asymmetric terminations, respectively. The gap becomes direct; 4.536 eV; for SrO termination. The contour in the (110) diagonal plane indicates a partial covalent character between Zr and O atoms for the SrO terminated surface.

A theoretical analysis of the TiO 2/Sn doped (1 1 0) surface properties

We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO 2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO 2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sn, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices.

First principles studies of relativistic and spin-orbit effects on the HfO2 band structures

Electronic band and structural properties of the cubic, tetragonal, and monoclinic phases of HfO2 are calculated. The all electron ab initio self-consistent linear augmented plane-wave method within the framework of the local-density approximation was used, and full-relativistic contributions were taken into account. For the cubic phase, the lattice constant was obtained through a total energy minimization. For the tetragonal phase, the lattice parameters were calculated from the equilibrium volume obtained by variation of the ratio c/a, besides the relaxation of internal parameters (atomic positions). The monoclinic phase was studied using parameters as extracted from previous theoretical calculations. The electronic band structures for the three different structural phases present the same chemical trends relative to energy band gaps and valence-band states. The relativistic and spin–orbit effects are more pronounced for deep valence states. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum-well states in ultrathin Ag(111) films deposited onto H-passivated Si(111)-(1×1) surfaces

Ag(111) films were deposited at room temperature onto H-passivated $\mathrm{Si}(111)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ substrates, and subsequently annealed at $300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ An abrupt nonreactive Ag/Si interface is formed, and very uniform nonstrained Ag(111) films of 6--12 ML have been grown. Angle-resolved photoemission spectroscopy was used to study the valence band electronic properties of these films. Well-defined Ag $\mathrm{sp}$ quantum-well states (QWS's) have been observed at discrete energies between 0.5--2 eV below the Fermi level, and their dispersions have been measured along the $\overline{\ensuremath{\Gamma}K},$ $\overline{\ensuremath{\Gamma}{M(M}^{\ensuremath{'}})},$ and $\ensuremath{\Gamma}L$ symmetry directions. QWS's show a parabolic bidimensional dispersion, with in-plane effective mass of $(0.38--{0.50)m}_{o},$ along the $\overline{\ensuremath{\Gamma}K}$ and $\overline{\ensuremath{\Gamma}{M(M}^{\ensuremath{'}})}$ directions, whereas no dispersion has been found along the $\ensuremath{\Gamma}L$ direction, indicating the low-dimensional electronic character of these states. The binding energy dependence of the QWS as a function of the Ag film thickness has been analyzed in the framework of the phase accumulation model. A good agreement between experimental data and the above-mentioned model is obtained for the Ag/H/Si(111)-(1\ifmmode\times\else\texttimes\fi{}1) system. Hydrogen at the interface not only enhances the Ag film uniformity, but also acts as a barrier modifying the phase change of the $\mathrm{Ag}\ensuremath{-}sp$ electron wave upon reflection at the Ag/Si interface.

Calculated electronic structure of metastable phases of Cu.

The electronic energy band and ground-state properties for the existing body-centered-cubic (bcc) and body-centered-tetragonal (bct) crystals, and the predicted hexagonal-close-packed (hcp) structure of elemental copper have been calculated by using first-principles density-functional linear muffin-tin orbital methods in a unified scheme. Results are presented in the form of the energy-band structure in k space and the total energy as a function of the lattice constant. A recent proposed generalized gradient approximation scheme gives more accurate values than the standard local-density approximation. The calculated band structure of bct-Cu is in good agreement with that measured in photoemission experiments, on Cu films grown epitaxialy on Pd{001} and on Pt{001}. The equilibrium lattice constants given by us are in good agreement with those obtained from experiments on bct-Cu and bcc-Cu films. The possibility of the existence of an artificial structure of hcp-Cu has been discussed.

Electronic structure, charge transfer excitations, and high-temperature superconducting oxides (invited)

We present high-precision results on the electronic band structure and properties of YBa2Cu3O7−δ, YB2Cu3O6, GdBa2Cu3O7−δ, and La2−xMxCuO4 as obtained from highly precise state-of-the-art local density calculations. The results obtained demonstrate the close relation of the band structure to the structural arrangements of the constituent atoms and provide an integrated chemical and physical picture of the interactions and their possible relation to superconductivity. The ionic character of the Y is proven by similar detailed highly precise local density calculations for high TC GdBa2Cu3O7, and explains the coexistence of magnetism and superconductivity in the high TC rare-earth superconductors. Surprising features are the low density of states (DOS) at EF, especially for δ≥0.1 which is lower per Cu atom than that in La2−xSrxCuO4—in agreement with experiment and a relatively large magnetic Stoner factor. Strong indications are demonstrated for the inadequacy of a conventional phonon mechanism for obtaining the higher TC. Charge transfer excitations of occupied Cu1−O dpπ antibonding orbitals into their empty Cu1-O dpσ antibonding orbital partners, result in poorly screened ‘‘Cu3+-Cu4+’’ -like charge fluctuations. These charge transfer excitations (excitons) thus lead to strong polarization effects in these poorly screened (highly ionic) materials and induce attractive interactions among the 2D electrons. Thus, these interactions via exchange of excitons enhance the electron pairing and serve to enhance the TC proposed for the quasi-2D superconductors.

Electronic band properties and superconductivity in La2-yXyCuO4.

The results of electronic-structure calculations for tetragonal ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ provide insight concerning the origin of high-temperature superconductivity in the ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{X}}_{\mathrm{y}}$${\mathrm{CuO}}_{4}$ alloys. A half-filled Cu(3d)-O(2p) band with two-dimensional character and a nearly square Fermi surface produces a Peierls instability for y=0 that opens a semiconductor gap over the Fermi surface. Alloying with divalent or tetravalent atoms should spoil the nesting features while maintaining the strong coupling of O phonons to the conduction electrons.

Electronic band structure and properties of α-U

We present some results of a detailed study of the electronic band structure and some physical properties of orthorhombic uranium using a relativistic version of the LAPW method. Total and projected (by orbital angular momentum) densities of states are used to discuss the photo-emission data of Veal and Lam. The Fermi surfaces of the dominant 6th and 7th bands are found to have high symmetry (and some nesting features) of interest to dHvA measurements being pursued by Arko and Schirber. The theoretical magnetic field induced neutron form factor is found to be in good agreement with the measurements of Maglic et al.

Thermal conductivity reviewed

Thermal conductivity conferences arose out of a need to improve measurements and ensure the consistency of results, particularly at high temperatures, where the elimination of major errors is no trivial task. This inevitably leads to the need to understand the factors which control thermal conductivity, and recent conferences have placed increasing emphasis on basic studies of thermal conductivity, particularly at low temperatures, where defect structures and electronic band properties are important.