Prediction of Γ-L and L-X crossovers in Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>Sb alloys by empirical pseudopotential method

Using empirical pseudopotential method Γ-L crossover is found for the Ga_0.74Al_0.26Sb. The conduction band minimum is observed to switch at the (0.87, 0, 0) point for Ga_0.51Al_0.49Sb which shifts to the X point for Ga_0.21Al_0.79Sb and remains at X leading finally to indirect band gap in AlSb. Band structure calculations for a large number of alloys are performed and bowing parameters b_X and b_L are proposed for the E_X and E_L respectively. Our findings may serve as directive to select the materials in a range of composition to examine the bowing parameters and thereby effective mass experimentally for the Ga_xAl_1-xSb alloys.     

Evolution of the electronic structures of NixSiy ordered systems: experimental and theoretical investigations

We study the change of the electronic structures of nickel silicides, Ni₃Si and NiSi₂, as well as nickel and silicon through the evolution of their valence band X-ray photoelectron spectra (v-XPS) both experimentally and theoretically. The experimental spectra are compared to the total and partial densities of states using tight-binding linear muffin-tin orbital method (TB-LMTO) in the atomic sphere approximation (ASA). Good agreement is found between theory and experiment. Received 10 July 2000     

Electronic structure of barium hexaboride

The electronic structure and the Fermi surface BaB₆ are studied as a function of doping in a conventional bandstructure approach based on the local density approximation to density functional theory (LDA). In particular it is shown that magnetic breakthrough can occur between the two sheets of the Fermi surface disconnected by electron-hole mixing and the spin-orbit interaction. It is further suggested that angle-resolved photoemission will see a single ellipse in the (100) plane for certain doping values, due to the finite energy/momentum resolution of the method. Transport properties, computed within Bloch-Boltzmann theory, are also presented. The bare plasma energy is found to be 0.6 eV in the stoichiometric compound. Received 23 May 2000     

Soft norm-conserving pseudopotential for carbon

A soft, norm-conserving pseudopotential for carbon is presented and its performance tested by calculations on atomic states and on diamond: electronic energy levels of different atomic configurations, equilibrium lattice constant, bulk modulus, and the TA(X) frozen-phonon frequency are accurately reproduced. Convergence of the total energy of diamond with the size of the plane-wave basis set is compared for several frequently used pseudopotentials, and it is shown that calculations with the reported pseudopotential can be performed at considerably lower cost than with the other norm-conserving pseudopotentials, without loosing the accuracy of the latter in predicting structural and dynamical properties. The rapid convergence of the results with the plane-wave cutoff is comparable to the performance of the Vanderbilt's ultrasoft pseudopotentials. The transferability of the pseudopotential to other electronic configurations is discussed. Received 8 September 1999     

First-principles electronic structure of rare-earth arsenides

The electronic properties of rare-earth arsenides have been calculated from first principles. In the calculations we have treated the rare-earth f electrons both as core-like and as valence-like electrons. We consider the changes in the energy bands and in the density of states near the Fermi level which are found to be relevant, except for the case of LuAs, and discuss this in relation with the role played from the rare-earth 5d derived states. Moreover we show that the rare-earth 5d related bands are particularly sensitive to the variation of the lattice constant; change in the lattice constant of less than 1% leads to a different behaviour with respect to the crossing of the rare-earth 5d derived bands and the As 4p derived bands along the Δ-direction. This point is discussed in connection with the possibility of having a semimetal-semiconductor transition in the rare-earth arsenides. Received 22 February 2001     

Spin density distribution of the conduction electrons in diperylene hexafluorophosphate analysed by high-resolution NMR

High-resolution ¹³C nuclear magnetic resonance with ¹H cross polarization and ¹H decoupling under magic angle spinning is measured for the quasi-one dimensional organic conductor diperylene hexafluorophosphate (including tetrahydrofurane solvent molecules) at temperatures between 160 K and 270 K. Ab initio molecular orbital calculations are used for chemical shift analysis and for assignment of Knight shifted lines and individual carbon positions. The coexistence of neutral perylene molecules and perylene radicals in the same radical cation salt is revealed. From Knight and chemical shifts we were able to distinguish two inequivalent perylene radicals within the conducting stack. The spin density distribution of the molecular electronic wave function is determined quantitatively for these radicals. Received 29 June 1999 and Received in final form 4 November 1999     

On the origin of surface states in a correlated local-moment film

The electronic quasiparticle structure of a ferromagnetic local moment film is investigated within the framework of the s-f model. For the special case of a single electron in an otherwise empty energy band being exchange coupled to a fully ordered localised spin system the problem can be solved exactly and, for the spin- electron, some marked correlation effects can be found. We extend our model to incorporate the influence of the surface on the electronic structure. Therefore we modify the hopping integrals in the vicinity of the surface. This leads to the existence of surface states, both for the spin- and the spin- spectral density of states. The interplay between the modification of the hopping integrals and the existence of surface states and correlation effects is discussed in detail. Received: 22 September 1997 / Revised: 12 December 1997 / Accepted: 15 December 1997     

Electronic structure of LiMnO : X-ray emission and photoelectron spectra and band structure calculations

The electronic structure of LiMnO₂ and Li₂MnO₃ was studied by means of X-ray photoelectron and soft X-ray emission spectroscopy. For LiMnO₂, LSDA and LSDA+U calculations were carried out. The LSDA+U calculations are in rather good agreement with the measured valence-band structure as well as with the magnetic and electrical properties of LiMnO₂. It is shown that the band gap in LiMnO₂ is determined by the charge-transfer effect. Received 15 March 1999 and Received in final form 14 July 1999     

Electronic structural and magnetic properties of cadmium chalcogenide beryllosilicate sodalites

Density functional self-consistent spin-polarized calculations with the Discrete Variational Method were performed to obtain the electronic structures of cadmium chalcogenide beryllosilicate sodalites Cd₈[BeSiO₄]₆Z₂ and their bulk materials CdZ (Z = S, Se, Te). Similar as their bulk materials, these sodalites are also semiconductors. From their density of states and charge distributions, it was found that there is greater localization of the electron density in the sodalites compared with that of the bulk. The band gaps in their bulk compounds are different from those in the sodalite structures due to the electrons of Si, Be and O filled in. Different from their bulk materials, the d electrons of Cd play an important role to form an unseparated valence band with all orbitals of other elements. The calculated magnetic moments of these three materials are very small since all d atomic orbitals are fully occupied. Received: 28 March 1997 / Revised: 18 July 1997 / Accepted: 9 September 1997     

Understanding of surface states in a correlated electron system

The aim of this work is to find a simple analytic model to explain some principal aspects of the behavior of surface states in correlated electron systems. We start from the analytic expression for the Green function of the semi-infinite tight binding linear chain. This Green function in case of modification of the center of gravity of the first atom and the change in coupling between the first and the second atom is evaluated as an exact analytic expression. Conditions for the existence and classification of surface states are given. The spectral weight of surface states and the local density of states are evaluated. The method is applied to a s.c. (100) surface of a local moment crystal. Conditions for the existence of surface states are derived and their locations in the Brillouin zone are predicted. It is shown that it is possible to include correlation effects within the framework of the discussed model. The comparison with former numerical results is performed. Received 8 March 2000     

Electronic structure and lattice properties of zinc-blende InN under high pressure

Dependencies of electronic structure and lattice properties of InN with zinc-blende structure on hydrostatic pressure are presented based on band structures computed using the empirical pseudopotential method. The pressure behavior of the pseudopotential form factors have been analyzed. The effect of pressure on the density of states has been examined. Trends in bonding and ionicity under pressure are also discussed. Our results show as well that the absolute value of the Fourier transform of the valence charge density might be useful in the prediction of the phase transition in zinc-blende materials. Received 25 May 2001 and Received in final form 16 January 2002     

A first principles study of cubic IrO<Subscript>2</Subscript> polymorph

We have studied structural, thermodynamic, elastic, and electronic properties of cubic IrO₂ polymorph via ab initio calculations within the LDA and GGA approximations. Basic physical properties, such as lattice constant, bulk modulus, second-order elastic constants (C_ij), and the electronic band structures are calculated, and compared with available experimental values. We have, also, predicted the Young's modulus, Poison's ratio (ν), Anisotropy factor (A), sound velocities, and Debye temperature.     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Band gaps and charge distribution in quasi-binary (GaSb) (InAs) crystals

Pseudopotential investigation of energy band gaps and charge distribution in quasi-binary (GaSb)_1-x(InAs)_x crystals has been reported. To the best of our knowledge, there had been no reported theoretical work on these materials. In agreement with experiment, the quasi-binary crystals of interest showed a significant narrowing of the optical band gap compared to the conventional Ga_xIn_1-xAs_ySb_1-y quaternary alloys (with x = 1 - y). Moreover, the absorption at the optical gaps indicated that (GaSb)_1-x(InAs)_x is a direct Γ to Γ band-gap semiconductor within a whole range of the x composition. The information derived from the present study predicts that the band gaps cross very important technological spectral regions and could be useful for thermophotovoltaic applications. Received 30 August 2002 Published online 1st April 2003 RID="a" ID="a"Present address: Physics Department, University of M'sila, 28000 M'sila, Algeria e-mail: N_Bouarissa@yahoo.fr     

Born effective charge reversal and metallic threshold state at a band insulator-Mott insulator transition

We study the quantum phase transition between a band (“ionic”) insulator and a Mott-Hubbard insulator, realized at a critical value in a bipartite Hubbard model with two inequivalent sites, whose on-site energies differ by an offset . The study is carried out both in D=1 and D=2 (square and honeycomb lattices), using exact Lanczos diagonalization, finite-size scaling, and Berry's phase calculations of the polarization. The Born effective charge jump from positive infinity to negative infinity previously discovered in D=1 by Resta and Sorella is confirmed to be directly connected with the transition from the band insulator to the Mott insulating state, in agreement with recent work of Ortiz et al. In addition, symmetry is analysed, and the transition is found to be associated with a reversal of inversion symmetry in the ground state, of magnetic origin. We also study the D=1 excitation spectrum by Lanczos diagonalization and finite-size scaling. Not only the spin gap closes at the transition, consistent with the magnetic nature of the Mott state, but also the charge gap closes, so that the intermediate state between the two insulators appears to be metallic. This finding, rationalized within Hartree-Fock as due to a sign change of the effective on-site energy offset for the minority spin electrons, underlines the profound difference between the two insulators. The band-to-Mott insulator transition is also studied and found in the same model in D=2. There too we find an associated, although weaker, polarization anomaly, with some differences between square and honeycomb lattices. The honeycomb lattice, which does not possess an inversion symmetry, is used to demonstrate the possibility of an inverted piezoelectric effect in this kind of ionic Mott insulator. Received 21 May 1999     

Aharonov-Bohm effect in one-channel weakly disordered rings

A new diagrammatic method, which is a reformulation of Berezinskii's technique, is constructed to study the density of electronic states of a one-channel weakly disordered ring, threaded by an external magnetic flux. The exact result obtained for the density of states shows an oscillation of with a period of the flux quantum . As the sample length (or the impurity concentration) is reduced, a transition takes place from the weak localization regime () to the ballistic one (). The analytical expression for the density of states shows the exact dependence of on the ring's circumference and on disorder strength for both regimes. Received 27 December 1999     

Phase evolution of layered cobalt oxides versus varying corrugation of the cobalt-oxygen basal plane

A general spin-state model and a qualitative physical picture have been proposed for a class of lately synthesized layered cobalt oxides (LCOs) by means of density functional calculations. As the plane corrugation of the cobalt-oxygen layer decreases, the LCOs evolve from a high-spin (HS) superexchange-coupled antiferromagnetic (AFM) insulator to an almost-HS AFM/ferromagnetic (FM) competing system where the FM coupling is mediated via the p-d exchange by an increasing amount of delocalized pdσ holes having mainly the planar O 2p character. It is tentatively suggested that the delocalized holes more than 0.3 per CoO₂ basal square are likely necessary for the insulator-metal and/or AFM-FM transitions in the corrugation-weakened LCOs. A phase control may be realized in LCOs by varying the plane corrugation (thus modifying the hole concentration) through an ionic-size change of the neighboring layers on both sides of the cobalt-oxygen layer. In addition, a few experiments are suggested for a check of the present model and picture. Received 28 July 2002 / Received in final form 10 October 2002 Published online 31 December 2002     

Influence of vacancies on the electronic structure of Co ZrSn Heusler alloys

The electronic structure of the Co_2-xZrSn Heusler alloys has been studied by X-ray photoelectron spectroscopy (XPS). XPS valence band spectra can be compared with ab initio electronic structure calculations using the linearized muffin-tin orbital (LMTO) method. The calculated magnetic moments per Co atom agree well with the moments obtained from experiment. The LMTO calculations also show the energy shifts of the Co, Zr and Sn valence electron states towards the Fermi level when the concentration of vacancies increases in these alloys. Received 9 March 1999 and Received in final form 6 May 1999     

Ab initio calculation of Electron Energy Loss spectra of clean and 1ML Fe-covered Ni(111)

The dielectric and the loss functions of Ni(111) are calculated by the Full Potential Linear Muffin Tin Orbitals (FPLMTO) ab initio method whithin the three-layers model (vacuum/surface/bulk). Particular attention is devoted to determine surface and bulk state contributions to the spectra. Good agreement is found with recent experimental EELS data on the Fe-covered Ni(111) surface. Received 10 July 2002 Published online 19 November 2002