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     

Electronic structure and magnetism of disordered bcc Fe alloys

We study the electronic structure and magnetic properties of disordered bcc Co_xFe_1-x, Cr_xFe_1-x and Mn_xFe_1-x alloys in their ferromagnetic phases using the Augmented Space Recursion (ASR) technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. We calculate the density of states and magnetic moment of these alloys to show the variation upon alloying Fe with the other neighbouring 3d transition metals using arguments based on charge transfer, exchange splitting and hybridization effects. Received 10 April 2001 and Received in final form 15 August 2001     

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     

Electronic structure of LaNi and its hydride LaNi H

The electronic structure of LaNi₅ and its hydride LaNi₅H₇ are obtained using the self-consistent cluster-embedding calculation method. The Fermi level of LaNi₅H₇ is 5.172 eV higher than that of LaNi₅. In both materials, the La 5d electrons locate nearby the Fermi levels, and make only a small contribution to the density of states (DOS) of the valence bands. There is no significant charge transfer from La to Ni in LaNi₅. But for LaNi₅H₇, there is a charge transfer of 1.16 electrons from La to H, and H atoms are combined mainly with Ni to form hybridized orbitals in the energy regions far below the Fermi level. An explanation of hydrogenation of LaNi₅ is proposed: It is easy for hydrogens to take off some localized La 5 d electrons near the Fermi level, and combine with Ni to form hybridized orbitals in lower energy regions. This process is therefore in favor of energy, and forces a lattice expansion until the Fermi level rises to zero. Received 13 July 2001 / Received in final form 10 December 2001 Published online 17 September 2002     

Oxygen K-edge in vanadium oxides: simulations and experiments

Band-structure (BS) calculations of the density of states (DOS) using the full potential augmented plane waves code WIEN97 were performed on the four single-valence vanadium oxides VO, V₂O₃, VO₂ and V₂O₅. The DOS are discussed with respect to the distortions of the VO₆ octahedra, the oxidation states of vanadium and the orbital hybridisations of oxygen atoms. The simulated oxygen K-edge fine structures (ELNES) calculated with the TELNES program were compared with experimental results obtained by electron energy-loss spectrometry (EELS), showing good agreement. We show that changes in the fine structures of the investigated vanadium oxides mainly result from changes in the O-p DOS and not from the shift of the DOS according to a rigid band model. Received 17 December 2001 / Received in final form 19 June 2002 Published online 13 August 2002     

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     

Electrical resistivity of the Ti\mathsf{_4}O\mathsf{_7} Magneli phase under high pressure

The electronic and positronic properties of the pentanary semiconductor alloys Ga_xIn_1-xP_ySb_zAs_1-y-z lattice matched to GaSb have been studied. The electron wave function is calculated semiempirically using the pseudopotential band model under the virtual crystal approximation. The positron wave function is evaluated under the point core approximation for the ionic potential. Electronic and positronic quantities namely, electronic structure and band gaps, positron band structure, effective mass and affinity, and electron-positron momentum densities have been predicted and their dependence on the phosphorus composition has been discussed. Received 30 August 2002 / Received in final form 12 February 2003 Published online 24 April 2003     

Observation of magnetic splitting in XPS MnL-spectra of Co2MnSn and Pd2MnSn Heusler alloys

The XPS MnL-spectra of Co₂MnSn, a nearly half-metallic ferromagnet (HMF) and Pd₂MnSn were investigated. The most drastical feature of the spectra observed is the well-defined magnetic splitting of the Mn 2p _3/2 , 2p _1/2 lines. This gives direct evidence of the existence of well-defined local magnetic moments in Heusler alloys in comparison with other itinerant-electron ferromagnets. The calculations of Mn2p XPS spectra of these materials were carried out using a fully relativistic generalization of the one-step model of photoemission and show excellent agreement with experiment. Received: 31 October 1997 / Received in final form: 28 November 1997 / Accepted: 18 December 1997     

Magnetic reconstructions at the surface of the B2 FeV alloy

We present an ab initio study of the magnetic surface reconstructions of the B2 FeV alloy using a self-consistent tight-binding linearized muffin tin orbital method developed in the atomic spheres approximation. For (001) and (111), the surface reconstruction stabilizes configurations unstable in the bulk alloy. When Fe is at the (001) surface, a c(2×2) in-plane antiferromagnetic order is found to be the ground state with magnetic moments of -2.32 and 2.27. A p(1×1) ↓ ferromagnetic order is displayed in case of V toplayer with a magnetic moment of -1.83. At the (111) surface, we obtain for Fe toplayer two solutions p(1×1)↑ and p(2×1). The configuration p(1×1)↑ is found to be the ground state with a magnetic moment per atom of 2.34. For V toplayer, only the p(1×1) ↓ solution is obtained with a moment of -0.84. In all cases, the Fe-V coupling is always antiparallel like in the bulk. Our results are discussed and compared to experiments. Received 11 August 2000 and Received in final form 8 June 2001     

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     

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     

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     

A density functional study of plutonium dioxide

The electronic and geometric structures of bulk PuO₂ and its (110) surface have been studied using a periodic model within the generalized gradient approximation (GGA) of density functional theory (DFT). The sixty core electrons of the Pu atom have been represented by a relativistic effective core potential and scalar relativistic effects have been incorporated on the valence orbitals. For bulk PuO₂, we predict an equilibrium lattice constant of 10.10 a.u. and a cohesive energy of 17.28 eV, in good agreement with experimental data. For the (110) surface, upon relaxation, the distance between the top layer and the next layer is found to decrease by 0.12 ?, i.e. 5.3% of the corresponding interlayer distance in the bulk. The distance between the two oxygen atoms on the top layer is found to increase by 0.15 ?, i.e. 5.6% of the corresponding bulk value. The small surface relaxation energy of 0.268 eV per unit cell indicates the fair stability of this surface. The effective charges on Pu and O atoms show that the chemical bonding in this system is not purely ionic. Together with the metallic feature of the density of states (DOS) on the surface, the effective charge distribution provides some basis for understanding surface reactivity and corresponding support for catalysis. Received 16 June 2000     

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     

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     

The electronic properties of SiCAlN quaternary compounds

We have investigated the properties of SiCAlN quaternary compounds composed of SiC and AlN polytypes by first-principle calculations. We find that their band gaps have a large tunability and are sensitive to the polytype structures. Their electronic properties vary from wide-band-gap semiconducting to metallic due to the complex charge transfer between the two constituents SiC and AlN. The formation energies are also calculated. These results show SiCAlN quaternary compounds have potential applications in the electronic devices that can be tuned over a large wavelength range.     

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.     

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     

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     

Optical investigation of the quasi two-dimensional monophosphate tungsten bronzes K x P 4 W 8 O 32 (x = 0 - 1.57)

The potassium doped monophosphate tungsten bronzes K_xP₄W₈O₃₂ are two-dimensional metals which show a metal-to-metal transition at a critical temperature which depends on the doping level. The metal-to-metal transition is accompanied by the formation of a commensurate charge density wave with wave vector (π/b,0) which is independent of the doping level. Undoped P₄W₈O₃₂, on the other hand, has two metal-to-metal transitions which are connected to the formation of incommensurate charge density waves. We measured the infrared reflectivity of the series K_xP₄W₈O₃₂ (x = 0 - 1.57) in the spectral range from 100 to 10 000 cm^-1 for room temperature and well below the critical temperature. Polarization-dependent infrared spectra find a two-dimensional behavior in the normal and the charge density wave state and show signatures of hybridization between one- and two-dimensional conduction bands. In undoped P₄W₈O₃₂ the essentials of the charge density wave state can be understood from the nesting vectors of the calculated Fermi surface and two gaps are observed in the infrared spectra. The gap sizes are a factor of about 2.5 bigger than the predictions from mean-field theory in the weak-coupling limit which suggests medium- or strong electron-phonon coupling. For potassium doped K_xP₄W₈O₃₂ one gap is observed in the charge density wave state. The energetics of the charge density formation may be dominated by the energy required for the lattice modulation. Received 27 April 2001 and Received in final form 21 September 2001