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 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     

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     

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 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     

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     

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     

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     

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     

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 of spin-chain compounds: common features

The incommensurate composite systems M₁₄Cu₂₄O₄₁ (M = Ca, Sr, La) are based on two fundamental structural units: CuO₂ chains and Cu₂O₃ ladders. We present electronic structure calculations within density functional theory in order to address the interrelations between chains and ladders. The calculations account for the details of the crystal structure by means of a unit cell comprising 10 chain and 7 ladder units. It turns out that chains and ladders can be treated independently, which allows us to introduce a model system based on a reduced unit cell. For the CuO₂ chains, we find two characteristic bands at the Fermi energy. Tight binding fits yield nearest and next-nearest neighbour interactions of the same order of magnitude.     

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     

Viable improvement of the full potential linearized augmented plane wave (FLAPW) method

A characteristic feature of the Full Potential Linearized Augmented Plane Wave (FLAPW)-method consists in the spatial subdivision of the charge density analogous to that of the one-particle wavefunctions, i.e. into a portion that is expanded in terms of spherical harmonics Y_lm inside the muffin-tin spheres and into a plane wave expansion of the interstitial charge density. To obtain the Hartree potential inside the spheres one is hence forced to solve a boundary value problem at the sphere surface. In addition, in all non-equivalent spheres each (l,m)-component of the charge density is mapped onto 300-400 radial grid points. To ensure an accelerated convergence of the calculation, the pertinent schemes require this rather large data set to be stored and mixed within 3-6 iteration steps. We show and illustrate for the example of a spin-polarized Ni-film with and without an oxygen overlayer and for bulk Si that this data set can be compressed by at least two orders of magnitude if one partitions the charge density in a different way so that the relevant portion determining the interatomic bonding can be Fourier expanded throughout the lattice cell. One thereby arrives at a modified FLAPW-scheme that combines favorable features of the original method with virtues of the pseudopotential method which consist in the simple construction of the Hartree potential and the efficient way of achieving self-consistency. These advantages can be exploited to the fullest by using Fast Fourier Transform. Moreover, forces that atoms experience in off-equilibrium positions attain a particularly simple form in terms of the charge density expansion coefficients. Received: 31 May 1997 / Revised: 12 December 1997 / Accepted: 18 December 1997     

Photoreflectance characterization of InAs/GaAs self-assembled quantum dots grown by ALMBE

We report on a photoreflectance investigation in the 0.8-1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. We observed clear and well-resolved structures, which we attribute to the optical response of different QD families. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and discussed considering vertical coupling between dots of the same column. It is shown that Coulomb interaction can account for the observed optical response of QD families with different morphology coexisting in the same sample. Received 17 November 1999     

Geometrical derivation of a new ground state formula for the n-electron Friedel resonance model

The n-electron ground state of the Friedel resonance model can be written as a single Slater determinant of n s-electrons plus d-electron-s-hole companion. This new formula is derived geometrically in the Hilbert space. The derivation uses the fact that a n-electron Slater determinant, built from N band states, corresponds to a n-dimensional subspace in the N-dimensional Hilbert space. Received: 4 November 1997 / Accepted: 19 November 1997     

Magneto-electronic properties of the AA- and ABC-stacked graphites

In this study, we apply the tight-binding method to magneto-electronic properties of the AA- and ABC-stacked graphites, which are strongly dependent on the interlayer interactions, the magnetic field, and the stacking sequences. First of all, the interlayer interactions induce the significant changes in the energy dispersions, the band symmetry about the Fermi level, the overlap between valence and conduction bands, the band width, and the band-edge states or the symmetry points. Then, the magnetic field induces the Peierls phase in the Bloch functions and thus strongly affects the energy dispersions of the Landau Levels, the subband spacings, the energy width, and the special structures in density of states (DOS). Finally, the stacking sequences dominates over the low-energy band overlap and the anisotropy of energy bands. The effects mentioned above are exactly reflected in the density of state. Here, DOS exhibits the 3D, 2D, and 1D characteristics.     

Multi-orbital Anderson models and the Kondo effect: a NCA study enhanced by vertex corrections

The low energy region of certain transition metal compounds reveals dramatic correlation effects between electrons, which can be studied by photoelectron spectroscopy. Theoretical investigations are often based on multi-orbital impurity models, which exhibit modified versions of the Kondo effect. We present a systematic study of a multi-orbital Anderson-like model, based on a new semi-analytical impurity solver which goes beyond simple modifications of the well known NCA. We discuss one-particle excitation spectra and in particular the role of level positions and Coulomb-matrix elements. It is shown that the low-energy region as well as the overall features of spectra critically depend on the model parameters and on the quality of the approximations used. Recent photoelectron experiments and corresponding existing calculations are put into perspective. An interesting crossover scenario between different regimes of ground states with characteristically different local correlations is uncovered.     

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     

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     

Electronic structure of GaxIn1-xAsySb1-y quaternary alloy by recursion method

Summary  This work reports the electronic structure of GaInAsSb quaternary alloy by recursion method. A five-orbital sp₃s^* per atom model was used in the tight-binding representation of the Hamiltonian. The local density of states (LDOS), integrated density of states (IDOS) and structural energy (ST.E) were calculated for Ga, In, As and Sb sites in Ga_0.5 In_0.5 As_0.5 Sb_0.5 and GaInAsSb lattice matched to GaAs and the same alloy lattice matched to GaSb. There are 216 atoms in our cluster arranged in a zincblend structure. The results are in good agreement with available information about the alloy.