Pseudogaps in the three-band Hubbard model

Using the strong coupling diagram technique, the energy spectrum of the three-bandHubbard model is investigated. In these calculations, the series in powers of thecopper-oxygen hybridization for the irreducible part is approximated by two lowest-orderterms. For parameters of hole-doped cuprates the calculated energy spectrum consists oflower and upper Hubbard subbands of predominantly copper nature, oxygen bands with someadmixture of copper states and the Zhang-Rice states of mixed nature. The spectrumcontains two pseudogaps, the lower of which separates the Hubbard subband from Zhang-Riceand oxygen bands. The pseudogaps arise due to multiple reabsorption of carriers in stateswith double occupancy of sites by holes or electrons.     

Ultraviolet photoemission spectroscopy of (TaSe4)2I

The vacuum ultraviolet photoemission spectra of quasi-one-dimensional charge density wave ( CDW ) system, (TaSe₄)₂I, were measured for photon energies between 32 and 100 eV at room temperature ( in the normal phase ) and at about 100 K ( in the CDW phase ). The spectrum of Ta 4f core-levels has shown no additional splitting due to the two different Ta sites. The spectra of the valence and conduction bands have revealed the resonant enhancement for the excitation of the Ta 5p core states, which demonstrates the remarkable hybridization of Ta 5d orbitals with Se 4p orbitals with binding energies smaller than 4 eV. In the CDW phase, the partial cross section decreases for both Ta 5d bands and Se 4p bands with Ta 5d components.     

The charge transfer via a double helix DNA

This paper reports a double helix model of charge transfer in DNA molecule. The calculated results show that the transmission characteristics of DNA are not only related to the longitudinal transfer but also to the helicity of molecule. It finds that there are four transmission bands centred at 0.92, 3.07, 7.75 and 8.87eV, also the width and intensity of bands corresponding to the helix direction are less than that of the longitudinal direction. With the increase of hopping energy, transmission bands become wide so that the transmission ability is enhanced.     

THE BOUND STATES AROUND THE DOPANTS I N POLYACETYLENE

The electron bound states caused by dopants in plyacetylene are investigated. When the dopant sits nesr the double bond, there will be two bound states in the gap for positive U and outside the bands for negative U. Whereas when the dopant is absorbed at the single bond, four bound states are formed, two of them in the gap and the other too outside the bands.     

Cesium-filled single wall carbon nanotubes as conducting nanowires: scanning tunneling spectroscopy study

Metal-filled single wall carbon nanotubes (SWCNTs) are examined for possible application to conducting wires in nanoelectronics architecture. The local electronic structure of SWCNTs partially filled with cesium atoms is studied with scanning tunneling spectroscopy. The conduction and valence bands are shifted downward with two localized states in the gap at the location where the Cs atoms are filled. From a first-principles calculation, we confirm that these two gap states are bound states originating from the two lowest conduction bands.     

Role of defects on the electronic and magnetic properties of CrAs, CrSe and CrSb zinc-blende compounds

We present an extended study of single impurity atoms and atomic swaps in half-metallic CrAs, CrSb and CrSe zinc-blende compounds. Although the perfect alloys present a rather large gap in the minority-spin band, all defects under study, with the exception of void impurities at Cr and sp sites and Cr impurities at sp sites (as long as no swap occurs), induce new states within the gap. The Fermi level can be pinned within these new minority states depending on the lattice constant used for the calculations and the electronegativity of the sp atoms. Although these impurity states are localized in space around the impurity atoms and very fast we regain the bulk behavior, their interaction can lead to wide bands within the gap and thus loss of the half-metallic character.     

Quasibound states in the continuum in a two channel quantum wire with an adatom

We report the prediction of quasibound states (resonant states with very long lifetimes) that occur in the eigenvalue continuum of propagating states for certain systems in which the continuum is formed by two overlapping energy bands. We illustrate this effect using a quantum wire system with two channels and an attached adatom. When the energy bands of the two channels overlap, a would-be bound state that lays just below the upper energy band is slightly destabilized by the lower energy band and thereby becomes a resonant state with a very long lifetime (a second such state lays above the lower energy band). Unlike the bound states in continuum predicted by von Neumann and Wigner, these states occur for a wide region of parameter space.     

Chiral bands, dynamical spontaneous symmetry breaking, and the selection rule for electromagnetic transitions in the chiral geometry

A model for a special configuration in triaxial odd-odd nuclei is constructed which exhibits degenerate chiral bands with a sizable rotation, a manifestation of dynamical spontaneous symmetry breaking. A quantum number obtained from the invariance of the model Hamiltonian, which characterizes observable states, is given and selection rules for electromagnetic transition probabilities in chiral bands is derived in terms of this quantum number. The degeneracy of the lowest two bands is indeed obtained in the numerical diagonalization of the Hamiltonian at an intermediate spin range, over which electromagnetic transitions follow exactly the selection rule expected for the chiral geometry.     

Scaling and the center-of-band anomaly in a one-dimensional Anderson model with diagonal disorder

We resolve the problem of the violation of single parameter scaling at the zero energy of the Anderson tight-binding model with diagonal disorder. It follows from the symmetry properties of the tight-binding Hamiltonian that this spectral point is, in fact, a boundary between two adjacent bands. The states in the vicinity of this energy behave similarly to states at other band boundaries, which are known to violate single parameter scaling.     

Local spectroscopy of two-dimensional states in Na films on Cu(111)

Alkali overlayers on noble metal surfaces exhibit two-dimensional electronic states. We investigated the electronic structure of Na on Cu(111) at T=5 K with scanning tunneling spectroscopy. In agreement with photoelectron spectroscopy we found two electronic bands close to the Fermi energy which correspond to quantum well states (QWS). In second monolayer islands, confinement of the QWS was observed.     

Effect of gamma softness on the stability of chiral geometry: spectroscopy of 106Ag

A study of the nucleus 106Ag has revealed the presence of two strongly coupled negative-parity rotational bands up to the 19- and 20- states, respectively, which cross each other at spin I approximately 14. The data suggest that near the crossover point the bands correspond to different shapes, which is different to the behavior expected from a pair of chiral bands. Inspection of the properties of these bands indicates a triaxial and a planar nature of rotation for the two structures. Possible causes for this may be understood in terms of a shape transformation resulting from the large degree of gamma softness of 106Ag. These data, along with the systematics of the odd-odd structures in the mass 100 region, suggest that gamma softness has marked implications for the phenomenon of nuclear chirality.     

Theoretical Electronic Structure of Oxide BaBiO3

In this paper, we present a model simulation to understand the electronic structure of BaBiO₃. By treating two types of Bi sites with different valence configurations (6s² 6p³ 6d⁰ and 7s⁰6p³6d⁰),t he valence instability in BaBiO₃ is simulated. We shorv that an LMTO-ASA calculation can then give a semiconducting feature for this material and a good estimation of the gap, which is 1.6 eV from the present model compared with 2.0 eV from experiments. The existence of Bi³⁺ sites reduces greatly the overlapping integrals between Bi_6s-O_2p states and reduces correspondingly their band widths. The band gap appears because the antibonding bands of Bi_6s-O_2p now separate from the rest bands of Bi-O_2p.     

LuI3 闪烁晶体的第一性原理研究

采用基于第一性原理的赝势和平面波方法计算了新型闪烁晶体基质材料 LuI₃的结构特性和电子特性. 计算结果表明: -4.4 eV附近有一个宽度约为0.2 eV的窄带, 主要是Lu的4f态; -3.55–0 eV之间的态组成了价带, 这主要是I的5p态; 2.44–12.35 eV之间的态组成了LuI₃的导带, 这主要来源于Lu的5d态, 其中还含有少量的Lu的6s态的贡献. 在-3.46 eV处, Lu的6s态、4f态和I的5p态同时出现了尖峰, 说明相邻的Lu原子的6s态, 4f态与I原子的5p态之间的相互作用强, 出现了杂化峰. 估算出LuI₃晶体的理论光产额约为100000 ph/MeV, 主要得益于LuI₃合适的带隙和能带结构. 关键词： 3')" href="#">LuI₃ 第一性原理 电子结构     

Electronic structure and local interactions on a S(100) 2×1 surface with submonolayer Ba overlayers

The electronic structure and ionization energy for the system Ba/Si(100)2×1 have been studied as functions of the submonolayer coverage. It is found that there is an energy gap in the surface states spectrum and that the Ba/Si(100)2×1 interface is semiconducting up to 1.5 monolayers of Ba. Two surface bands induced by Ba adsorption have been detected. The evolution of the spectrum with increasing degree of Ba coverage points to the existence of two nonequivalent “adsorption sites,” which differ in binding energy by 0.11 eV. The development of the Ba-induced bands is found to terminate at a coverage corresponding to the minimum ionization energy and close to one monolayer. The adsorption bond is shown to have a primarily covalent character. Zh. éksp. Teor. Fiz. 114, 2145–2152 (December 1998)     

Resonance Raman scattering by localized and resonant modes of Ag2 molecules in rare gas matrices

We demonstrate for the first time that excitation of silver containing rare gas matrices at 406.74 nm results in resonance Raman spectra which show low energy localized and resonant modes. As they combine with the Ag₂ stretching vibration and its overtones, they can unambiguously be attributed to the Ag₂ molecule. In Xe-matrices the coupling of lattice phonons and impurity vibrations is documented by side bands which resemble the phonon density of states of the host lattice. Two trapping sites are observed in Kr-matrices.     

拓扑石墨烯电极分子器件输运特性

利用密度泛函理论结合非平衡格林函数方法,研究了不同拓扑能带结构的石墨烯电极分子器件输运特性.结果表明器件导通电压与电极禁带宽度正相关,同时器件在输运过程中表现出负微分电阻特性,峰谷电流比可达2697.分析认为器件导通源自于偏压升高过程中两电极能带匹配.器件负微分电阻特性源自于偏压升高过程中两电极能带交错.散射态分析表明,能带匹配后散射态分布较为离域,有利于电子通过器件.能带交错后散射态局域于电极处,表明电子输运受到抑制.     

Flat bands and Wigner crystallization in the honeycomb optical lattice

We study the ground states of cold atoms in the tight-binding bands built from p orbitals on a two dimensional honeycomb optical lattice. The band structure includes two completely flat bands. Exact many-body ground states with on-site repulsion can be found at low particle densities, for both fermions and bosons. We find crystalline order at n=1/6 with a sqrt[3] x sqrt[3] structure breaking a number of discrete lattice symmetries. In fermionic systems, if the repulsion is strong enough, we find the bonding strength becomes dimerized at n=1/2. Experimental signatures of crystalline order can be detected through the noise correlations in time of flight experiments.     

Full inclusion of symmetry in constructing Wannier functions: Chemical bonding in MgO and TiO<Subscript>2</Subscript> crystals

Chemical bonding in MgO and TiO₂ crystals is analyzed using a minimal basis consisting of atomic-type Wannier functions (AWFs) centered at atomic sites in the crystal and constructed from Bloch states of the energy bands originating from the valence states of the atoms. A method proposed earlier for constructing Wannier functions is improved. Symmetrization of the initial Bloch function basis and symmetrical orthogonalization of the generalized Bloch functions greatly reduces computational effort. The prior symmetrization of the Bloch function basis is of fundamental importance in constructing AWFs, because the latter functions have to be centered at the atomic sites and possess the symmetry of the atomic functions in the crystal. The principle that should be followed in selecting the conduction bands originating from the valence states of the atoms of the crystal is formulated. The Bloch functions are calculated using the LCAO approximation within the Hartree-Fock method and density-functional theory. Using the calculated Bloch functions, a minimal valence AWF basis is constructed and calculations of the local characteristics of the electronic structure (atomic charge, bond order, atomic valence) are performed for the MgO and TiO₂ crystals. According to the analysis performed, the covalent component of the chemical bonding in the MgO crystal is negligibly small and TiO₂ is a mixed ionic and covalent crystal with pronounced covalent bonding.     

Effect of temperature and magnetic field on disorder in semiconductor structures

We present the results of consistent theoretical analysis of various factors that may lead to influence of temperature and external magnetic field on disorder in semiconductor structures. Main attention is paid to quantum well (QW) structures in which only QWs or both QW and barriers are doped (the doping level is assumed to be close to the value corresponding to the metal–insulator transition). The above factors include (i) ionization of localized states to the region of delocalized states above the mobility edge, which is presumed to exist in the impurity band; (ii) the coexistence in the upper and lower Hubbard bands (upon doping of QWs as well as barriers); in this case, in particular, the external magnetic field determines the relative contribution of the upper Hubbard band due to spin correlations at doubly filled sites; and (iii) the contribution of the exchange interaction at pairs of sites, in which the external magnetic field can affect the relation between ferromagnetic and antiferromagnetic configurations. All these factors, which affect the structure and degree of disorder, lead to specific features in the temperature dependence of resistivity and determine specific features of the magnetoresistance. Our conclusions are compared with available experimental data.