Origin of ferromagnetism and its pressure and doping dependence in Tl2Mn2O7

Using the Nth order muffin-tin obital downfolding technique, we investigate the origin of ferromagnetism in pyrochlore Tl2Mn2O7. It is found to be driven by a hybridization induced spin polarization of delocalized charge carriers derived from Tl-s and O-p states. The mean-field estimate of the ferromagnetic transition temperature Tc estimated using computed exchange integrals are found to be in good agreement with measurements. We find an enhancement of Tc for moderate doping with nonmagnetic Sb and a suppression of Tc upon application of pressure, both in agreement with experimental findings.     

Magnetic and charge orders in zigzag nanographene ribbons

We theoretically study the electronic states in graphene ribbons which are strongly affected by the edge states, the peculiar non-bonding molecular orbitals localized along the zigzag edges of the ribbons. New kinds of edge localized electronic states with spin and charge polarizations are found in the mean field solutions of the extended Hubbard model with onsite and nearest-neighbor Coulomb repulsions. These novel states appear due to the interplay between the edge states and the Fermi instabilities. We also examine the competition between the charge polarized state and the spin polarized state to draw a phase diagram depending on Coulomb parameters. The results obtained by the mean field calculations with the extended Hubbard model modified to include Coulomb integrals provide useful insights to understand and functionalize the nanoscale materials.     

Even and Odd Charge Coherent States: Higher-Order Nonclassical Properties and Generation Scheme

We examine the higher-order nonclassical properties of the even and odd charge coherent states as well as proposing a scheme to generate these states whose modes can freely travel in open space. We show that the even and odd charge coherent states exhibit both higher-order antibunching and higher-order squeezing. While the two-mode higher-order antibunching occurs in any order and essentially depends on the charge number, the two-mode higher-order squeezing appears only in the even orders. We also prove that these states are genuinely entangled, and they can be generated by means of cross-Kerr media, beam splitters, phase shifts and threshold detectors. We find that the fidelity and the corresponding success probability to generate these states are dependent on the correlative parameters.     

Electronic structure of bases in DNA duplexes characterized by resonant photoemission spectroscopy near the Fermi level

The electronic structure of bases in DNA duplexes was investigated by resonant photoemission spectroscopy near the Fermi level, in order to specify charge migration mechanisms. We observed a kinetic energy shift of N-KLL Auger electrons and an intensity enhancement of valence electrons on the resonant photoemission spectra for both poly(dG).poly(dC) and poly(dA).poly(dT) DNAs. These directly show the localized unoccupied states of the bases. We conclude that the charge hopping model is pertinent for electric conduction in a DNA duplex, when electrons pass through the unoccupied states.     

Tuning the effects of Landau level mixing on anisotropic transport in quantum Hall systems

Electron-electron interactions in half-filled high Landau levels in two-dimensional electron gases in a strong perpendicular magnetic field can lead to states with anisotropic longitudinal resistance. This longitudinal resistance is generally believed to arise from broken rotational invariance, which is indicated by charge density wave order in Hartree-Fock calculations. We use the Hartree-Fock approximation to study the influence of externally tuned Landau level mixing on the formation of interaction-induced states that break rotational invariance in two-dimensional electron and hole systems. We focus on the situation when there are two non-interacting states in the vicinity of the Fermi level and construct a Landau theory to study coupled charge density wave order that can occur as interactions are tuned and the filling or mixing are varied. We consider numerically a specific example where mixing is tuned externally through Rashba spin-orbit coupling. We calculate the phase diagram and find the possibility of ordering involving coupled striped or triangular charge density waves in the two levels. Our results may be relevant to recent transport experiments on quantum Hall nematics in which Landau level mixing plays an important role.     

Lattice Green's Function in the General Glasser Case

We have investigated the lattice Green's function for the general Glasser cubic lattice. Expressions for its density of states, phase shift, and scattering cross section in terms of complete elliptic integrals of the first kind are derived.     

The singularities of the integrals in Mayer's ionic solution theory

The terms of the expression previously obtained by Friedman for the cluster integral sum in Mayer's theory are examined as to order in total ion concentration, c, at the limit, c = 0. The order of the singularity at c = 0 is calculated for several of those terms of the irreducible cluster integrals that correspond to graphs of low connectivity but with an arbitrary number of vertices. On the basis of these calculations and two postulates concerning the relation of the singularities of these integrals to others, it is concluded that the terms of the cluster integral sum, when arranged in order of increasing index, are also in increasing order of concentration, and that the first term, κ ³/12π, has a lower order than any other. The order in concentration of the higher terms depends on whether the third moment of the concentration of charge types, σc₈z₈ ³, vanishes, as it does in solutions of electrolytes of symmetrical charge type.     

Analytic integration of the continued fraction expansion of a density of states

We show that the continued fraction expansion of a density of states can be analytically integrated and expressed in terms of simple elementary integrals.     

Charge and spin order on the triangular lattice: NaxCoO2 at x=0.5

The nature of electronic states due to strong correlation and geometric frustration on the triangular lattice is investigated in connection to the unconventional insulating state of NaxCoO2 at x=0.5. We study an extended Hubbard model using a spatially unrestricted Gutzwiller approximation. We find a new class of charge and spin ordered states at x=1/3 and x=0.5 where antiferromagnetic (AFM) frustration is alleviated via weak charge inhomogeneity. At x=0.5, we show that the square root of 3a x 2a off-plane Na dopant order induces weak square root of 3a x 1a charge order in the Co layer. The symmetry breaking enables successive square root of 3a x 1a AFM and 2a x 2a charge- or spin-ordering transitions at low temperatures. The Fermi surface is truncated by the 2a x 2a hexagonal zone boundary into small electron and hole pockets. We study the phase structure and compare to recent experiments.     

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     

Unitarity of the Knizhnik-Zamolodchikov-Bernard connection and the Bethe Ansatz for the elliptic Hitchin systems

We work out finite-dimensional integral formulae for the scalar product of genus one states of the groupG Chern-Simons theory with insertions of Wilson lines. Assuming convergence of the integrals, we show that unitarity of the elliptic Knizhnik-Zamolodchikov-Bernard connection with respect to the scalar product of CS states is closely related to the Bethe Ansatz for the commuting Hamiltonians building up the connection and quantizing the quadratic Hamiltonians of the elliptic Hitchin system.     

On the quantization of the hall impedance

The quantized Hall impedence is shown to relate to the ratio of two period integrals which are identified as the familiar period integrals of flux and electric charge if their domain of definition is extended to the realm of spacetime.     

Charge resonance enhanced ionization of CO2 probed by laser Coulomb explosion imaging

The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance R(c) is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for CO(2), by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry ≈ 2.1 ? and <θ(OCO)> ≈ 163° (equilibrium CO(2) geometry is ≈ 1.16 ? and <θ(OCO)> ≈ 172°). The CO(2)(3+) molecule, however, appears always to begin dissociation from closer than 1.7 ? indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO(2) and identify the electronic states responsible for CREI.     

Effects of quantitative disorder on the electronic structure of tetrahedrally-bonded amorphous semiconductors

We have studied the effects of quantitative disorder on the valence band in the electronic density of states (DOS) of tetrahedrally-bonded amorphous semiconductors on the basis of a simple Weaire-Thorpe model. The coherent potential approximation is applied to the random variation of the transfer integrals V₁ and V₂ of the model. Our numerical results indicate that it is not always necessary to assume the existence of five-membered rings in order to explain the characteristic features of the DOS.