Coulomb interactions of massless Dirac fermions in graphene; pair-distribution functions and exchange-driven spin-polarized phases

The quasi-2D electrons in graphene behave as massless fermions obeying a Dirac-Weyl equation in the low-energy regime near the two Fermi points. The stability of spin-polarized phases (SPP) in graphene is considered. The exchange energy is evaluated from the analytic pair-distribution functions, and the correlation energies are estimated via a closely similar four-component 2D electron fluid which has been investigated previously. SPPs appear for sufficiently high doping, when the exchange energy alone is considered. However, the inclusion of correlations is found to {\it suppress} the spin-phase transition in ideal graphene.Comment: 5 pages, 3 figures, Revte

The Classical-Map Hyper-Netted-Chain (CHNC) technique for inhomogeneous electron systems. Application to quantum dots

The Classical-map Hyper-Netted-Chain (CHNC) technique is a simple method of calculating quantum pair-distribution functions, spin-dependent energies, etc., of strongly-interacting {\it uniform} systems. We present CHNC calculations of charge densities and energies of {\it non-uniform} systems, viz., quantum dots, and compare with quantum Monte Carlo and density -functional results. Results for up to 210 electrons are reported.Comment: Latex manuscript and one figure (colour online). New calculations extending to 210 electrons are include

Magnetism and structure at a vacancy in graphene

The electronic structure, bonding and magnetism in graphene containing vacancies are studied using density-functional methods. The single-vacancy graphene ground state is spin polarized and structurally flat. The unpolarized state is non planar only for finite segments. Systems containing periodic arrays of vacancies displays magnetic transitions and metal-insulator transitions.Comment: 4 pages, four figure

The Equation of State and the Hugoniot of Laser Shock-Compressed Deuterium

The equation of state and the shock Hugoniot of deuterium are calculated using a first-principles approach, for the conditions of the recent shock experiments. We use density functional theory within a classical mapping of the quantum fluids [ Phys. Rev. Letters, {\bf 84}, 959 (2000) ]. The calculated Hugoniot is close to the Path-Integral Monte Carlo (PIMC) result. We also consider the {\it quasi-equilibrium} two-temperature case where the Deuterons are hotter than the electrons; the resulting quasi-equilibrium Hugoniot mimics the laser-shock data. The increased compressibility arises from hot $D^+-e$ pairs occuring close to the zero of the electron chemical potential.Comment: Four pages; One Revtex manuscript, two postscipt figures; submitted to PR

Local environment of Nitrogen in GaN{y}As{1-y} epilayers on GaAs (001) studied using X-ray absorption near edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is used to study the N environment in bulk GaN and in GaN{y}As{1-y} epilayers on GaAs (001), for y \~5%. Density-functional optimized structures were used to predict XANES via multiple-scattering theory. We obtain striking agreement for pure GaN. An alloy model with nitrogen pairs on Ga accurately predicts the threshold energy, the width of the XANES ``white line'', and features above threshold, for the given X-ray polarization. The presence of N-pairs may point to a role for molecular N_2 in epitaxial growth kinetics.Comment: Four pages (PRL style) with two figure

Saturation of dephasing time in mesoscopic devices produced by a ferromagnetic state

We consider an exchange model of itinerant electrons in a Heisenberg ferromagnet and we assume that the ferromagnet is in a fully polarized state. Using the Holstein-Primakoff transformation we are able to obtain a boson-fermion Hamiltonian that is well-known in the interaction between light and matter. This model describes the spontaneous emission in two-level atoms that is the proper decoherence mechanism when the number of modes of the radiation field is taken increasingly large, the vacuum acting as a reservoir. In the same way one can see that the interaction between the bosonic modes of spin waves and an itinerant electron produces decoherence by spin flipping with a rate proportional to the size of the system. In this way we are able to show that the experiments on quantum dots, described in D. K. Ferry et al. [Phys. Rev. Lett. {\bf 82}, 4687 (1999)], and nanowires, described in D. Natelson et al. [Phys. Rev. Lett. {\bf 86}, 1821 (2001)], can be understood as the interaction of itinerant electrons and an electron gas in a fully polarized state.Comment: 10 pages, no figure. Changed title. Revised version accepted for publication in Physical Review

Spin magnetization of strongly correlated electron gas confined in a two-dimensional finite lattice

The influence of disorder and interaction on the ground state polarization of the two-dimensional (2D) correlated electron gas is studied by numerical investigations of unrestricted Hartree-Fock equations. The ferromagnetic ground state is found to be plausible when the electron number is lowered and the interaction and disorder parameters are suitably chosen. For a finite system at constant electronic density the disorder induced spin polarization is cut off when the electron orbitals become strongly localized to the individual network sites. The fluctuations of the interaction matrix elements are calculated and brought out as favoring the ferromagnetic instability in the extended and weak localization regime. The localization effect of the Hubbard interaction term is discussed.Comment: 7 pages, 9 figure

Probing Ion-Ion and Electron-Ion Correlations in Liquid Metals within the Quantum Hypernetted Chain Approximation

We use the Quantum Hypernetted Chain Approximation (QHNC) to calculate the ion-ion and electron-ion correlations for liquid metallic Li, Be, Na, Mg, Al, K, Ca, and Ga. We discuss trends in electron-ion structure factors and radial distribution functions, and also calculate the free-atom and metallic-atom form-factors, focusing on how bonding effects affect the interpretation of X-ray scattering experiments, especially experimental measurements of the ion-ion structure factor in the liquid metallic phase.Comment: RevTeX, 19 pages, 7 figure

Determination of the pair potential and the ion-electron pseudopotential for aluminum from experimental structure-factor data for liquid aluminum

A method of inverting a given structure factor [S(k)]expt of a liquid metal using a hypernetted-chain equation containing bridge-diagram contributions is presented. Starting from parametrized local pseudopotential and a parametrized model local field (or a theoretical local field), a pair potential is constructed. The S(k) calculated from it is fitted to the given [S(k)]expt. The method is first applied to a molecular-dynamics-generated S(k) derived from an ab initio aluminum potential and shown to yield the pair potential, the pseudopotential, and the charge density in excellent quantitative agreement with the original ab initio potential and other quantities. The method is then applied to the experimental S(k) of aluminum from x-ray data at 943 K. The Al-Al pair potential, Al-electron pseudopotential, and electron charge densities as well as the electron-gas response function (i.e., the model local field) are obtained self-consistently, to within the accuracy of the experimental data. The calculated electrical resistivity is in excellent agreement with experiment. These investigations provide a comparative examination of the electron-gas local fields of Geldart-Taylor, Vashishta-Singwi, Ichimaru-Utsumi, and the density-functional local-density approximation. The hard-sphere parameter defining the bridge term is found to be essentially the same for the different ion-ion potentials determined from all but one of the different local fields, thus supporting the "universality" hypothesis of Rosenfeld and Ashcroft.Peer reviewed: YesNRC publication: Ye