Nuclear matter theory revisited

Claiming that the usual Fermi sea of plane waves Hartree-Fock vacuum state cannot be the true (or stablest) ground vacuum state for nuclear matter, since it violates a monotonicity requirement, we discuss some recent attempts in facing the problem of how to find such a (zero-order) state. The one-dimensional Fermi gas with zero- and finite-range interactions, as well as self-consistent alpha-clustering effects in three-dimensional nuclear matter, are employed as examples to illustrate the concepts involved in such a search.     

Potential energy shift for a screened Coulomb potential

Utilizing Hartree-Fock self-consistent field wave functions, accurate values of the potential energy shift at the nucleus due to atomic electrons have been determined for a large number of atoms. An extensive numerical table of values of the shift,V ₀, is given and comparison is made with earlier approximate results obtained from the Thomas-Fermi and Thomas-Fermi-Dirac statistical models and with estimates fromK binding energy data. In the calculation of screening corrections to Fermi functions, the Hartree-Fock results present a considerable improvement over earlier estimates from the aforementioned methods.     

Ab initio study of the Electron Momentum Distribution of metallic lithium

The Electron Momentum Distribution (EMD) of metallic lithium has been described starting from the wave function obtained by means of an accurate Hartree-Fock (HF) computation. The quality of the HF solution is first ascertained in terms of total energy and Fermi surface anisotropy. It is also shown that there is appreciable transfer of electrons toward ionic interstices with respect to atomic superposition. Much structure is left, however, in the distribution of conduction electrons with respect to an electron gas model. This is reflected in the structure of the EMD where there is appreciable population of the secondary Fermi spheres: a detailed analysis shows that this is due partly to core orthogonality, partly to atomic-like structural effects and partly to genuine crystal structural effects. Correlation effects are added according to a procedure suggested by Lundqvist on the EMD of conduction electrons. The resulting Compton Profiles and autocorrelation function are calculated and compared with the available, unfortunately scarce, experimental information.     

A self-consistent model calculation of the electronic structure of ordered,disordered and hydrogenated Pd3Fe

We study, within the Bethe lattice approximation, the electronic structure of the ordered, disordered and hydrogenated intermetallic Pd₃Fe. We employ a simple one-orbital per site model hamiltonian, which includes a Hubbard-like Coulomb interaction term. This is treated in the Hartree-Fock approximation. We present results for the number of electrons, magnetic moments, and density of states at the Fermi level. Good agreement with available experimental data is obtained.     

Dependence of the density distribution of 208Pb on the occupation probabilities of shell-model orbits

An independent-particle model for the proton density distribution of ²⁰⁸Pb is constructed; it closely approximates the Hartree-Fock calculation of Dechargé and Gogny. We investigate the modifications which arise when one introduces a depletion of the Fermi sea of the amount suggested by analyses of recent electron scattering data and by nuclear-matter calculations. The main effect of the depletion is to flatten the density distribution in the nuclear interior. The calculated density is in good agreement with the empirical one near the nuclear centre but is too small in the vicinity of 5 fm. The main consequences of the depletion are shown to be largely independent of the details of the model. It is concluded that Hartree-Fock single-particle wave functions which yield good agreement with empirical density distributions are rather different from the natural orbitals. Accordingly they should not be expected to yield a good approximation to the off-diagonal elements of the one-body density matrix, e.g. to the momentum distribution.     

Anisotropic states of two-dimensional electrons in high magnetic fields

We study the collective states formed by two-dimensional electrons in Landau levels of index n > or = near half filling. By numerically solving the self-consistent Hartree-Fock (HF) equations for a set of oblique two-dimensional lattices, we find that the stripe state is an anisotropic Wigner crystal (AWC), and determine its precise structure for varying values of the filling factor. Calculating the elastic energy, we find that the shear modulus of the AWC is small but finite (nonzero) within the HF approximation. This implies, in particular, that the long-wavelength magnetophonon mode in the stripe state vanishes q(3/2) like as in an ordinary Wigner crystal, and not like q(5/2) as was found in previous studies where the energy of shear deformations was neglected.     

Screening of Local Magnetic Moment by Electrons of Disordered Graphene

Based on the Anderson impurity model and self-consistent approach, we investigate the condition for the screening of a local magnetic moment by electrons in graphene and the influence of the moment on electronic properties of the system. The results of numerical calculations carried out on a finite sheet of graphene show that when the Fermi energy is above the single occupancy energy and below the double occupancy energy of the local impurity, a magnetic state is possible. A phase diagram in a parameter space spanned by the Coulomb energy U and the Fermi energy is obtained to distinguish the parameter regions for the magnetic and nonmagnetic states of the impurity. We find that the combined effect of the impurity and finite size effect results in a large charge density near the edges of the finite graphene sheet. The density of states exhibits a peak at the Dirac point which is caused by the appearance of the edge states localized at the zigzag edges of the sheet.     

Electron correlation and fermi surface topology of NaxCoO2

The electronic structure of NaxCoO2 revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by local-density approximation calculations have not been observed as the associated e'(g) band fails to cross the Fermi level for a wide range of sodium doping concentration x. In addition, significant bandwidth renormalizations of the t(2g) complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multiorbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments.     

Self-consistent green function approach for calculation of electronic structure in transition metals

We present an approach for self-consistent calculations of the many-body Green function in transition metals. The distinguishing feature of our approach is the use of one-site approximation and the self-consistent quasiparticle wave function basis set obtained from the solution of the Schr?dinger equation with a nonlocal potential. We analyze several sets of skeleton diagrams as generating functionals for the Green function self-energy, including GW and fluctuating exchange sets. Calculations for Fe and Ni revealed stronger energy dependence of the effective interaction and self-energy of the d electrons near the Fermi level compared to s and p electron states.     

Microscopic verification of topological electron-vortex binding in the lowest Landau-level crystal state

When two-dimensional electrons are subjected to a very strong magnetic field, they are believed to form a triangular crystal. By a direct comparison with the exact wave function, we demonstrate that this crystal is not a simple Hartree-Fock crystal of electrons but an inherently quantum mechanical crystal characterized by a nonperturbative binding of quantized vortices to electrons. It is suggested that this has qualitative consequences for experiment.     

过渡金属原子在离子晶体上的化学吸附

本文研究了一种描写过渡金属原子在离子晶体上的化学吸附模型。其中离子晶体用半无限A—B交替原子链代表,而吸附原子d轨道电子间的库仑排斥作用则用Anderson-Newns方法表述。对d轨道电子与晶体表面相互作用的几种不同耦合常数,用自洽格林函数方法计算了化学吸附能和吸附原子的电荷转移量。讨论了各种自洽解的存在条件和性质,得到了一些有趣的定性结论。 关键词：     

Local enhancement of the effective mass near the Fermi surface in nuclear matter

It is shown that the effective mass of nucleons in nuclear matter has a pronounced and narrow maximum near the Fermi surface, in contradistinction with previous belief that this local enhancement exists only in finite nuclei. Implications of this finding for self-consistent Hartree-Fock calculations of finite nuclei are pointed out.     

Conductance of nano-systems with interactions coupled via conduction electrons: effect of indirect exchange interactions

A nano-system in which electrons interact and in contact with Fermi leads gives rise to an effective one-body scattering which depends on the presence of other scatterers in the attached leads. This non local effect is a pure many-body effect that one neglects when one takes non interacting models for describing quantum transport. This enhances the non-local character of the quantum conductance by exchange interactions of a type similar to the RKKY-interaction between local magnetic moments. A theoretical study of this effect is given assuming the Hartree-Fock approximation for spinless fermions of Fermi momentum k_F in an infinite chain embedding two scatterers separated by a segment of length L_c. The fermions interact only inside the two scatterers. The dependence of one scatterer onto the other exhibits oscillations of period π/k_F which decay as 1/L_c and which are suppressed when L_c exceeds the thermal length L_T. The analytical results given by the Hartree-Fock approximation are compared with exact numerical results obtained with the embedding method and the DMRG algorithm.     

Application of the self-consistent field method to the theory of surface states of electrons in a crystal

It is shown under what conditions there exist solutions to the Hartree-Fock equation, which are localized in the neighbourhood of the surface of the crystal. The results obtained earlier as to the number and character of the localized solutions of the single electron Schrödinger equation of electrons in a limited crystal, can be applied to the localized solutions of the Hartree-Fock equation after accepting plausible assumptions. A linear combination, having analogical properties to the Wannier function, can be formed from the solution of the Hartree-Fock equation for an infinite crystal.     

Spin-charge density waves in the Hubbard model

The ground state of the Hubbard model in a square lattice is examined in the Hartree-Fock mean field approximation at zero temperature. At small finite hole doping, the system has periodically distributed soliton like structures whose modulations are incommensurate. In a self-consistent way, the Fermi energy can always be located in a gap. The incommensurate states have lower energies than the commensurate antiferromagnetic states calculated at the same filling. These soliton structures persist even when a sizeable nearest neighbor repulsive interaction is included.     

The boundary condition integration technique: results for the Hubbard model in 1D and 2D

We study models of strongly correlated electrons in one-and two dimensions. We exactly diagonalize small clusters with general boundary conditions (BC) and integrate over all possible BC. This technique recovers the kinetic energy part of the (extended lattice) Hamiltonianexactly in a grand-canonical formulation. A continuous range of particle densities may be described with this technique and the momentum space can be probed for arbitrary momenta. For the Hubbard Hamiltonian we recover details of the Mott-insulating behaviour for the momentum distribution function at half filling, both in 1D and 2D. Off half-filling the shape of thecanonical Fermi surface is strongly distorted in 2D with respect to thegrand canonical Fermi surface. The shape of the grand canonical Fermi surface obtained by this finite-size technique reduces in the weak-coupling limit exactly to that of the infinite-lattice Fermi sea.email: UPH 301 at DDOHR Z11     

Hartree-Fock studies of quantum dots and the 0.7 anomaly

We apply unrestricted Hartree-Fock to modelling two systems:

(1)

We calculate the spin structure and addition spectra of small symmetric quantum dots (often called 2D “artificial atoms”), improving the accuracy considerably by including, for the first time, second-order correlation corrections. We compare the results to experiment and to previous numerical works, and find that our spin structure in some cases disagrees with that calculated within mean-field theories, such as Hartree-Fock without correlation corrections, or density-functional theory [C. Sloggett, O.P. Sushkov, Phys. Rev. B 71 (2005) 235326].

(2)

We model the well-known 0.7 anomaly in the conductance of a quantum point contact. We calculate the conductance using direct calculation of scattering phases on a ring, within Hartree-Fock. We observe strong localisation of the Fermi electrons on the barrier, and suggest a mechanism for the observed temperature-dependent conductance anomaly.

     

Renormalization of relativistic self-consistent Hartree-Fock approximation

The renormalization of the relativistic self-consistent Hartree-Fock approximation is restudied. It is shown that the renormalization procedure suggested by Bielajew and Serot can be greatly simplified and the renormalization achieved in a way no more complicated than that of the relativistic self-consistent Fock approximation, if the parameters in the counterterms are allowed to be density-dependent and the renormalization of the tadpole self-energy is treated appropriately. A transformation relation between the four- and three-dimensional representation of the baryon self-energy is presented and a self-consistent Hartree-Fock scheme different from that considered by Bielajew and Serot studied. The renormalized integral equations for the baryon self-energy which includes effects from the Dirac sea are reformulated in a three-dimensional form. Explicit expressions are derived. Received: 29 August 1997 / Revised version: 30 April 1998     

Galvanomagnetic measurements on n- and p-type α-tin electron-irradiated at 5 K

Galvanomagnetic measurements were obtained for pure and doped n- and p-type α-tin filaments irradiated by 1 MeV electrons at ～ 5 K. Variations in the Hall coefficient for highly degenerate samples were in qualitative agreement with theory. Carrier removal rates are reported; changes in these as a function of the initial position of the Fermi level indicate the presence of donor defect sites near E_F = 0 and acceptor defect sites deeper in the valence band. Isochronal annealing data have been obtained.     

Dispersionless modes and the superconductivity of ultrathin films

A self-consistent analytical solution of the problem of the superconductivity of ultrathin metal films is found within the tight-binding model for normal-metal electrons with a simple example of a film of three atomic layers. Superconductivity is not destroyed in atomically thin films if the energies of the electron subsystem lie near the Fermi surface at least for some values of the quasimomentum component along the film. A substantial increase in the critical temperature of an ultrathin metal film as compared to its bulk value is possible if the electron excitation spectrum contains low-energy modes with an anomalously weak dispersion.