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We study the temperature-dependent zone-boundary surface phonons across the c(2 x 2)..-->..1 x 1 reconstruction phase transition of the clean W(001) surface. Velocity-velocity correlations and hence the phonon spectral densities are calculated by molecular dynamics for the surface atoms of a finite-thickness (001) slab, with interatomic potentials established in a previous study of the surface statics. Our calculated k = ((1/2, 1) / 2 )(2..pi../a) surface phonon spectra are dominated by three main low-frequency modes. Of these, the longitudinal and the shear horizontal modes are reconstruction related and display critical broadening and softening at the phase transition, while the third, the shear vertical mode, is basically unaffected. The reconstruction phase mode, the shear horizontal, appears to be responsible for the phase fluctuations which destroy long-range order at the transition.

A new method for simulating strongly correlated fermionic systems has been applied to the study of the ground state properties of the 2D Hubbard model at various fillings. Comparison has been made with exact diagonalization in the 2×2, 3×3 and 4×4 lattices where very good agreement has been verified. Numerical results concerning a)magnetization, b)momentum distribution, c)charge, spin and pairing correlations have been obtained with this method for sizes up to 16×16 sites and several fillings. The 2D antiferromangetic order is destroyed by the presence of few holes and is very sensitive to the boundary conditions. The momentum distribution of the doped system does not have a clear Fermi surface. This is also supported by the exponential decay of the density matrix at large enough distances. Particular attention will be paid to the single hole properties and hole-hole correlations.

Spin-flip Stoner excitations have been detected in ${\mathrm{Fe}}_{80}$${\mathrm{B}}_{15}$${\mathrm{Si}}_{5}$ using simple electron-energy-loss spectroscopy via the strong energy dependence of the exchange process involved. The Stoner-excitation contribution, obtained as the difference between energy-loss spectra taken with low-energy and high-energy primary electrons, is in good agreement with the results of spin-polarized electron-energy-loss spectroscopy and with theoretical predictions.

Based on a lattice instability theory, it is predicted that melting should commence on the surface layer of a crystal. In addition it is shown that for a fcc crystal governed by a two-body potential, the loose-packed surface melts prior to a close-packed one, while this order gets reversed for fcc metals like copper, where many-body electronic cohesive forces are important.

We have developed a lattice theory of surface melting based on minimization of the free energy with respect to two spatially varying order parameters---density and ``crystallinity.'' The partition function is evaluated using mean-field and free-volume approximations on a lattice. Direct application is made to (100) and (110) Lennard-Jones crystal surfaces. It is shown that on the coexistence line and very close to the triple-point temperature ${T}_{M}$ a quasiliquid layer forms on the crystal-gas interface. The thickness of the layer grows asymptotically as (${T}_{M}$-T${)}^{\mathrm{\ensuremath{-}}1/3}$, in good agreement with the recent experiments on Ar films. A change from long- to short-range interparticle attraction reduces the growth behavior to logarithmic, while a switch of the potential tail from attractive to repulsive can block altogether the growth of the quasiliquid layer. It is further shown that in cases where no in-plane disorder can arise no surface melting occurs and the crystal-vapor interface can even be overheated. Within the present mean-field approximation, surface melting is found to be continuous without any singularities below ${T}_{M}$ in the surface free energy, which is explicitly calculated. The decay of the ``crystallinity'' order parameter at the quasi-liquid-gas interface is predicted to be a ``stretched exponential'' in the long-range case and power law in the short-range case.

Etude par un modele de force a plusieurs corps et par dynamique moleculaire de la structure et de l'energetique des surfaces Au(100) et Au(111). La reconstruction de surface a lieu sur les terrasses entre les gradins favorisant les orientations «magiques» chaque fois que les terrasses contiennent un nombre entier de cellule de reconstruction. Ce phenomene a des implications sur le comportement du facettage de ces surfaces

Among all metals, gold has the extra peculiarity (besides others!) that none of its low-index surfaces is stable with its “ideal” periodic structure. Reconstruction takes place systematically, apparently dictated by the strong necessity of a better surface packing than that attainable by simple relaxation.

In the glue model the total cohesion of a metal is determined by a pairwise atom-atom effective interaction plus a many-body force (the glue) which is introduced to ensure optimal coordination. Using parameters optimized for gold, we have studied the structural behaviour of the low index surfaces Au(100), Au(110) and Au(111). We have used a simulated annealing strategy based on molecular dynamics to search the lowest surface energy configuration. In all cases the optimal structures are found to be reconstructed, and remarkably similar to some experimentally suggested reconstruction models. The main driving mechanism is the formation of close-packed triangular surface layers favoured by the glue term.

Thermomodulation studies of the optical properties of Sm 1− x Gd x S alloys are presented for x ⩽ 0.15. A new interpretation of the low energy absorption peaks as excitons, where a Sm 5 d electron is bound to a localized hole in the 4 f shell, is strongly suggested.

Many well known difficulties associated with the use of two-body forces for the description of metallic systems may be overcome by using an expression for the total energy of the form V = ½Σ ij φ(r ij ) + Σ i U(ni), where n i = Σiρ(r ij ) is a generalized atomic coordination. The three functions φ(r), U(n) and ρ(r) are constructed empirically, by fitting several physical quantities including thermal and surface properties. This simple many-body force scheme can be used in molecular-dynamics simulations with few overheads compared with pair-wise systems. We present our realization for gold and summarize the results of recent structural and dynamical studies of Au surfaces.

Physical Review Letters

The expulsion dynamics of the last liquid monolayer of molecules confined between two surfaces has been analyzed by solving the two-dimensional (2D) Navier-Stokes equation for a compressible liquid. We find that the squeeze-out is characterized by the parameter g 0 P 0 /ρc 2 , where P 0 is the average perpendicular (squeezing) pressure, p is the liquid (3D) density, and c is the longitudinal sound velocity in the monolayer film. When g 0 <

First-principles calculations are used to investigate the electronic properties of the surfaces of polyethylene. The calculations support the experimental evidence of a negative electron affinity, with calculated values of -0.17 eV and -0.10 eV for surfaces with chains perpendicular and parallel to the surface normal, respectively. Both surfaces exhibit a surface state with binding energy -1.2 +/- 0.5 eV with respect to the bulk polyethylene conduction band minimum. Implications of these findings on spectroscopy, as well as on the transport and aging properties of polyethylene for high-voltage applications, are discussed.

We consider the exchange Hamiltonian H_ST = -J Sum_{} (2 S_r S_r' - 1/2) (2 T_r T_r' - 1/2) describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on equivalent bonds. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit. To investigate the ground state properties we use a Green's Function Monte Carlo, calculating energy gaps and correlation functions, the latter through the forward walking technique. In one dimension we find that the ground state is a ``crystal'' of valence bond dimers. In two dimensions, the spin gap appears to remain finite in the thermodynamic limit, and, consistently, the staggered magnetization -- signal of Neel long range order -- seems to vanish. From the analysis of dimer-dimer correlation functions, however, we find no sign of a valence bond crystal. A spin liquid appears as a plausible scenario compatible with our findings., Comment: 3 pages, 3 figures

We consider an exchange model describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on the square lattice. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit, and is known to show a spin-Peierls-like dimerization in one dimension. In two dimensions we calculate energy gaps, susceptibilities, and correlation functions with a Green's Function Monte Carlo. We find a finite spin gap and no evidence of any kind of order. We conclude that the ground state is, most likely, a spin liquid of resonating valence bonds., Comment: 4 pages, 4 figures, Revtex

The Gutzwiller projector technique has long been known as a method to include correlations in electronic structure calculations. We describe a model implementation for a Gutzwiller + LDA calculation in a localized-orbital restricted basis framework, emphasizing the protocol step by step and illustrating our specific procedure for this and future applications. We demonstrate the method with a classic problem, the ferromagnetism of bulk bcc Fe, whose nature is attracting fresh interest. In the conventional Stoner-Wohlfarth model, and in spin-polarized LDA calculations, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion, at the cost of some increase of electron kinetic energy. This balance may, however, be altered by correlations, which are strong for localized d orbitals. The present localized basis Gutzwiller + LDA calculation demonstrates how the ferromagnetic ordering of Fe may, in fact, entrain a decrease of kinetic energy at the cost of some increase of potential energy. This happens because, as foreshadowed long ago by Goodenough and others and more recently supported by LDA-DMFT calculations, correlations cause e(g) and t(2g) d orbitals to behave differently, with the weakly propagating e(g) states fully spin polarized and almost localized, and only t(2g) states forming a broad partly filled itinerant band. Owing to an intra-atomic Hund's rule exchange that aligns e(g) and t(2g) spins, the propagation of itinerant t(2g) holes is favored when different atomic spins are ferromagnetically aligned. This suggests a strong analogy with double exchange in iron ferromagnetism.

Superconductivity and ferromagnetism have been believed to be incompatible over any extended temperature range until certain specific examples - RuSr2GdCu2O8 and UGe2- have arisen in the past 2-3 years. The discovery of superconductivity above 8 K in MgCNi3, which is primarily the ferromagnetic element Ni and is strongly exchange-enhanced, provides a probable new and different example. This compound is shown here to be near ferromagmnetism, requiring only hole-doping by 12% substitution of Mg by Na or Li. This system will provide the means to prode coupling, and possible coexistence, of these two forms of collective behavior without the requirement of pressure.

We discuss a hopping model of electrons between idealized molecular sites with local orbital degeneracy and dynamical Jahn-Teller effect, for crystal field environments of sufficiently high symmetry. For the Mott-insulating case (one electron per site and large Coulomb repulsions), in the simplest two-fold degenerate situation, we are led to consider a particular exchange hamiltonian, describing two isotropic spin-1/2 Heisenberg problems coupled by a quartic term on equivalent bonds. This twin-exchange hamiltonian applies to a physical regime in which the inter-orbital singlet is the lowest-energy intermediate state available for hopping. This regime is favored by a relatively strong electron-phonon coupling. Using variational arguments, a large-N limit, and exact diagonalization data, we find that the ground state, in the one dimensional case, is a solid valence bond state. The situation in the two dimensional case is less clear. Finally, the behavior of the system upon hole doping is studied in one dimension., 11 pages, 5 figures

Surface melting is strongly influenced by long-range dispersion forces which crucially control the thickness l of the liquid layer that forms at the crystal surface, through a free-energy term of asymptotic form V(l)=H/l 2 . In isotropic crystals, the face-independent Hamaker constant H is usually calculated by a well-known formula by Lifshitz et al. that relates dispersion forces to the optical conductivity of the solid and the liquid. In this paper we generalize this result to anisotropic crystals specializing in uniaxial crystal surfaces normal or parallel to the optical c axis, and to surfaces of orthorhombic biaxial crystals normal to one principal axis. The main result is that the Hamaker constant becomes face dependent, and may, in principle, even change sign from one face to another

Using mappings between one-dimensional models, we argue in this note that any additional spin-lattice coupling to the Hubbard model and the ensuing tendency towards spin-Peierls state is damaging, rather than favorable to the RVB-type. superconductivity.

The problems of classical diffusion in a random environment and of quantum mechanics with a random potential are related by the transformation of the Fokker-Planck equation into the imaginary time Schrodinger equation. The hypothesis of dynamic scaling allows to connect the long-time properties of diffusion to the low-energy spectrum of the quantum problem. For the specific model where the drift force is a white noise, the anomalous sublinear diffusion found by Sinai is related to logarithmic anomalies in the inverse localization length and in the density of states.

The correlated basis function method, in its Fermi hypernetted chain (FHNC) version, has been generalized for a study of the discrete Hubbard model. We report preliminary results for the one-dimensional case, where comparison is possible with the exact solution and other approximate methods.

We present a comparative study of the temperature-dependent surface phonons of the Mo(001) and W(001) surfaces, across the reconstruction phase transitions. The aim is to point out analogies and differences, the latter resulting from the incommensurate nature of Mo(001). Our results, based on extensive slab molecular dynamics simulations, indicate a definite, if somewhat subtle, difference of phonon dispersion for T >Tc. Very large differences, with the presence of incommensurate acoustical phasons, are predicted below Tc for Mo(001).

The effect of multilayer relaxation on melting is investigated here for a high cohesion metal like copper. The method involves finding the quasi-harmonic instability of (100), (110) and (111) surfaces of copper, where the electronic cohesive force is incorporated in a phenomenological way. It is seen that surface contraction has drastic effect on melting making a loose-packed surface melt later than a close-packed one.

The melting behaviour of a metal surface is of considerable experimental interest, but has not so far been studied theoretically at a quantitative level. We have undertake a molecular dynamics study of Au(111) based on the empirical but thoroughly tested “glue” many-body force scheme. Using a slab geometry, we consider both reconstructed and unreconstructed surfaces. The reconstructed surface (which has a lower surface energy) remains stable in its “crystalline” form up to the bulk melting temperature Tm. It further survives as a metastable state for another ∼150 K above Tm. Hence, we conclude that microscopic surface melting is absent in this case. The two outermost layers of the unreconstructed surface “melt’ ∼100 K below Tm. This state of the surface remains stable up to Tm, and it further survives as a metastable state again as high as ∼100 K above Tm. Within the severe limits imposed by our finite-size fluectuations, we observe no other stable surface melting behaviour. These results for Au(111) are contrasted with those for Lennard-Jones crystal surfaces.

Two- and three-dimensional alloy surfaces of the Ag-Au(110) system have been studied by evaporation of Ag onto a Au(110) substrate. Annealing up to 200°C caused surface diffusion and ordering, while annealing to 250°C or higher induced bulk diffusion with the formation of a three-dimensional alloy. At 20°C, Ag caused streaking of the LEED pattern, followed by a (1×1) structure with high background. Heating to 150–200°C allowed the observation of the sequence 1×2→streaks→1×3→streaks→1×1. The results are interpreted within the framework of the “glue” model, in which classical many-body forces play a dominant role.

We have studied the static properties (surface tension, density profile, structure and correlations) of a model liquid metal surface. Classical molecular dynamics has been used, in conjunction with a many-body force scheme (the “glue” model) optimized to describe gold. A general tendency of this liquid surface to be more short-range ordered than the bulk is found, and characterized in detail.

We have studied theoretically the optimal low-temperature surface structure of Au(100), Au(110) and Au(111). This has been done by using molecular dynamics as a tool to search, through suitable annealing and quenching cycles, the lowest surface energy states of slabs. The interatomic forces used consist of a two-body term, plus a many-body term (the “glue” term). The latter mimics the electronic forces, and tends to increase atom coordination at the surface. In the case of gold, we show that this simple mechanism, with a unique choice of forces for bulk and surface, is capable of explaining the reconstruction phenomena of all the low-index surfaces in considerable detail.

We study the general conditions for the occurrence of surface melting by describing the solid-vapour interface Gibbs free energy as a Cahn-Hilliard functional of density ρ and crystal order y . The Euler equations of this variational problem yield one or more physical trajectories in the (ρ, y ) plane running from vapour to solid at a variable distance from the “liquid point” L . For some systems only one physical trajectory exists, approaching L more and more closely as the triple point is approached: surface melting occurs and the thickness of the molten layer increases logarithmically. For other systems there exist both surface melting trajectories of the above type and a sublimation trajectory (never approaching L ): the latter has a lower Gibbs free energy and is physically realized. The van der Waals forces replace the logarithmic thickness increase of the molten layer with a power law or with a bounded function, depending on their sign.

Unreconstructed intrinsic semiconductor (111) surfaces are, to a good approximation, two-dimensional metals. The Fermi surface has flat portions, leading to a very large density of states at the Fermi energy, and to a quasi-1 dimensional electron dynamics. The condition for the stability of the surface metallic state is examined and shown to be violated; a surface phase transition to an insulating state follows. The insulating state can be either a magnetically ordered state (spin density wave) or a Peierls superlattice (charge density wave), depending on the relative strength of the surface electron-hole and electron-phonon interaction. The periodicities of many of the higher n×n reconstructions observed on Si and Ge (111) agree with those expected for the charge density wave case. It is pointed out that SDW magnetic ordering on surfaces might be detectable by polarized LEED, or otherwise.