Fabio Siringo

Extension of the Anderson-Newns model for hydrogen chemisorption on Ni(100), Ni(111) and W(110)

Abstract Extensive calculations of the ground state properties of hydrogen chemisorbed on Ni(100), Ni(111) and W(110) surfaces are presented. The calculations are performed using an extension of the Anderson-Newns model, which includes the core-core repulsion term. The variation of the adatom-metal hopping matrix elements with the hydrogen atom position is considered in the framework of the extended Huckel model. The asymptotic behaviour is well reproduced considering corrections to the adatom Coulomb repulsion U due to the charge image potential. The following properties are discussed: (a) chemisorption energy; (b) preferred site; (c) bond lengths; (d) vibrational frequencies; (e) charge transfer; (f) adatom electron density of states. Comparison with available experimental results shows that our method can give a very good overall picture of H chemisorption on the above mentioned systems. The energy contour plot for a plane parallel to the W(110) surface at the equilibrium distance shows the presence of an elongated region of almost constant energy connecting the trigonal and long bridge sites. This result is in agreement with the experimental findings of Blanchet et al. [Surface Sci. 118 (1982) 496], which have shown that the two sites compete. As a consequence of this picture the calculation predict a parallel vibrational mode, whose frequency turns out to be in good agreement with the experimental value (160 meV) given by Blanchet et al.

Gluon propagator in Feynman gauge by the method of stationary variance

The low-energy limit of pure Yang-Mills SU(3) gauge theory is studied in Feynman gauge by the method of stationary variance, a genuine second-order variational method that is suited to deal with the minimal coupling of fermions in gauge theories. In terms of standard irreducible graphs, the stationary equations are written as a set of coupled non-linear integral equations for the gluon and ghost propagators. A physically sensible solution is found for any strength of the coupling. The gluon propagator is finite in the infrared, with a dynamical mass that decreases as a power at high energies. At variance with some recent findings in Feynman gauge, the ghost dressing function does not vanish in the infrared limit and a decoupling scenario emerges as recently reported for the Landau gauge.

Self-consistent variational approach to the minimal left-right symmetric model of electroweak interactions

The problem of mass generation is addressed by a Gaussian variational method for the minimal left-right symmetric model of electroweak interactions. Without any scalar bidoublet, the Gaussian effective potential is shown to have a minimum for a broken symmetry vacuum with a finite expectation value for both the scalar Higgs doublets. The symmetry is broken by the fermionic coupling that destabilizes the symmetric vacuum, yielding a self-consistent fermionic mass. In this framework a light Higgs is only compatible with the existence of a new high energy mass scale below 2 TeV.

Symmetry breaking and restoring under high pressure: the amazing behaviour of the “simple” alkali metals

Abstract:We argue that an ionic lattice surrounded by a Fermi liquid changes phase several times under pressure, oscillating between the symmetric phase and a low-symmetry dimerized structure, as a consequence of Friedel oscillations in the pair potential. Phase oscillations explain the tendency towards dimerization which has been recently reported for the light alkali metals under high pressure. Moreover, a restoring of the symmetric phase is predicted for such elements at an even higher density.

Analytical study of Yang–Mills theory in the infrared from first principles

Abstract Pure Yang–Mills SU(N) theory is studied in the Landau gauge and four dimensional space. While leaving the original Lagrangian unmodified, a double perturbative expansion is devised, based on a massive free-particle propagator. In dimensional regularization, all diverging mass terms cancel exactly in the double expansion, without the need to include mass counterterms that would spoil the symmetry of the Lagrangian. No free parameters are included that were not in the original theory, yielding a fully analytical approach from first principles. The expansion is safe in the infrared and is equivalent to the standard perturbation theory in the UV. At one-loop, explicit analytical expressions are given for the propagators and the running coupling and are found in excellent agreement with the data of lattice simulations. A universal scaling property is predicted for the inverse propagators and shown to be satisfied by the lattice data. Higher loops are found to be negligible in the infrared below 300 MeV where the coupling becomes small and the one-loop approximation is under full control.

Second order gluon polarization for SU(N) theory in a linear covariant gauge

The gluon polarization functional is evaluated for a generic linear covariant gauge and for any space dimension in pure Yang-Mills SU(N) theory up to second order in a generalized perturbation theory, where the zeroth order action is freely chosen and can be determined by some variational method. Some numerical data are given for the gluon propagator in Landau gauge and compared with Feynmann gauge. A comparison is given for several variational methods that can be set up by the knowledge of the second-order polarization.

Higher order extensions of the Gaussian effective potential

A variational method is discussed, extending the Gaussian effective potential to higher orders. The single variational parameter is replaced by trial unknown two-point functions, with infinite variational parameters to be optimized by the solution of a set of integral equations. These stationary conditions are derived by the self-energy without having to write the effective potential, making use of a general relation between self-energy and functional derivatives of the potential. This connection is proven to any order and verified up to second order by an explicit calculation for the scalar theory. Among several variational strategies, the methods of minimal sensitivity and of minimal variance are discussed in some detail. For the scalar theory, at variance with other post-Gaussian approaches, the pole of the second-order propagator is shown to satisfy the simple first-order gap equation that seems to be more robust than expected. By the method of minimal variance, nontrivial results are found for gauge theories containing fermions, where the first-order Gaussian approximation is known to be useless.

Gaussian Effective Potential and superconductivity

Abstract:The Gaussian Effective Potential in a fixed transverse unitarity gauge is studied for the static three-dimensional U(1) scalar electrodynamics (Ginzburg-Landau phenomenological theory of superconductivity). In the broken-symmetry phase the mass of the electromagnetic field (inverse penetration depth) and the mass of the scalar field (inverse correlation length) are both determined by solution of the coupled variational equations. At variance with previous calculations, the choice of a fixed unitarity gauge prevents from the occurrence of any unphysical degree of freedom. The theory provides a nice interpolation of the experimental data when approaching the critical region, where the standard mean-field method is doomed to failure.

Metalinsulator transition induced by pressure in chemically bonded solids

Abstract A review is given of metal-insulator transitions induced by pressure in chemically bonded solids. After an introduction to energy band overlap mechanisms, which appear very useful to characterize the insulator-metal transitions in some solids (the rare gases He and Xe, the iso-electronic sequence of Xe, H2 and I2), some discussion is given of electron correlation in relation to molecular dissociation. However, it is to be emphasized that such dissociation is a sufficient, but not a necessary, condition for metallization in H. and I2. For N2, the above considerations, which focussed dominantly on ground-state properties as a function of pressure, need transcending because the observation of metallization in this system plainly involves thermal effects in an important way. The final part of the review considers the way various low-dimensional solids (polyacetylene, TTF-TCNQ, and appropriate phases of H2 and I2) are altered by application of high pressures. Present understanding leads to the conclus...

Analytic structure of QCD propagators in Minkowski space

Analytical functions for the propagators of QCD, including a set of chiral quarks, are derived by a one-loop massive expansion in the Landau gauge, deep in the infrared. By analytic continuation, the spectral functions are studied in Minkowski space, yielding a direct proof of positivity violation and confinement from first principles.The dynamical breaking of chiral symmetry is described on the same footing of gluon mass generation, providing a unified picture. While dealing with the exact Lagrangian, the expansion is based on massive free-particle propagators, is safe in the infrared and is equivalent to the standard perturbation theory in the UV. By dimensional regularization, all diverging mass terms cancel exactly without including mass counterterms that would spoil the gauge and chiral symmetry of the Lagrangian. Universal scaling properties are predicted for the inverse dressing functions and shown to be satisfied by the lattice data. Complex conjugated poles are found for the gluon propagator, in agreement with the i-particle scenario.