4,036 research outputs found
Dynamically induced scalar quark confinement
We employ a functional approach to investigate the confinement problem in
quenched Landau gauge QCD. We demonstrate analytically that a linear rising
potential between massive quarks is generated by infrared singularities in the
dressed quark-gluon vertex. The selfconsistent mechanism that generates these
singularities is driven by the scalar Dirac amplitudes of the full vertex and
the quark propagator. These can only be present when chiral symmetry is broken.
We have thus uncovered a novel mechanism that directly links chiral symmetry
breaking with confinement.Comment: 12 pages, 2 figures; v2: clarifications added and typos corrected,
version to be published by MPL
Probing molecular dynamics at the nanoscale via an individual paramagnetic center
Understanding the dynamics of molecules adsorbed to surfaces or confined to
small volumes is a matter of increasing scientific and technological
importance. Here, we demonstrate a pulse protocol using individual paramagnetic
nitrogen vacancy (NV) centers in diamond to observe the time evolution of 1H
spins from organic molecules located a few nanometers from the diamond surface.
The protocol records temporal correlations among the interacting 1H spins, and
thus is sensitive to the local system dynamics via its impact on the nuclear
spin relaxation and interaction with the NV. We are able to gather information
on the nanoscale rotational and translational diffusion dynamics by carefully
analyzing the time dependence of the NMR signal. Applying this technique to
various liquid and solid samples, we find evidence that liquid samples form a
semi-solid layer of 1.5 nm thickness on the surface of diamond, where
translational diffusion is suppressed while rotational diffusion remains
present. Extensions of the present technique could be adapted to highlight the
chemical composition of molecules tethered to the diamond surface or to
investigate thermally or chemically activated dynamical processes such as
molecular folding
Spectral Density of the Two-Impurity Anderson Model
We investigate static and dynamical ground-state properties of the
two-impurity Anderson model at half filling in the limit of vanishing impurity
separation using the dynamical density-matrix renormalization group method. In
the weak-coupling regime, we find a quantum phase transition as function of
inter-impurity hopping driven by the charge degrees of freedom. For large
values of the local Coulomb repulsion, the transition is driven instead by a
competition between local and non-local magnetic correlations. We find evidence
that, in contrast to the usual phenomenological picture, it seems to be the
bare effective exchange interactions which trigger the observed transition.Comment: 18 pages, 6 figures, submitted to J. Phys.:Condens. Matte
Instabilities in asymmetric nuclear matter
The existence of phase transitions from liquid to gas phases in asymmetric
nuclear matter (ANM) is related with the instability regions which are limited
by the spinodals. In this work we investigate the instabilities in ANM
described within relativistic mean field hadron models, both with constant and
density dependent couplings at zero and finite temperatures. In calculating the
proton and neutron chemical potentials we have used an expansion in terms of
Bessel functions that is convenient at low densities. The role of the isovector
scalar -meson is also investigated in the framework of relativistic
mean field models and density dependent hadronic models. It is shown that the
main differences occur at finite temperature and large isospin asymmetry close
to the boundary of the instability regions.Comment: 13 pages, 5 figures; to appear in Phys. Rev.
Systematics of collective correlation energies from self-consistent mean-field calculations
The collective ground-state correlations stemming from low-lying quadrupole
excitations are computed microscopically. To that end, the self-consistent
mean-field model is employed on the basis of the Skyrme-Hartre-Fock (SHF)
functional augmented by BCS pairing. The microscopic-macroscopic mapping is
achieved by quadrupole-constrained mean-field calculations which are processed
further in the generator-coordinate method (GCM) at the level of the Gaussian
overlap approximation (GOA).
We study the correlation effects on energy, charge radii, and surface
thickness for a great variety of semi-magic nuclei. A key issue is to work out
the influence of variations of the SHF functional. We find that collective
ground-state correlations (GSC) are robust under change of nuclear bulk
properties (e.g., effective mass, symmetry energy) or of spin-orbit coupling.
Some dependence on the pairing strength is observed. This, however, does not
change the general conclusion that collective GSC obey a general pattern and
that their magnitudes are rather independent of the actual SHF parameters.Comment: 13 pages, 13 figure
Microscopic Description of Nuclear Fission Dynamics
We discuss possible avenues to study fission dynamics starting from a
time-dependent mean-field approach. Previous attempts to study fission dynamics
using the time-dependent Hartree-Fock (TDHF) theory are analyzed. We argue that
different initial conditions may be needed to describe fission dynamics
depending on the specifics of the fission phenomenon and propose various
approaches towards this goal. In particular, we provide preliminary
calculations for studying fission following a heavy-ion reaction using TDHF
with a density contraint. Regarding prompt muon-induced fission, we also
suggest a new approach for combining the time-evolution of the muonic wave
function with a microscopic treatment of fission dynamics via TDHF
Ionic structure and photoabsorption in medium sized sodium clusters
We present ground-state configurations and photoabsorption spectra of Na-7+,
Na-27+ and Na-41+. Both the ionic structure and the photoabsorption spectra of
medium-size sodium clusters beyond Na-20 have been calculated self-consistently
with a nonspherical treatment of the valence electrons in density functional
theory. We use a local pseudopotential that has been adjusted to experimental
bulk properties and the atomic 3s level of sodium. Our studies have shown that
both the ionic structure of the ground state and the positions of the plasmon
resonances depend sensitively on the pseudopotential used in the calculation,
which stresses the importance of its consistent use in both steps.Comment: 4 pages, 3 figures. Accepted for publication in PRB, tentatively July
15th, 1998 some typos corrected, brought to nicer forma
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