1,660 research outputs found
Goldstone boson counting in linear sigma models with chemical potential
We analyze the effects of finite chemical potential on spontaneous breaking
of internal symmetries within the class of relativistic field theories
described by the linear sigma model. Special attention is paid to the emergence
of ``abnormal'' Goldstone bosons with quadratic dispersion relation. We show
that their presence is tightly connected to nonzero density of the Noether
charges, and formulate a general counting rule. The general results are
demonstrated on an SU(3)xU(1) invariant model with an SU(3)-sextet scalar
field, which describes one of the color-superconducting phases of QCD.Comment: 10 pages, REVTeX4, 4 eps figures, v2: general discussion in Sec. IV
expanded and improved, references added, other minor corrections throughout
the tex
Hartree-Fock and Many-Body Perturbation Theory with Correlated Realistic NN-Interactions
We employ correlated realistic nucleon-nucleon interactions for the
description of nuclear ground states throughout the nuclear chart within the
Hartree-Fock approximation. The crucial short-range central and tensor
correlations, which are induced by the realistic interaction and cannot be
described by the Hartree-Fock many-body state itself, are included explicitly
by a state-independent unitary transformation in the framework of the unitary
correlation operator method (UCOM). Using the correlated realistic interaction
V_UCOM resulting from the Argonne V18 potential, bound nuclei are obtained
already on the Hartree-Fock level. However, the binding energies are smaller
than the experimental values because long-range correlations have not been
accounted for. Their inclusion by means of many-body perturbation theory leads
to a remarkable agreement with experimental binding energies over the whole
mass range from He-4 to Pb-208, even far off the valley of stability. The
observed perturbative character of the residual long-range correlations and the
apparently small net effect of three-body forces provides promising
perspectives for a unified nuclear structure description.Comment: 14 pages, 8 figures, 3 tables, using REVTEX
Spontaneously Broken Spacetime Symmetries and Goldstone's Theorem
Goldstone's theorem states that there is a massless mode for each broken
symmetry generator. It has been known for a long time that the naive
generalization of this counting fails to give the correct number of massless
modes for spontaneously broken spacetime symmetries. We explain how to get the
right count of massless modes in the general case, and discuss examples
involving spontaneously broken Poincare and conformal invariance.Comment: 4 pages; 1 figure; v2: minor corrections. version to appear on PR
Exploring academic development programme evaluation at a university: A systematic scholarly approach
The systematic review, critique and evaluation of Academic Development (henceforth AD) programmes are increasingly becoming important for informing ongoing and future programme design. Ensuring the relevance requires a thorough understanding of the programme to be evaluated (Chalmers and Gardiner, 2015). This paper reports on the findings from a study exploring the nature of AD programmes at the University â with the aim of constructing a conceptual framework for thinking about AD programmes. This is necessary, in order to determine and evaluate the characteristics of nine programmes; and to determine whether these include the promotion of conceptual change â to thinking differently about teaching and learning in Higher Education.  The process of re-contextualising teaching, learning and assessment at the University from a common-sense approach â to a more scholarly approach â includes evaluative-research methodologies, as well as critical reflection within a paradigm of system theories. The research outcomes include a conceptual framework and a blueprint for a generic AD programme design.
Nuclear Structure Calculations and Modern Nucleon-Nucleon Potentials
We study ground-state properties of the doubly magic nuclei 4He, 16O, and
40Ca employing the Goldstone expansion and using as input four different
high-quality nucleon-nucleon (NN) potentials. The short-range repulsion of
these potentials is renormalized by constructing a smooth low-momentum
potential V-low-k. This is used directly in a Hartree-Fock approach and
corrections up to third order in the Goldstone expansion are evaluated.
Comparison of the results shows that they are only slightly dependent on the
choice of the NN potential.Comment: 5 pages, submitted to Physical Review
Brueckner-Goldstone perturbation theory for the half-filled Hubbard model in infinite dimensions
We use Brueckner-Goldstone perturbation theory to calculate the ground-state
energy of the half-filled Hubbard model in infinite dimensions up to fourth
order in the Hubbard interaction. We obtain the momentum distribution as a
functional derivative of the ground-state energy with respect to the bare
dispersion relation. The resulting expressions agree with those from
Rayleigh-Schroedinger perturbation theory. Our results for the momentum
distribution and the quasi-particle weight agree very well with those obtained
earlier from Feynman-Dyson perturbation theory for the single-particle
self-energy. We give the correct fourth-order coefficient in the ground-state
energy which was not calculated accurately enough from Feynman-Dyson theory due
to the insufficient accuracy of the data for the self-energy, and find a good
agreement with recent estimates from Quantum Monte-Carlo calculations.Comment: 15 pages, 8 fugures, submitted to JSTA
Effects of Monovalent and Divalent Cations on Ca2+ Fluxes Across Chromaffin Secretory Membrane Vesicles
Abstract: Bovine chromaffin secretory vesicle ghosts loaded with Na+ were found to take up Ca2+ when incubated in K+ media or in sucrose media containing micromolar concentrations of free Ca2+. Li+- or choline+loaded ghosts did not take up Ca2+. The Ca2+ accumulated by Na+-loaded ghosts could be released by the Ca2+ ionophore A23187, but not by EGTA. Ca2+ uptake was inhibited by external Sr2+, Na +, Li +, or choline +. All the 45Ca2+ accumulated by Na+-dependent Ca2+ uptake could be released by external Na +, indicating that both Ca2+ influx and efflux occur in a Na+-dependent manner. Na + -dependent Ca2+ uptake and release were only slightly inhibited by Mg2+. In the presence of the Na+ ionophore Monensin the Ca2+ uptake by Na +-loaded ghosts was reduced. Ca2+ sequestered by the Na+-dependent mechanism could also be released by external Ca2+ or Sr2+ but not by Mg2+, indicating the presence of a Ca2+/Ca2+ exchange activity in secretory membrane vesicles. This Ca2+/Ca2+ exchange system is inhibited by Mg2+, but not by Sr2+. The Na + -dependent Ca2+ uptake system in the presence of Mg2+ is a saturable process with an apparent Km of 0.28 ÎŒM and a Vmax= 14.5 nmol minâ1 mg proteinâ1. Ruthenium red inhibited neither the Na+/Ca2+ nor the Ca2+/Ca2+ exchange, even at high concentrations
Chiral Symmetry Breaking and Pion Wave Function
We consider here chiral symmetry breaking through nontrivial vacuum structure
with quark antiquark condensates. We then relate the condensate function to the
wave function of pion as a Goldstone mode. This simultaneously yields the pion
also as a quark antiquark bound state as a localised zero mode in vacuum. We
illustrate the above with Nambu Jona-Lasinio model to calculate different
pionic properties in terms of the vacuum structure for breaking of exact or
approximate chiral symmetry, as well as the condensate fluctuations giving rise
to mesons.Comment: latex, revtex, 16 page
Unfrustrated Qudit Chains and their Ground States
We investigate chains of 'd' dimensional quantum spins (qudits) on a line
with generic nearest neighbor interactions without translational invariance. We
find the conditions under which these systems are not frustrated, i.e. when the
ground states are also the common ground states of all the local terms in the
Hamiltonians. The states of a quantum spin chain are naturally represented in
the Matrix Product States (MPS) framework. Using imaginary time evolution in
the MPS ansatz, we numerically investigate the range of parameters in which we
expect the ground states to be highly entangled and find them hard to
approximate using our MPS method.Comment: 5 pages, 5 figures. Typos correcte
Effects of dissipation in an adiabatic quantum search algorithm
We consider the effect of two different environments on the performance of
the quantum adiabatic search algorithm, a thermal bath at finite temperature,
and a structured environment similar to the one encountered in systems coupled
to the electromagnetic field that exists within a photonic crystal. While for
all the parameter regimes explored here, the algorithm performance is worsened
by the contact with a thermal environment, the picture appears to be different
when considering a structured environment. In this case we show that, by tuning
the environment parameters to certain regimes, the algorithm performance can
actually be improved with respect to the closed system case. Additionally, the
relevance of considering the dissipation rates as complex quantities is
discussed in both cases. More particularly, we find that the imaginary part of
the rates can not be neglected with the usual argument that it simply amounts
to an energy shift, and in fact influences crucially the system dynamics.Comment: 18 pages, 9 figure
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