1,603 research outputs found
Stability in the instantaneous Bethe-Salpeter formalism: harmonic-oscillator reduced Salpeter equation
A popular three-dimensional reduction of the Bethe-Salpeter formalism for the
description of bound states in quantum field theory is the Salpeter equation,
derived by assuming both instantaneous interactions and free propagation of all
bound-state constituents. Numerical (variational) studies of the Salpeter
equation with confining interaction, however, observed specific instabilities
of the solutions, likely related to the Klein paradox and rendering (part of
the) bound states unstable. An analytic investigation of this problem by a
comprehensive spectral analysis is feasible for the reduced Salpeter equation
with only harmonic-oscillator confining interactions. There we are able to
prove rigorously that the bound-state solutions correspond to real discrete
energy spectra bounded from below and are thus free of any instabilities.Comment: 23 pages, 3 figures, extended conclusions, version to appear in Phys.
Rev.
Quantum phase transitions in photonic cavities with two-level systems
Systems of coupled photonic cavities have been predicted to exhibit quantum
phase transitions by analogy with the Hubbard model. To this end, we have
studied topologies of few (up to six) photonic cavities each containing a
single two-level system. Quantum phase space diagrams are produced for these
systems, and compared to mean-field results. We also consider finite effective
temperature, and compare this to the notion of disorder. We find the extent of
the Mott lobes shrink analogously to the conventional Bose-Hubbard model.Comment: 11 pages, 11 figures, updated typo
Linking entanglement and quantum phase transitions via density functional theory
Density functional theory (DFT) is shown to provide a novel conceptual and
computational framework for entanglement in interacting many-body quantum
systems. DFT can, in particular, shed light on the intriguing relationship
between quantum phase transitions and entanglement. We use DFT concepts to
express entanglement measures in terms of the first or second derivative of the
ground state energy. We illustrate the versatility of the DFT approach via a
variety of analytically solvable models. As a further application we discuss
entanglement and quantum phase transitions in the case of mean field
approximations for realistic models of many-body systems.Comment: 6 pages, 2 figure
New Method to Calculate Electrical Forces Acting on a Sphere in an Electrorheological Fluid
We describe a method to calculate the electrical force acting on a sphere in
a suspension of dielectric spheres in a host with a different dielectric
constant, under the assumption that a spatially uniform electric field is
applied. The method uses a spectral representation for the total electrostatic
energy of the composite. The force is expressed as a certain gradient of this
energy, which can be expressed in a closed analytic form rather than evaluated
as a numerical derivative. The method is applicable even when both the spheres
and the host have frequency-dependent dielectric functions and nonzero
conductivities, provided the system is in the quasistatic regime. In principle,
it includes all multipolar contributions to the force, and it can be used to
calculate multi-body as well as pairwise forces. We also present several
numerical examples, including host fluids with finite conductivities. The force
between spheres approaches the dipole-dipole limit, as expected, at large
separations, but departs drastically from that limit when the spheres are
nearly in contact. The force may also change sign as a function of frequency
when the host is a slightly conducting fluid.Comment: 29 pages, 8 figures, Accepted for Publication in Physical Review
Sampling of quantum dynamics at long time
The principle of energy conservation leads to a generalized choice of
transition probability in a piecewise adiabatic representation of
quantum(-classical) dynamics. Significant improvement (almost an order of
magnitude, depending on the parameters of the calculation) over previous
schemes is achieved. Novel perspectives for theoretical calculations in
coherent many-body systems are opened.Comment: Revised versio
Point Interaction in two and three dimensional Riemannian Manifolds
We present a non-perturbative renormalization of the bound state problem of n
bosons interacting with finitely many Dirac delta interactions on two and three
dimensional Riemannian manifolds using the heat kernel. We formulate the
problem in terms of a new operator called the principal or characteristic
operator. In order to investigate the problem in more detail, we then restrict
the problem to one particle sector. The lower bound of the ground state energy
is found for general class of manifolds, e.g., for compact and Cartan-Hadamard
manifolds. The estimate of the bound state energies in the tunneling regime is
calculated by perturbation theory. Non-degeneracy and uniqueness of the ground
state is proven by Perron-Frobenius theorem. Moreover, the pointwise bounds on
the wave function is given and all these results are consistent with the one
given in standard quantum mechanics. Renormalization procedure does not lead to
any radical change in these cases. Finally, renormalization group equations are
derived and the beta-function is exactly calculated. This work is a natural
continuation of our previous work based on a novel approach to the
renormalization of point interactions, developed by S. G. Rajeev.Comment: 43 page
Stability and phase coherence of trapped 1D Bose gases
We discuss stability and phase coherence of 1D trapped Bose gases and find
that inelastic decay processes, such as 3-body recombination, are suppressed in
the strongly interacting (Tonks-Girardeau) and intermediate regimes. This is
promising for achieving these regimes with a large number of particles.
"Fermionization" of the system reduces the phase coherence length, and at T=0
the gas is fully phase coherent only deeply in the weakly interacting
(Gross-Pitaevskii) regime.Comment: published versio
Measuring Perceived Realistic Physical Threat Imposed by Migrants: Scale Development and Validation
Individuals differ in the extent to which they perceive threat imposed by out-groups like migrants. An established distinction in intergroup threat research is between symbolic and realistic threat. While symbolic threats concern a perceived menace against societal values, realistic threats jeopardize in-group members’ well-being more directly. Typically applied realistic threat conceptions explicitly include the aspect of physical integrity, but most empirical research captures only realistic economic threats, arguably also due to a lack of appropriate measures. Therefore, we have developed the Perceived Realistic Physical Threat scale (PRPT) with samples from Germany and the UK (total N = 1,391). Moreover, we conducted follow-up analyses with data from a subsample (N = 473) of the initial UK sample. Factor analyses indicated an 8-item one-factorial solution for the PRPT scale. We further identified measurement invariance across samples and over time and stability across 21 months. We found convincing evidence for its convergent and divergent validity and for its predictive and, importantly, incremental validity, above and beyond the prediction of relevant criteria by other threat types. The PRPT scale appears to be a distinct, comprehensive, and psychometrically sound measure of perceived realistic physical threat, complementing the existing body of available measures
Pion-delta sigma-term
We use a configuration space chiral model in order to evaluate nucleon and
delta sigma-terms. Analytic expressions are consistent with chiral counting
rules and give rise to expected non-analytic terms in the chiral limit. We
obtain the results MeV and MeV, which are
very close to values extracted from experiment and produced by other groups.Comment: 18 pages, 4 figure
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