316 research outputs found
Lifshitz and Excited State Quantum Phase Transitions in Microwave Dirac Billiards
We present experimental results for the density of states (DOS) of a
superconducting microwave Dirac billiard which serves as an idealized model for
the electronic properties of graphene. The DOS exhibits two sharp peaks which
evolve into van Hove singularities with increasing system size. They divide the
band structure into regions governed by the \emph{relativistic} Dirac equation
and by the \emph{non-relativistic} Schr\"odinger equation, respectively. We
demonstrate that in the thermodynamic limit a topological transition appears as
a neck-disrupting Lifshitz transition in the number susceptibility and as an
excited state transition in the electronic excitations. Furthermore, we recover
the finite-size scaling typical for excited state quantum phase transitions
involving logarithmic divergences and identify a quasi-order parameter
Color-superconductivity in the strong-coupling regime of Landau gauge QCD
The chirally unbroken and the superconducting 2SC and CFL phases are
investigated in the chiral limit within a Dyson-Schwinger approach for the
quark propagator in QCD. The hierarchy of Green's functions is truncated such
that at vanishing density known results for the vacuum and at asymptotically
high densities the corresponding weak-coupling expressions are recovered. The
anomalous dimensions of the gap functions are analytically calculated. Based on
the quark propagator the phase structure is studied, and results for the gap
functions, occupation numbers, coherence lengths and pressure differences are
given and compared with the corresponding expressions in the weak-coupling
regime. At moderate chemical potentials the quasiparticle pairing gaps are
several times larger than the extrapolated weak-coupling results.Comment: 14 pages, 9 figures; v2: one reference adde
On the appearance of hyperons in neutron stars
By employing a recently constructed hyperon-nucleon potential the equation of
state of \beta-equilibrated and charge neutral nucleonic matter is calculated.
The hyperon-nucleon potential is a low-momentum potential which is obtained
within a renormalization group framework. Based on the Hartree-Fock
approximation at zero temperature the densities at which hyperons appear in
neutron stars are estimated. For several different bare hyperon-nucleon
potentials and a wide range of nuclear matter parameters it is found that
hyperons in neutron stars are always present. These findings have profound
consequences for the mass and radius of neutron stars.Comment: 12 pages, 12 figures, RevTeX4; summary and conclusions are
strengthened, to appear in PR
semileptonic decay in covariant quark models \`a la Bakamjian Thomas
Once chosen the dynamics in one frame, for example the rest frame, the
Bakamjian and Thomas method allows to define relativistic quark models in any
frame. These models have been shown to provide, in the heavy quark limit, fully
covariant current form factors as matrix elements of the quark current
operator. They also verify the Isgur-Wise scaling and give a slope parameter
for all the possible choices of the dynamics. In this paper we
study the excited states and derive the general formula, valid for any
dynamics, for the scaling invariant form factors and
. We also check the Bjorken-Isgur-Wise sum rule already
demonstrated elsewhere in this class of models.Comment: 14 pages, Latex2e, AMS-LaTe
Gene therapy potential for genetic disorders of surfactant dysfunction
Pulmonary surfactant is critically important to prevent atelectasis by lowering the surface tension of the alveolar lining liquid. While respiratory distress syndrome (RDS) is common in premature infants, severe RDS in term and late preterm infants suggests an underlying genetic etiology. Pathogenic variants in the genes encoding key components of pulmonary surfactant including surfactant protein B (SP-B
Model--space approach to neutron and proton pairing with the Bonn meson--exchange potentials
In this work we calculate neutron and proton energy gaps in neutron star
matter, using the Bonn meson--exchange interactions and a model--space approach
to the gap equation. This approach allows a consistent calculation of energy
gaps and single particle energies with the model--space
Brueckner--Hartree--Fock (MBHF) method, without double counting of
two--particle correlations. Neutron energy gaps are calculated at zero and
finite temperature. Proton energy gaps are calculated at beta equilibrium, and
it is shown that the inclusion of muons has a significant effect. The results
are compared with those of other works, and the implications for neutron star
physics are briefly discussed.Comment: uuencoded file, 19 pages, 15 figures include
Olefin Hydroarylation Catalyzed by (Pyridyl-Indolate)Pt(II) Complexes: Catalytic Efficiencies and Mechanistic Aspects
A series of Pt(II) complexes of the type (NâN)PtPh(SR_2) (NâN = 2,2â˛-pyridyl-indolate) were prepared, and their performance as catalysts for the hydroarylation of olefins was assessed. Evidence that the catalysis is homogeneous and is Pt-mediated is provided by control experiments with added hindered base (2,6-di-tert-butyl-4-methylpyridine) and Hg(0). Two potential catalytic intermediates, (^tBuPyInd)PtPh(C_2H_4) and (^tBuPyInd)Pt(CH_2CH_2Ph)(C_2H_4), were synthesized, and their catalytic efficacy was explored. Additionally, decomposition and deactivation pathways, including styrene formation via β-hydride elimination and ligand reductive demetalation, were identified
Medium Modifications of the Rho Meson at CERN/SPS Energies
Rho meson propagation in hot hadronic matter is studied in a model with
coupling to states. Medium modifications are induced by a change of
the pion dispersion relation through collisions with nucleons and in
the fireball. Maintaining gauge invariance dilepton production is calculated
and compared to the recent data of the CERES collaboration in central S+Au
collisions at 200 GeV/u. The observed enhancement of the rate below the rho
meson mass can be largely accounted for.Comment: 10 pages RevTeX and 2 figures (uuencoded .ps-files
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