60,307 research outputs found
Studying pion effects on the chiral phase transition
We investigate the chiral phase transition at finite temperatures and zero
chemical potential with Dyson-Schwinger equations. Our truncation for the
quark-gluon interaction includes mesonic degrees of freedom, which allows us to
study the impact of the pions on the nature of the phase transition. Within the
present scheme we find a five percent change of the critical temperature due to
the pion backreaction whereas the mean field character of the transition is not
changed.Comment: 2 pages, 2 figures, talk given by J.A.M. at the 30th International
School of Nuclear Physics, Erice, Sicily from 16 - 24 September 200
Smooth relativistic Hartree-Fock pseudopotentials for H to Ba and Lu to Hg
We report smooth relativistic Hartree-Fock pseudopotentials (also known as
averaged relativistic effective potentials or AREPs) and spin-orbit operators
for the atoms H to Ba and Lu to Hg. We remove the unphysical extremely
non-local behaviour resulting from the exchange interaction in a controlled
manner, and represent the resulting pseudopotentials in an analytic form
suitable for use within standard quantum chemistry codes. These
pseudopotentials are suitable for use within Hartree-Fock and correlated wave
function methods, including diffusion quantum Monte Carlo calculations.Comment: 13 pages, 3 figure
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
Thermal Conductivity of Single Wall Carbon Nanotubes: Diameter and Annealing Dependence
The thermal conductivity, k(T), of bulk single-wall carbon nanotubes (SWNT's)
displays a linear temperature dependence at low T that has been attributed to
1D quantization of phonons. To explore this issue further, we have measured the
k(T) of samples with varying average tube diameters. We observe linear k(T) up
to higher temperatures in samples with smaller diameters, in agreement with a
quantization picture. In addition, we have examined the effect of annealing on
k(T). We observe an enhancement in k(T) for annealed samples which we attribute
to healing of defects and removal of impurities. These measurements demonstrate
how the thermal properties of an SWNT material can be controlled by
manipulating its intrinsic nanoscale properties.Comment: Proc. of the XV. Int. Winterschool on Electronic Properties of Novel
Materials, Kirchberg/Tirol, Austria, 200
Enhanced tracer transport by the spiral defect chaos state of a convecting fluid
To understand how spatiotemporal chaos may modify material transport, we use
direct numerical simulations of the three-dimensional Boussinesq equations and
of an advection-diffusion equation to study the transport of a passive tracer
by the spiral defect chaos state of a convecting fluid. The simulations show
that the transport is diffusive and is enhanced by the spatiotemporal chaos.
The enhancement in tracer diffusivity follows two regimes. For large Peclet
numbers (that is, small molecular diffusivities of the tracer), we find that
the enhancement is proportional to the Peclet number. For small Peclet numbers,
the enhancement is proportional to the square root of the Peclet number. We
explain the presence of these two regimes in terms of how the local transport
depends on the local wave numbers of the convection rolls. For large Peclet
numbers, we further find that defects cause the tracer diffusivity to be
enhanced locally in the direction orthogonal to the local wave vector but
suppressed in the direction of the local wave vector.Comment: 11 pages, 12 figure
What the Infrared Behaviour of QCD Vertex Functions in Landau gauge can tell us about Confinement
The infrared behaviour of Landau gauge QCD vertex functions is investigated
employing a skeleton expansion of the Dyson-Schwinger and Renormalization Group
equations. Results for the ghost-gluon, three-gluon, four-gluon and quark-gluon
vertex functions are presented. Positivity violation of the gluon propagator,
and thus gluon confinement, is demonstrated. Results of the Dyson-Schwinger
equations for a finite volume are compared to corresponding lattice data. It is
analytically demonstrated that a linear rising potential between heavy quarks
can be generated by infrared singularities in the dressed quark-gluon vertex.
The selfconsistent mechanism that generates these singularities necessarily
entails the scalar Dirac amplitudes of the full vertex and the quark
propagator. These can only be present when chiral symmetry is broken, either
explicitly or dynamically.Comment: 13 pages, 13 figures; to appear in the Proceedings of ``X Hadron
Physics 2007'', Florianopolis, Brazil, March 26 - 31, 200
The nonrelativistic limit of Dirac-Fock codes: the role of Brillouin configurations
We solve a long standing problem with relativistic calculations done with the
widely used Multi-Configuration Dirac-Fock Method (MCDF). We show, using
Relativistic Many-Body Perturbation Theory (RMBPT), how even for relatively
high-, relaxation or correlation causes the non-relativistic limit of states
of different total angular momentum but identical orbital angular momentum to
have different energies. We show that only large scale calculations that
include all single excitations, even those obeying the Brillouin's theorem have
the correct limit. We reproduce very accurately recent high-precision
measurements in F-like Ar, and turn then into precise test of QED. We obtain
the correct non-relativistic limit not only for fine structure but also for
level energies and show that RMBPT calculations are not immune to this problem.Comment: AUgust 9th, 2004 Second version Nov. 18th, 200
Observations of solar small-scale magnetic flux-sheet emergence
Aims. Moreno-Insertis et al. (2018) recently discovered two types of flux
emergence in their numerical simulations: magnetic loops and magnetic sheet
emergence. Whereas magnetic loop emergence has been documented well in the last
years, by utilising high-resolution full Stokes data from ground-based
telescopes as well as satellites, magnetic sheet emergence is still an
understudied process. We report here on the first clear observational evidence
of a magnetic sheet emergence and characterise its development.
Methods. Full Stokes spectra from the Hinode spectropolarimeter were inverted
with the SIR code to obtain solar atmospheric parameters such as temperature,
line-of-sight velocities and full magnetic field vector information.
Results. We analyse a magnetic flux emergence event observed in the quiet-sun
internetwork. After a large scale appearance of linear polarisation, a magnetic
sheet with horizontal magnetic flux density of up to 194 Mx/cm hovers in
the low photosphere spanning a region of 2 to 3 arcsec. The magnetic field
azimuth obtained through Stokes inversions clearly shows an organised structure
of transversal magnetic flux density emerging. The granule below the magnetic
flux-sheet tears the structure apart leaving the emerged flux to form several
magnetic loops at the edges of the granule.
Conclusions. A large amount of flux with strong horizontal magnetic fields
surfaces through the interplay of buried magnetic flux and convective motions.
The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic
flux at the foot points of the order of Mx. This is one to two
orders of magnitude larger than what has been reported for small-scale magnetic
loops. The convective flows feed the newly emerged flux into the pre-existing
magnetic population on a granular scale.Comment: 6 pages, 5 figures, accepted as a letter in A&
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