28 research outputs found
Self-consistent approximations: application to a quasiparticle description of the thermodynamic properties of relativistic plasmas
We generalize the concept of conserving,\Phi-derivable, approximations to
relativistic field theories. Treating the interaction field as a dynamical
degree of freedom, we derive the thermodynamical potential in terms of fully
dressed propagators, an approach which allows us to resolve the entropy of a
relativistic plasma into contributions from its interacting elementary
excitations. We illustrate the derivation for a hot QED plasma of massless
particles. We also discuss how the self-consistency of the treatment manifests
itself into relationships between the contributions from interaction and matter
fields.Comment: 9 pages, 1 eps figure, to appear in "Progress in Nonequilibrium
Green's functions.", M. Bonitz (Ed.), World Scientific, Singapore 200
Microscopic spectral density in random matrix models for chiral and diquark condensation
We examine random matrix models of QCD which are capable of supporting both
chiral and diquark condensation. A numerical study of the spectral densities
near zero virtuality shows that the introduction of color in the interactions
does not alter the one-body results imposed by chiral symmetry. A model with
three colors has the spectral density predicted for the chiral ensemble with a
Dyson index beta = 2; a pseudoreal model with two colors exhibits the spectral
density of the chiral ensemble with beta = 1.Comment: 6 pages, 3 eps figures, uses revtex4 and graphicx. v2 : minor
editions, Fig. 3 shows relative deviations rather than absolute. Version to
appear in PR
Random matrix model for antiferromagnetism and superconductivity on a two-dimensional lattice
We suggest a new mean field method for studying the thermodynamic competition
between magnetic and superconducting phases in a two-dimensional square
lattice. A partition function is constructed by writing microscopic
interactions that describe the exchange of density and spin-fluctuations. A
block structure dictated by spin, time-reversal, and bipartite symmetries is
imposed on the single-particle Hamiltonian. The detailed dynamics of the
interactions are neglected and replaced by a normal distribution of random
matrix elements. The resulting partition function can be calculated exactly.
The thermodynamic potential has a structure which depends only on the spectrum
of quasiparticles propagating in fixed condensation fields, with coupling
constants that can be related directly to the variances of the microscopic
processes. The resulting phase diagram reveals a fixed number of phase
topologies whose realizations depend on a single coupling-parameter ratio,
alpha. Most phase topologies are realized for a broad range of values of alpha
and can thus be considered robust with respect to moderate variations in the
detailed description of the underlying interactions.Comment: 21 pages, 8 figures, RevTex 4. Minor grammatical errors corrected in
the last versio
Random matrix models for chiral and diquark condensation
We consider random matrix models for the thermodynamic competition between
chiral symmetry breaking and diquark condensation in QCD at finite temperature
and finite baryon density. The models produce mean field phase diagrams whose
topology depends solely on the global symmetries of the theory. We discuss the
block structure of the interactions that is imposed by chiral, spin, and color
degrees of freedom and comment on the treatment of density and temperature
effects. Extension of the coupling parameters to a larger class of theories
allows us to investigate the robustness of the phase topology with respect to
variations in the dynamics of the interactions. We briefly study the phase
structure as a function of coupling parameters and the number of colors.Comment: 6 pages, 2 figures, proceedings of the workshop "Three Days of
Hadronic Physics", Joint Meeting Heidelberg-Liege-Paris-Rostock,
16/12/2004-18/12/2004, Sol Cress, Spa, Belgium. v2: typographical errors
corrected in reference
Self-consistent approximations in relativistic plasmas: Quasiparticle analysis of the thermodynamic properties
We generalize the concept of conserving, Phi-derivable, approximations to
relativistic field theories. Treating the interaction field as a dynamical
degree of freedom, we derive the thermodynamic potential in terms of fully
dressed propagators, an approach which allows us to resolve the entropy of a
relativistic plasma into contributions from its interacting elementary
excitations. We illustrate the derivation for a hot relativistic system
governed by electromagnetic interactions.Comment: 22 pages, 3 figures, submitted to J. Stat. Phys. 98-74, a special
issue dedicated to Leo Kadanoff on his sixtieth birthda
Numerical simulation of the magnetization of high-temperature superconductors: 3D finite element method using a single time-step iteration
We make progress towards a 3D finite-element model for the magnetization of a
high temperature superconductor (HTS): We suggest a method that takes into
account demagnetisation effects and flux creep, while it neglects the effects
associated with currents that are not perpendicular to the local magnetic
induction. We consider samples that are subjected to a uniform magnetic field
varying linearly with time. Their magnetization is calculated by means of a
weak formulation in the magnetostatic approximation of the Maxwell equations
(A-phi formulation). An implicit method is used for the temporal resolution
(Backward Euler scheme) and is solved in the open source solver GetDP. Picard
iterations are used to deal with the power law conductivity of HTS. The finite
element formulation is validated for an HTS tube with large pinning strength
through the comparison with results obtained with other well-established
methods. We show that carrying the calculations with a single time-step (as
opposed to many small time-steps) produce results with excellent accuracy in a
drastically reduced simulation time. The numerical method is extended to the
study of the trapped magnetization of cylinders that are drilled with different
arrays of columnar holes arranged parallel to the cylinder axis
Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields
We have experimentally studied the magnetic shielding properties of a
cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic
fields. The magnetic response has been investigated as a function of the
dimensions of the tube, the magnitude of the applied field and the frequency.
These results are explained quantitatively by employing the method of Brandt
(1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline
material. Specifically, we observe that the applied field can sweep into the
central region either through the thickness of the shield or through the
opening ends, the latter mechanism being suppressed for long tubes. For the
first time, we systematically detail the spatial variation of the shielding
factor (the ratio of the applied field over the internal magnetic field) along
the axis of a high-temperature superconducting tube. The shielding factor is
shown to be constant in a region around the centre of the tube, and to decrease
as an exponential in the vicinity of the ends. This spatial dependence comes
from the competition between two mechanisms of field penetration. The frequency
dependence of the shielding factor is also discussed and shown to follow a
power law arising from the finite creep exponent n.Comment: 22 pages, 10 figure
Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release by reversible phosphorylation and dephosphorylation
AbstractThe inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a universal intracellular Ca2+-release channel. It is activated after cell stimulation and plays a crucial role in the initiation and propagation of the complex spatio-temporal Ca2+ signals that control cellular processes as different as fertilization, cell division, cell migration, differentiation, metabolism, muscle contraction, secretion, neuronal processing, and ultimately cell death. To achieve these various functions, often in a single cell, exquisite control of the Ca2+ release is needed. This review aims to highlight how protein kinases and protein phosphatases can interact with the IP3R or with associated proteins and so provide a large potential for fine tuning the Ca2+-release activity and for creating efficient Ca2+ signals in subcellular microdomains
Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor
We use macroscopic holes drilled in a bulk YBCO superconductor to probe its
magnetic properties in the volume of the sample. The sample is subjected to an
AC magnetic flux with a density ranging from 30mT to 130mT and the flux in the
superconductor is probed by miniature coils inserted in the holes. In a given
hole, three different penetration regimes can be observed: (i) the shielded
regime, where no magnetic flux threads the hole; (ii) the gradual penetration
regime, where the waveform of the magnetic field has a clipped sine shape whose
fundamental component scales with the applied field; and (iii) the flux
concentration regime, where the waveform of the magnetic field is nearly a sine
wave, with an amplitude exceeding that of the applied field by up to a factor
of two. The distribution of the penetration regimes in the holes is compared
with that of the magnetic flux density at the top and bottom surfaces of the
sample, and is interpreted with the help of optical polarized light micrographs
of these surfaces. We show that the measurement of the magnetic field inside
the holes can be used as a local characterization of the bulk magnetic
properties of the sample