201 research outputs found
Classical analogy for the deflection of flux avalanches by a metallic layer
Sudden avalanches of magnetic flux bursting into a superconducting sample
undergo deflections of their trajectories when encountering a conductive layer
deposited on top of the superconductor. Remarkably, in some cases flux is
totally excluded from the area covered by the conductive layer. We present a
simple classical model that accounts for this behaviour and considers a
magnetic monopole approaching a semi-infinite conductive plane. This model
suggests that magnetic braking is an important mechanism responsible for
avalanche deflection.Comment: 14 pages, 5 figure
Strange quark matter: mapping QCD lattice results to finite baryon density by a quasi-particle model
A quasi-particle model is presented which describes QCD lattice results for
the 0, 2 and 4 quark-flavor equation of state. The results are mapped to finite
baryo-chemical potentials. As an application of the model we make a prediction
of deconfined matter with appropriate inclusion of strange quarks and consider
pure quark stars.Comment: invited talk at Strangeness 2000, Berkeley; prepared version for the
proceedings, 5 page
Random Matrix Theory and Chiral Symmetry in QCD
Random matrix theory is a powerful way to describe universal correlations of
eigenvalues of complex systems. It also may serve as a schematic model for
disorder in quantum systems. In this review, we discuss both types of
applications of chiral random matrix theory to the QCD partition function. We
show that constraints imposed by chiral symmetry and its spontaneous breaking
determine the structure of low-energy effective partition functions for the
Dirac spectrum. We thus derive exact results for the low-lying eigenvalues of
the QCD Dirac operator. We argue that the statistical properties of these
eigenvalues are universal and can be described by a random matrix theory with
the global symmetries of the QCD partition function. The total number of such
eigenvalues increases with the square root of the Euclidean four-volume. The
spectral density for larger eigenvalues (but still well below a typical
hadronic mass scale) also follows from the same low-energy effective partition
function. The validity of the random matrix approach has been confirmed by many
lattice QCD simulations in a wide parameter range. Stimulated by the success of
the chiral random matrix theory in the description of universal properties of
the Dirac eigenvalues, the random matrix model is extended to nonzero
temperature and chemical potential. In this way we obtain qualitative results
for the QCD phase diagram and the spectrum of the QCD Dirac operator. We
discuss the nature of the quenched approximation and analyze quenched Dirac
spectra at nonzero baryon density in terms of an effective partition function.
Relations with other fields are also discussed.Comment: invited review article for Ann. Rev. Nucl. Part. Sci., 61 pages, 11
figures, uses ar.sty (included); references added and typos correcte
Design of a superconducting magnetic shield closed on both ends for a high-sensitivity particle detector
peer reviewedThis work deals with the numerical design of a
high-efficiency superconducting magnetic shield required for a
high-sensitivity particle detector. This research was carried out
in the context of the ‘ABRACADABRA’ project aiming at
detecting hypothetical elementary particles called axions.
Axions are promising candidates to explain the particle nature
of the dark matter. The detector relies on a SQUID for
measuring the ultra-small oscillating magnetic field resulting
from the interaction between the axions and a toroidal DC field.
A magnetic shield is mandatory to reduce the ambient magnetic
field noise. Given the operating temperature (~ 1.2 K), the shield
is made of type-I superconductor. In this work we use numerical
modelling to determine the best topology for the shield and its
ability to screen both axial and transverse fields. Amongst the
geometries investigated (tubes or ‘swiss-rolls’ closed on both
ends) the best results are obtained with two semi-closed tubes
inserted in one another. This geometry is close to the shield of
the final prototype, made of two closed Cu tubes, spin-coated
with Sn (Tc = 3.72 K) and welded shut
Random matrix model for chiral symmetry breaking and color superconductivity in QCD at finite density
We consider a random matrix model which describes the competition between
chiral symmetry breaking and the formation of quark Cooper pairs in QCD at
finite density. We study the evolution of the phase structure in temperature
and chemical potential with variations of the strength of the interaction in
the quark-quark channel and demonstrate that the phase diagram can realize a
total of six different topologies. A vector interaction representing
single-gluon exchange reproduces a topology commonly encountered in previous
QCD models, in which a low-density chiral broken phase is separated from a
high-density diquark phase by a first-order line. The other five topologies
either do not possess a diquark phase or display a new phase and new critical
points. Since these five cases require large variations of the coupling
constants away from the values expected for a vector interaction, we conclude
that the phase diagram of finite density QCD has the topology suggested by
single-gluon exchange and that this topology is robust.Comment: ReVTeX, 22 pages, 14 figures. An animated gif movie showing the
evolution of the phase diagram with the coupling constants can be viewed at
http://www.nbi.dk/~vdheyden/QCDpd.htm
An AC susceptometer for the characterization of large, bulk superconducting samples
The main purpose of this work was to design, develop and construct a simple,
low-cost AC susceptometer to measure large, bulk superconducting samples (up to
32 mm in diameter) in the temperature range 78-120 K. The design incorporates a
double heating system that enables a high heating rate (25 K/hour) while
maintaining a small temperature gradient (< 0.2 K) across the sample. The
apparatus can be calibrated precisely using a copper coil connected in series
with the primary coil. The system has been used successfully to measure the
temperature dependence of the AC magnetic properties of entire RE-Ba-Cu-O
[(RE)BCO] bulk superconducting domains. A typical AC susceptibility measurement
run from 78 K to 95 K takes about 2 hours, with excellent temperature
resolution (temperature step ~ 4 mK) around the critical temperature, in
particular.Comment: 25 pages, 7 figures. Accepted for publication in Measurement Science
and Technolog
Nonequilibrium Dynamics of Optical Lattice-Loaded BEC Atoms: Beyond HFB Approximation
In this work a two-particle irreducible (2PI) closed-time-path (CTP)
effective action is used to describe the nonequilibrium dynamics of a Bose
Einstein condensate (BEC) selectively loaded into every third site of a
one-dimensional optical lattice. The motivation of this work is the recent
experimental realization of this system at National Institute of Standards and
Technology (NIST) where the placement of atoms in an optical lattice is
controlled by using an intermediate superlattice. Under the 2PI CTP scheme with
this initial configuration, three different approximations are considered: a)
the Hartree-Fock-Bogoliubov (HFB) approximation, b) the next-to-leading order
1/ expansion of the 2PI effective action up to second order in the
interaction strength and c) a second order perturbative expansion in the
interaction strength. We present detailed comparisons between these
approximations and determine their range of validity by contrasting them with
the exact many body solution for a moderate number of atoms and wells. As a
general feature we observe that because the second order 2PI approximations
include multi-particle scattering in a systematic way, they are able to capture
damping effects exhibited in the exact solution that a mean field collisionless
approach fails to produce. While the second order approximations show a clear
improvement over the HFB approximation our numerical result shows that they do
not work so well at late times, when interaction effects are significant.Comment: 34 pages, 7 figure
Microscopic correlations of non-Hermitian Dirac operators in three-dimensional QCD
In the presence of a non-vanishing chemical potential the eigenvalues of the Dirac operator become complex. We calculate spectral correlation functions of complex eigenvalues using a random matrix model approach. Our results apply to non-Hermitian Dirac operators in three-dimensional QCD with broken flavor symmetry and in four-dimensional QCD in the bulk of the spectrum. The derivation follows earlier results of Fyodorov, Khoruzhenko and Sommers for complex spectra exploiting the existence of orthogonal polynomials in the complex plane. Explicit analytic expressions are given for all microscopic k-point correlation functions in the presence of an arbitrary even number of massive quarks, both in the limit of strong and weak non-Hermiticity. In the latter case the parameter governing the non-Hermiticity of the Dirac matrices is identified with the influence of the chemical potential
BCS vs Overhauser pairing in dense (2+1)d QCD
We compare the BCS and Overhauser effect as competing mechanisms for the
destabilization of the quark Fermi surface at asymptotically large chemical
potential, for the special case of 2 space and 1 time dimension. We use the
framework of perturbative one-gluon exchange, which dominates the pairing at
. With screening in matter, we show that in the weak coupling
limit the Overhauser effect can compete with the BCS effect only for a
sufficiently large number of colors. Both the BCS and the Overhauser gaps are
of order in Landau gauge.Comment: 10 pages, no figur
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