231 research outputs found
Use of 2G coated conductors for efficient shielding of DC magnetic fields
This paper reports the results of an experimental investigation of the
performance of two types of magnetic screens assembled from YBa2Cu3O7-d (YBCO)
coated conductors. Since effective screening of the axial DC magnetic field
requires the unimpeded flow of an azimuthal persistent current, we demonstrate
a configuration of a screening shell made out of standard YBCO coated conductor
capable to accomplish that. The screen allows the persistent current to flow in
the predominantly azimuthal direction at a temperature of 77 K. The persistent
screen, incorporating a single layer of superconducting film, can attenuate an
external magnetic field of up to 5 mT by more than an order of magnitude. For
comparison purposes, another type of screen which incorporates low critical
temperature quasi-persistent joints was also built. The shielding technique we
describe here appears to be especially promising for the realization of large
scale high-Tc superconducting screens.Comment: 8 pages, 3 figure
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Elementary school teachers\u27 perceptions of curriculum-based measures of written expression
Despite evidence indicating the general acceptability of curriculum-based measures (CBM) to teachers (e.g., Eckert, Shapiro, & Lutz, 1995), there has to date been no empirical evidence demonstrating the relative level of acceptability of measures of written expression that might set the stage for their increased adoption by teachers. Results of an exploratory survey designed to assess teacher\u27s perceptions of various measures of written expression are reported. The relative acceptability of the measures for this sample of 335 teachers from several locations throughout the United States, the relationship of the measures to demographics, and directions for future research are discussed. Accessed 25,349 times on https://pareonline.net from March 29, 2006 to December 31, 2019. For downloads from January 1, 2020 forward, please click on the PlumX Metrics link to the right
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
Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magnetic fields
We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets
can also act as efficient magnetic shields. The magnetic screening properties
under an axial DC magnetic field are characterized at several temperatures
below the liquid nitrogen temperature (77 K). Two main shielding properties are
studied and compared with those of Bi-2223, a material that has been considered
in the past for bulk magnetic shields. The first property is related to the
maximum magnetic flux density that can be screened, Blim; it is defined as the
applied magnetic flux density below which the field attenuation measured at the
centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall
thickness of 5 mm and a large ratio of length over radius, Blim is evaluated to
1 T at T = 10 K. This value largely exceeds the Blim value measured at the same
temperature on similar tubes of Bi-2223. The second shielding property that is
characterized is the dependence of Blim with respect to variations of the sweep
rate of the applied field, dBapp/dt. This dependence is interpreted in terms of
the power law E = Ec(J/Jc)^n and allows us to determine the exponent n of this
E(J) characteristics for Bi-2212. The characterization of the magnetic field
relaxation involves very small values of the electric field. This gives us the
opportunity to experimentally determine the E(J) law in an unexplored region of
small electric fields. Combining these results with transport and AC shielding
measurements, we construct a piecewise E(J) law that spans over 8 orders of
magnitude of the electric field.Comment: 16 pages, 7 figure
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
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
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
Color superconductivity in weak coupling
We derive perturbatively the gap equations for a color-superconducting
condensate with total spin J=0 in dense QCD. At zero temperature, we confirm
the results of Son for the dependence of the condensate on the coupling
constant, and compute the prefactor to leading logarithmic accuracy. At nonzero
temperature, we find that to leading order in weak coupling, the temperature
dependence of the condensate is identical to that in BCS-like theories. The
condensates for total spin J=1 are classified; to leading logarithmic accuracy
these condensates are of the same order as those of spin J=0.Comment: 30 pages, 3 figures, RevTeX, epsf and psfig style files require
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