334 research outputs found
Surface Superconductivity in Niobium for Superconducting RF Cavities
A systematic study is presented on the superconductivity (sc) parameters of
the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz
cavities for the linear collider project TESLA. Cylindrical Nb samples have
been subjected to the same surface treatments that are applied to the TESLA
cavities: buffered chemical polishing (BCP), electrolytic polishing (EP),
low-temperature bakeout (LTB). The magnetization curves and the complex
magnetic susceptibility have been measured over a wide range of temperatures
and dc magnetic fields, and also for di erent frequencies of the applied ac
magnetic field. The bulk superconductivity parameters such as the critical
temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found
to be in good agreement with previous data. Evidence for surface
superconductivity at fields above Bc2 is found in all samples. The critical
surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in
BCP-treated samples and increases even further if EP or LTB are applied. From
the field dependence of the susceptibility and a power-law analysis of the
complex ac conductivity and resistivity the existence of two different phases
of surface superconductivity can be established which resemble the Meissner and
Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing
sc shielding currents flowing around the entire cylindrical sample, for
external fields B in the range between Bc2 and Bcohc3, and (2) incoherent
surface superconductivity with disconnected sc domains between Bcohc3 and Bc3.
The coherent critical surface field separating the two phases is found to be
Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are
different for BCP and EP samples, pointing to different surface topologies.Comment: 15 pages, 21 figures, DESY-Report 2004-02
Pairs Emission in a Uniform Background Field: an Algebraic Approach
A fully algebraic general approach is developed to treat the pairs emission
and absorption in the presence of some uniform external background field. In
particular, it is shown that the pairs production and annihilation operators,
together with the pairs number operator, do actually fulfill the SU(2)
functional Lie algebra. As an example of application, the celebrated Schwinger
formula is consistently and nicely recovered, within this novel approach, for a
Dirac spinor field in the presence of a constant and homogeneous electric field
in four spacetime dimensions.Comment: 26 pages, no figure
Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa2Cu3O7 single crystals
We report on transport and ac susceptibility studies below the peak effect in
twinned YBa2Cu3O7 single crystals. We find that disorder generated at the peak
effect can be partially inhibited by forcing vortices to move with an ac
driving current. The vortex system can be additionally ordered below a
well-defined temperature where elastic interactions between vortices overcome
pinning-generated stress and a plastic to elastic crossover seems to occur. The
combined effect of these two processes results in vortex structures with
different mobilities that give place to history effects.Comment: 4 pages, 4 figures. Published in PRB Rapid Comm., February 1, 200
Dynamical Lorentz simmetry breaking from 3+1 Axion-Wess-Zumino model
We study the renormalizable abelian vector-field models in the presence of
the Wess-Zumino interaction with the pseudoscalar matter. The renormalizability
is achieved by supplementing the standard kinetic term of vector fields with
higher derivatives. The appearance of fourth power of momentum in the
vector-field propagator leads to the super-renormalizable theory in which the
-function, the vector-field renormalization constant and the anomalous
mass dimension are calculated exactly. It is shown that this model has the
infrared stable fixed point and its low-energy limit is non-trivial. The
modified effective potential for the pseudoscalar matter leads to the possible
occurrence of dynamical breaking of the Lorentz symmetry. This phenomenon is
related to the modification of Electrodynamics by means of the Chern-Simons
(CS) interaction polarized along a constant CS vector. Its presence makes the
vacuum optically active that has been recently estimated from astrophysical
data. We examine two possibilities for the CS vector to be time-like or
space-like, under the assumption that it originates from v.e.v. of some
pseudoscalar matter and show that only the latter one is consistent in the
framework of the AWZ model, because a time-like CS vector makes the vacuum
unstable under pairs creation of tachyonic photon modes with the finite vacuum
decay rate.Comment: 33 pages, no Figures, Plain TeX, submitted to Phys. Rev.
Baryogenesis vs. proton stability in theories with extra dimensions
We propose a new scenario of baryogenesis in the context of theories with
large extra dimensions. The baryon number is almost conserved at zero
temperature by means of a localization mechanism recently analyzed by
Arkani-Hamed and Schmaltz: leptons and quarks are located at two slightly
displaced positions in the extra space, and this naturally suppresses the
interactions which ``convert'' the latter in the former. We show that this is
expected to be no longer true when finite temperature effects are taken into
account. The whole scenario is first presented in its generality, without
referring to the bulk geometry or to the specific mechanism which may generate
the baryon asymmetry. As an example, we then focus on a baryogenesis model
reminiscent of GUT baryogenesis. The Sakharov out of equilibrium condition is
satisfied by assuming nonthermal production of the bosons that induce baryon
number violation.Comment: 10 pages, 1 figur
Super-acceleration on the Brane by Energy Flow from the Bulk
We consider a brane cosmological model with energy exchange between brane and
bulk. Parameterizing the energy exchange term by the scale factor and Hubble
parameter, we are able to exactly solve the modified Friedmann equation on the
brane. In this model, the equation of state for the effective dark energy has a
transition behavior changing from to , while
the equation of state for the dark energy on the brane has . Fitting data
from type Ia supernova, Sloan Digital Sky Survey and Wilkinson Microwave
Anisotropy Probe, our universe is predicted now in the state of
super-acceleration with .Comment: Revtex, 11 pages including 2 figures,v2: tpos fixed, references
added, to appear in JCA
Spin polarization of electrons by ultraintense lasers
Electrons in plasmas produced by next-generation ultraintense lasers (I>5×1022W/cm2) can be spin polarized to a high degree (10%-70%) by the laser pulses on a femtosecond time scale. This is due to electrons undergoing spin-flip transitions as they radiate γ-ray photons, preferentially spin polarizing in one direction. Spin polarization can modify the radiation reaction force on the electrons, which differs by up to 30% for opposite spin polarizations. Consequently, the polarization of the radiated γ-ray photons is also modified: the relative power radiated in the σ and π components increases and decreases by up to 30%, respectively, potentially reducing the rate of pair production in the plasma by up to 30%
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