18,091 research outputs found
Meissner-London currents in superconductors with rectangular cross section
Exact analytic solutions are presented for the magnetic moment and screening
currents in the Meissner state of superconductor strips with rectangular cross
section in a perpendicular magnetic field and/or with transport current. The
extension to finite London penetration is achieved by an elegant numerical
method which works also for disks. The surface current in the specimen corners
diverges as l^(-1/3) where l is the distance from the corner. This enhancement
reduces the barrier for vortex penetration and should increase the nonlinear
Meissner effect in d-wave superconductors
Non-linear electromagnetic interactions in thermal QED
We examine the behavior of the non-linear interactions between
electromagnetic fields at high temperature. It is shown that, in general, the
log(T) dependence on the temperature of the Green functions is simply related
to their UV behavior at zero-temperature. We argue that the effective action
describing the nonlinear thermal electromagnetic interactions has a finite
limit as T tends to infinity. This thermal action approaches, in the long
wavelength limit, the negative of the corresponding zero-temperature action.Comment: 7 pages, IFUSP/P-111
Thermodynamics and magnetic field profiles in low-kappa type-II superconductors
Two-dimensional low-kappa type-II superconductors are studied numerically
within the Eilenberger equations of superconductivity. Depending on the
Ginzburg-Landau parameter \kappa=\lambda/\xi vortex-vortex interaction can be
attractive or purely repulsive. The sign of interaction is manifested as a
first (second) order phase transition from Meissner to the mixed state.
Temperature and field dependence of the magnetic field distribution in
low-kappa type-II superconductors with attractive intervortex interaction is
calculated. Theoretical results are compared to the experiment.Comment: 4 pages, 3 figure
An overview of new supersymmetric gauge theories with 2-form gauge potentials
An overview of new 4d supersymmetric gauge theories with 2-form gauge
potentials constructed by various authors during the past five years is given.
The key role of three particular types of interaction vertices is emphasized.
These vertices are used to develop a connecting perspective on the new models
and to distinguish between them. One example is presented in detail to
illustrate characteristic features of the models. A new result on couplings of
2-form gauge potentials to Chern-Simons forms is presented.Comment: 11 pages; to appear in the proceedings of NATO ARW "Noncommutative
structures in mathematics and physics" (Kiev 09/00); table in section 3
correcte
Charge-ordered ferromagnetic phase in manganites
A mechanism for charge-ordered ferromagnetic phase in manganites is proposed.
The mechanism is based on the double exchange in the presence of diagonal
disorder. It is modeled by a combination of the Ising double-exchange and the
Falicov-Kimball model. Within the dynamical mean-field theory the charge and
spin correlation function are explicitely calculated. It is shown that the
system exhibits two successive phase transitions. The first one is the
ferromagnetic phase transition, and the second one is a charge ordering. As a
result a charge-ordered ferromagnetic phase is stabilized at low temperature.Comment: To appear in Phys. Rev.
Can the unresolved X-ray background be explained by emission from the optically-detected faint galaxies of the GOODS project?
The emission from individual X-ray sources in the Chandra Deep Fields and
XMM-Newton Lockman Hole shows that almost half of the hard X-ray background
above 6 keV is unresolved and implies the existence of a missing population of
heavily obscured active galactic nuclei (AGN). We have stacked the 0.5-8 keV
X-ray emission from optical sources in the Great Observatories Origins Deep
Survey (GOODS; which covers the Chandra Deep Fields) to determine whether these
galaxies, which are individually undetected in X-rays, are hosting the
hypothesised missing AGN. In the 0.5-6 keV energy range the stacked-source
emission corresponds to the remaining 10-20 per cent of the total background --
the fraction that has not been resolved by Chandra. The spectrum of the stacked
emission is consistent with starburst activity or weak AGN emission. In the 6-8
keV band, we find that upper limits to the stacked X-ray intensity from the
GOODS galaxies are consistent with the ~40 per cent of the total background
that remains unresolved, but further selection refinement is required to
identify the X-ray sources and confirm their contribution.Comment: 7 pages, 1 figure, accepted for publication in MNRA
Magnetic levitation force between a superconducting bulk magnet and a permanent magnet
The current density in a disk-shaped superconducting bulk magnet and the
magnetic levitation force exerted on the superconducting bulk magnet by a
cylindrical permanent magnet are calculated from first principles. The effect
of the superconducting parameters of the superconducting bulk is taken into
account by assuming the voltage-current law and the material law. The magnetic
levitation force is dominated by the remnant current density, which is induced
by switching off the applied magnetizing field. High critical current density
and flux creep exponent may increase the magnetic levitation force. Large
volume and high aspect ratio of the superconducting bulk can enhance the
magnetic levitation force further.Comment: 18 pages and 8 figure
The Luminosity Function of high-redshift QSOs - A combined analysis of GOODS and SDSS
Aims: In this work the luminosity function of QSOs is measured in the
redshift range 3.5<z<5.2 for the absolute magnitude interval -21<M_{145}<-28.
The determination of the faint end of the luminosity function at these
redshifts provides important constraints on models of joint evolution of
galaxies and AGNs. Methods: We have defined suitable criteria to select faint
QSOs in the GOODS fields, checking in detail their effectiveness and
completeness. Spectroscopic follow-up of the resulting QSO candidates has been
carried out. The confirmed sample of faint QSOs is compared with a brighter one
derived from the SDSS. We have used a Monte-Carlo technique to estimate the
properties of the luminosity function, checking various parameterizations for
its shape and evolution. Results: Models based on Pure Density Evolution show
better agreement with observation than models based on Pure Luminosity
Evolution. However a different break magnitude with respect to z~2.1 is
required at 3.5<z<5.2. Models with a steeper faint end score a higher
probability. We do not find any evidence for a flattening of the bright end at
redshift z>3.5. Conclusions: The estimated space density evolution of QSOs
indicates a suppression of the formation and/or feeding of Supermassive Black
Holes at these redshifts. The QSO contribution to the UV background is
insufficient to ionize the IGM at 3.5<z<5.2.Comment: 17 pages, 13 ps figures, A&A accepted. Updated to journal versio
Behavior of logarithmic branch cuts in the self-energy of gluons at finite temperature
We give a simple argument for the cancellation of the log(-k^2) terms (k is
the gluon momentum) between the zero-temperature and the temperature-dependent
parts of the thermal self-energy.Comment: 4 page
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