54,880 research outputs found
Magnetic collapse of a neutron gas: Can magnetars indeed be formed
A relativistic degenerate neutron gas in equilibrium with a background of
electrons and protons in a magnetic field exerts its pressure anisotropically,
having a smaller value perpendicular than along the magnetic field. For
critical fields the magnetic pressure may produce the vanishing of the
equatorial pressure of the neutron gas. Taking it as a model for neutron stars,
the outcome could be a transverse collapse of the star. This fixes a limit to
the fields to be observable in stable neutron star pulsars as a function of
their density. The final structure left over after the implosion might be a
mixed phase of nucleons and meson condensate, a strange star, or a highly
distorted black hole or black "cigar", but no any magnetar, if viewed as a
super strongly magnetized neutron star. However, we do not exclude the
possibility of a supersotrong magnetic fields arising in supernova explosions
which lead directly to strange stars. In other words, if any magnetars exist,
they cannot be neutron stars.Comment: 15 pages, 3 figures. European Physical Journal C in pres
Perpendicular transport properties of YBa_2Cu_3O_{7-\delta}/PrBa_2Cu_3O_{7-\delta} superlattices
The coupling between the superconducting planes of YBa2Cu3O{7-\delta}/
PrBa2Cu3O{7-\delta} superlattices has been measured by c-axis transport. We
show that only by changing the thickness of the superconducting
YBa2Cu3O{7-\delta} layers, it is possible to switch between quasi-particle and
Josephson tunneling. From our data we deduce a low temperature c-axis coherence
length of 0.27 nm.Comment: Presented at LT22, contains 2 pages and 2 figures. to appear in
Physica
Dynamics of a magnetic dimer with exchange, dipolar and Dzyalozhinski-Moriya interaction
We investigate the dynamics of a magnetic system consisting of two magnetic
moments coupled by either exchange, dipole-dipole, or Dzyalozhinski-Moriya
interaction. We compare the switching mechanisms and switching rates as induced
by the three couplings. For each coupling and each configuration of the two
anisotropy axes, we describe the switching modes and, using the kinetic theory
of Langer, we provide (semi-)analytical expressions for the switching rate. We
then compare the three interactions with regard to their efficiency in the
reversal of the net magnetic moment of the dimer. We also investigate how the
energy barriers vary with the coupling. For the dipole-dipole interaction we
find that the energy barrier may either increase or decrease with the coupling
depending on whether the latter is weak or strong. Finally, upon comparing the
various switching rates, we find that the dipole-dipole coupling leads to the
slowest magnetic dimer, as far as the switching of its net magnetic moment is
concerned.Comment: 20 pages, 18 Figures, 2 table
Obtaining Au thin films in atmosphere of reactive nitrogen through magnetron sputtering
4d and 5d series of the transition metals are used to the obtaining nitrides metallic, due to the synthesis of PtN, AgN and AuN in the last years. Different nitrides are obtained in the Plasma Assisted Physics Vapour Deposition system, due to its ionization energy which is necessary for their formation. In this paper a Magnetron Sputtering system was used to obtain Au thin films on Si wafers in Nitrogen atmosphere. The substrate temperature was varied between 500 to 950°C. The samples obtained at high temperatures (>500°C) show Au, Si and N elements, as it is corroborated in the narrow spectrum obtained for X-Ray Photoelectron Spectroscopy; besides the competition of orientation crystallographic texture between (111) and (311) directions was present in the X-Ray Diffraction analysis to the sample heated at 950°C
Nonlinear dynamic analysis of shells of revolution by matrix displacement method
Nonlinear dynamic analysis of shells of revolution by matrix displacement metho
Electroweak Absorptive Parts in NRQCD Matching Conditions
Electroweak corrections associated with the instability of the top quark to
the next-to-next-to-leading logarithmic (NNLL) total top pair threshold cross
section in e+e- annihilation are determined. Our method is based on absorptive
parts in electroweak matching conditions of the NRQCD operators and the optical
theorem. The corrections lead to ultraviolet phase space divergences that have
to be renormalized and lead to NLL mixing effects. Numerically, the corrections
can amount to several percent and are comparable to the known NNLL QCD
corrections.Comment: 17 pages, revtex4, 4 postscript figures included; minor changes in
text and references, title modified in printed versio
A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 A whose intensity and polarization profiles are very
sensitive to the presence of hyperfine structure. A theoretical investigation
of the magnetic sensitivity of this line to the magnetic field uncovers several
interesting properties. The most important one is that the presence of strong
Paschen-Back perturbations due to the hyperfine structure produces an intensity
line profile whose shape changes according to the absolute value of the
magnetic field strength. A line ratio technique is developed from the intrinsic
variations of the line profile. This line ratio technique is applied to
spectropolarimetric observations of the quiet solar photosphere in order to
explore the probability distribution function of the magnetic field strength.
Particular attention is given to the quietest area of the observed field of
view, which was encircled by an enhanced network region. A detailed theoretical
investigation shows that the inferred distribution yields information on the
average magnetic field strength and the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field strength
and a tentative value of ~0.45" for the spatial scale at which the observed
magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical
Journal. Figures 1 and 9 are in JPG forma
On Electroweak Matching Conditions for Top Pair Production at Threshold
We determine the real parts of electroweak matching conditions relevant for
top quark pair production close to threshold in e+e- annihilation at
next-to-next-to-leading logarithmic (NNLL) order. Numerically the corrections
are comparable to the NNLL QCD corrections.Comment: 12 pages, revtex4, 1 postscript figure included; minor changes in
text and references, version published in Phys. Rev.
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