881 research outputs found
ROSAT X-ray sources and exponential field decay in isolated neutron stars
In this paper we semianalyticaly evaluate influence of the exponential decay
of magnetic field on the fate of isolated neutron stars. The fact of ROSAT
observations of several X-ray sources, which can be accreting old isolated
neutron stars gives us an opportunity to put some limits on the parameters of
the exponential decay.
We argue, that, if most part of neutron stars have approximately the same
decay and initial parameters, then the combinations of the bottom magnetic
momentum, , in the range and
characteristic time scale, , in the range for
standard initial magnetic momentum, , can be
excluded, because for that sets of parameters neutron stars never come to the
stage when accretion of the interstellar medium on their surfaces is possible
even for low velocity of neutron stars and relatively high density of the
interstellar medium. The region of excluded parameters increases with
decreasing.Comment: 5 pages, 4 PostScript figures (uses A&A style
Evolution of isolated neutron stars in globular clusters: number of Accretors
With a simple model from the point of view of population synthesis we try to
verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim
sources in globular clusters (GCs) can be isolated accreting neutron stars
(NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated
NS per typical GC with in correspondence with observations.
Properties of old accreting isolated NSs in GCs are briefly discussed. We
suggest that accreting NSs in GCs experienced significant magnetic field decay.Comment: 6 pages, no figures. Submitted to Astronomical and Astrophysical
Transactions (style included
Constrains on parameters of magnetic field decay for accreting isolated neutron stars
The influence of exponential magnetic field decay (MFD) on the spin evolution
of isolated neutron stars is studied. The ROSAT observations of several X-ray
sources, which can be accreting old isolated neutron stars, are used to
constrain the exponential and power-law decay parameters. We show that for the
exponential decay the ranges of minimum value of magnetic moment, , and
the characteristic decay time, , , are excluded assuming the
standard initial magnetic moment, . For these
parameters, neutron stars would never reach the stage of accretion from the
interstellar medium even for a low space velocity of the stars and a high
density of the ambient plasma. The range of excluded parameters increases for
lower values of .
We also show, that, contrary to exponential MFD, no significant restrictions
can be made for the parameters of power-law decay from the statistics of
isolated neutron star candidates in ROSAT observations.
Isolated neutron stars with constant magnetic fields and initial values of
them less than never come to the stage
of accretion.
We briefly discuss the fate of old magnetars with and without MFD, and
describe parameters of old accreting magnetars.Comment: 18 pages, 6 PostScript figures, to be published in the Proceedings of
the XXVIII ITEP Winter Schoo
Phenomenological theory of phase transitions in highly piezoelectric perovskites
Recently discovered fine structure of the morphotropic phase boundaries in
highly piezoelectric mixture compounds PZT, PMN-PT, and PZN-PT demonstrates the
importance of highly non-linear interactions in these systems. We show that an
adequate Landau-type description of the ferroelectric phase transitions in
these compounds is achieved by the use of a twelfth-order expansion of the
Landau potential in terms of the phenomenological order parameter.
Group-theoretical and catastrophe-theory methods are used in constructing the
appropriate Landau potential. A complete phase diagram is calculated in
phenomenological parameter space. The theory describes both PZT and PZN-PT
types of phase diagrams, including the newly found monoclinic and orthorhombic
phases. Anomalously large piezoelectric coefficients are predicted in the
vicinity of the phase transition lines.Comment: RevTex4, 8 pages, 2 figures. Dramatically changed after referees'
Comments, to appear in Phys. Rev. B, 1 April 200
Statistics of speckle patterns
We develop a general method for calculating statistical properties of the
speckle pattern of coherent waves propagating in disordered media. In some
aspects this method is similar to the Boltzmann-Langevin approach for the
calculation of classical fluctuations. We apply the method to the case where
the incident wave experiences many small angle scattering events during
propagation, but the total angle change remains small. In many aspects our
results for this case are different from results previously known in the
literature. The correlation function of the wave intensity at two points
separated by a distance , has a long range character. It decays as a power
of and changes sign. We also consider sensitivities of the speckles to
changes of external parameters, such as the wave frequency and the incidence
angle.Comment: 4 pages, 2 figure
Uncertainty relations in curved spaces
Uncertainty relations for particle motion in curved spaces are discussed. The
relations are shown to be topologically invariant. New coordinate system on a
sphere appropriate to the problem is proposed. The case of a sphere is
considered in details. The investigation can be of interest for string and
brane theory, solid state physics (quantum wires) and quantum optics.Comment: published version; phase space structure discussion adde
Population synthesis of old neutron stars in the Galaxy
The paucity of old isolated accreting neutron stars in ROSAT observations is
used to derive a lower limit on the mean velocity of neutron stars at birth.
The secular evolution of the population is simulated following the paths of a
statistical sample of stars for different values of the initial kick velocity,
drawn from an isotropic Gaussian distribution with mean velocity . The spin-down, induced by dipole losses and the
interaction with the ambient medium, is tracked together with the dynamical
evolution in the Galactic potential, allowing for the determination of the
fraction of stars which are, at present, in each of the four possible stages:
Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky
Survey an upper limit of accreting neutron stars within pc
from the Sun, we infer a lower bound for the mean kick velocity, \ga
200-300 . The same conclusion is reached for both a constant
( G) and a magnetic field decaying exponentially with a
timescale yr. Present results, moreover, constrain the fraction of
low velocity stars, which could have escaped pulsar statistics, to \la 1%.Comment: 8 pages, 4 PostScript figures, to appear in the proceedings of IAU
Symposium 19
Lasing Effect in Symmetrical van der Waals Heterostructured Metasurfaces Due to Lattice-Induced Multipole Coupling
New practical ways to reach the lasing effect in symmetrical metasurfaces have been developed and theoretically demonstrated. Our approach is based on excitation of the resonance of an octupole quasi-trapped mode (OQTM) in heterostructured symmetrical metasurfaces composed of monolithic disk-shaped van der Waals meta-atoms featured by thin photoluminescent layers and placed on a substrate. We revealed that the coincidence of the photoluminescence spectrum maximum of these layers with the wavelength of high-quality OQTM resonance leads to the lasing effect. Based on the solution of laser rate equations and direct full-wave simulation, it was shown that lasing is normally oriented to the metasurface plane and occurs from the entire area of metasurface consisting of MoS2/hBN/MoTe2 disks with line width of generated emission of only about 1.4 nm near the wavelength 1140 nm. This opens up new practical possibilities for creating surface emitting laser devices in subwavelength material systems
Restrictions on parameters of power-law magnetic field decay for accreting isolated neutron stars
In this short note we discuss the influence of power-law magnetic field decay
on the evolution of old accreting isolated neutron stars. We show, that,
contrary to exponential field decay (Popov & Prokhorov 2000), no additional
restrictions can be made for the parameters of power-law decay from the
statistics of isolated neutron star candidates in ROSAT observations.
We also briefly discuss the fate of old magnetars with and without field
decay, and describe parameters of old accreting magnetars.Comment: 8 pages including 3 PostScript figure
Terahertz conductivity of Si and of Ge/Si(001) heterostructures with quantum dots
With an MBE technique, a Si/Ge heterostructures are prepared containing
layers of nanostructured Ge with quantum dots of size of several nanometers.
The effective conductivity of the layers is determined by a quasioptical
terahertz-subterahertz coherent source BWO spectroscopy. The conductivity is
found to be strongly enhanced compared with the conductivity of bulk germanium.
Possible microscopic mechanisms responsible for the enhancement will be
discussed. Application of BWO spectrometers for obtaining precise quantitative
information on of dielectric properties at THz-subTHz frequencies of
semiconducting layers and structures is demonstrated by presenting the
temperature dependences of dielectric characteristics of a commercial silicon
wafer at frequencies 0.3 to 1.2 THz and temperatures 5K-300K.Comment: Proc. 33rd International Conference on Infrared, Millimeter, and
Terahertz Waves (IRMMW-THz 2008), September 15-19, 2008, California Institute
of Technology, Pasadena, California, USA
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