12,624 research outputs found
Negative Group Velocity
The group velocity for pulses in an optical medium can be negative at
frequencies between those of a pair of laser-pumped spectral lines. The gain
medium then can amplify the leading edge of a pulse resulting in a time advance
of the pulse when it exits the medium, as has been recently demonstrated in the
laboratory. This effect has been called superluminal, but, as a classical
analysis shows, it cannot result in signal propgation at speeds greater than
that of light in vacuum.Comment: v3 adds discussion of "rephasing", and adds a figure. v4 adds
references to the early history of negative group velocity, and adds a
figure; thanks to Alex Grani
Two Bipolar Outflows and Magnetic Fields in a Multiple Protostar System, L1448 IRS 3
We performed spectral line observations of CO J=2-1, 13CO J=1-0, and C18O
J=1-0 and polarimetric observations in the 1.3 mm continuum and CO J=2-1 toward
a multiple protostar system, L1448 IRS 3, in the Perseus molecular complex at a
distance of ~250 pc, using the BIMA array. In the 1.3 mm continuum, two sources
(IRS 3A and 3B) were clearly detected with estimated envelope masses of 0.21
and 1.15 solar masses, and one source (IRS 3C) was marginally detected with an
upper mass limit of 0.03 solar masses. In CO J=2-1, we revealed two outflows
originating from IRS 3A and 3B. The masses, mean number densities, momentums,
and kinetic energies of outflow lobes were estimated. Based on those estimates
and outflow features, we concluded that the two outflows are interacting and
that the IRS 3A outflow is nearly perpendicular to the line of sight. In
addition, we estimated the velocity, inclination, and opening of the IRS 3B
outflow using Bayesian statistics. When the opening angle is ~20 arcdeg, we
constrain the velocity to ~45 km/s and the inclination angle to ~57 arcdeg.
Linear polarization was detected in both the 1.3 mm continuum and CO J=2-1. The
linear polarization in the continuum shows a magnetic field at the central
source (IRS 3B) perpendicular to the outflow direction, and the linear
polarization in the CO J=2-1 was detected in the outflow regions, parallel or
perpendicular to the outflow direction. Moreover, we comprehensively discuss
whether the binary system of IRS 3A and 3B is gravitationally bound, based on
the velocity differences detected in 13CO J=1-0 and C18O J=1-0 observations and
on the outflow features. The specific angular momentum of the system was
estimated as ~3e20 cm^2/s, comparable to the values obtained from previous
studies on binaries and molecular clouds in Taurus.Comment: ApJ accepted, 20 pages, 2 tables, 10 figure
Particle acceleration at ultrarelativistic shocks: an eigenfunction method
We extend the eigenfunction method of computing the power-law spectrum of
particles accelerated at a relativistic shock fronts to apply to shocks of
arbitrarily high Lorentz factor. In agreement with the findings of Monte-Carlo
simulations, we find the index of the power-law distribution of accelerated
particles which undergo isotropic diffusion in angle at an ultrarelativistic,
unmagnetized shock is s=4.23 (where s=-d(ln f)/dp with f the Lorentz invariant
phase-space density and p the momentum). This corresponds to a synchrotron
index for uncooled electrons of a=0.62 (taking cooling into account a=1.12),
where a=-d(ln F)/dn, F is the radiation flux and n the frequency. We also
present an approximate analytic expression for the angular distribution of
accelerated particles, which displays the effect of particle trapping by the
shock: compared with the non-relativistic case the angular distribution is
weighted more towards the plane of the shock and away from its normal. We
investigate the sensitivity of our results to the transport properties of the
particles and the presence of a magnetic field. Shocks in which the ratio of
Poynting to kinetic energy flux upstream is not small are less compressive and
lead to larger values of .Comment: Minor additions on publicatio
Electronic and optical properties of beryllium chalcogenides/silicon heterostructures
We have calculated electronic and optical properties of
Si/BeSeTe heterostructures by a semiempirical
tight-binding method. Tight-binding parameters and band bowing of
BeSeTe are considered through a recent model for highly
mismatched semiconductor alloys. The band bowing and the measurements of
conduction band offset lead to a type II heterostucture for
Si/BeSeTe with conduction band minimum in the Si layer and
valence band maximum in the BeSeTe layer. The electronic
structure and optical properties of various (Si/(BeSeTe [001] superlattices have been considered. Two
bands of interface states were found within the bandgap of bulk Si. Our
calculations indicate that the optical edges are below the fundamental bandgap
of bulk Si and the transitions are optically allowed.Comment: 16 pager, 7 figure
Electrical current distribution across a metal-insulator-metal structure during bistable switching
Combining scanning electron microscopy (SEM) and electron-beam-induced
current (EBIC) imaging with transport measurements, it is shown that the
current flowing across a two-terminal oxide-based capacitor-like structure is
preferentially confined in areas localized at defects. As the thin-film device
switches between two different resistance states, the distribution and
intensity of the current paths, appearing as bright spots, change. This implies
that switching and memory effects are mainly determined by the conducting
properties along such paths. A model based on the storage and release of charge
carriers within the insulator seems adequate to explain the observed memory
effect.Comment: 8 pages, 7 figures, submitted to J. Appl. Phy
Neutron Stars as Type-I Superconductors
In a recent paper by Link, it was pointed out that the standard picture of
the neutron star core composed of a mixture of a neutron superfluid and a
proton type-II superconductor is inconsistent with observations of a long
period precession in isolated pulsars. In the following we will show that an
appropriate treatment of the interacting two-component superfluid (made of
neutron and proton Cooper pairs), when the structure of proton vortices is
strongly modified, may dramatically change the standard picture, resulting in a
type-I superconductor. In this case the magnetic field is expelled from the
superconducting regions of the neutron star leading to the formation of the
intermediate state when alternating domains of superconducting matter and
normal matter coexist.Comment: 4 page
Recommended from our members
Methods for treating bone deficit conditions with benzothiazole
" Compounds containing two aromatic systems covalently linked through a linker containing one or more atoms, or ""linker"" defined as including a covalent bond per se so as to space the aromatic systems at a distance 1.5-15 .ANG., are effective in treating conditions associated with bone deficits. The compounds can be administered to vertebrate subjects alone or in combination with additional agents that promote bone growth or that inhibit bone resorption. They can be screened for activity prior to administration by assessing their ability to effect the transcription of a reporter gene coupled to a promoter associated with a bone morphogenetic protein and/or their ability to stimulate calvarial growth in model animal systems. "Board of Regents, University of Texas Syste
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