40,242 research outputs found
New Consequences of Induced Transparency in a Double-Lambda scheme: Destructive Interference In Four-wave Mixing
We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.Comment: Submitted to Phys. Rev. A, 7 pages, 4 figure
Nebular Spectra of SN 1998bw Revisited: Detailed Study by One and Two Dimensional Models
Refined one- and two-dimensional models for the nebular spectra of the
hyper-energetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray
burst GRB980425, from 125 to 376 days after B-band maximum are presented. One
dimensional, spherically symmetric spectrum synthesis calculations show that
reproducing features in the observed spectra, i.e., the sharply peaked [OI]
6300\AA doublet and MgI] 4570\AA emission, and the broad [FeII] blend around
5200\AA, requires the existence of a high-density O-rich core expanding at low
velocities (\lsim 8,000 km s) and of Fe-rich material moving faster
than the O-rich material. Synthetic spectra at late phases from aspherical
(bipolar) explosion models are also computed with a two-dimensional spectrum
synthesis code. The above features are naturally explained by the aspherical
model if the explosion is viewed from a direction close to the axis of symmetry
(), since the aspherical model yields a high-density O-rich
region confined along the equatorial axis. By examining a large parameter space
(in energy and mass), our best model gives following physical quantities: the
kinetic energy ergs \gsim 8 - 12 and the
main-sequence mass of the progenitor star M_{\rm ms} \gsim 30 - 35 \Msun. The
temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-,
and C-rich regions, and highly clumpy structure.Comment: 38 pages, 22 figures. ApJ, 640 (01 April 2006 issue), in pres
BCS-BEC crossover in a relativistic boson-fermion model beyond mean field approximation
We investigate the fluctuation effect of the di-fermion field in the
crossover from Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein
condensate (BEC) in a relativistic superfluid. We work within the boson-fermion
model obeying a global U(1) symmetry. To go beyond the mean field approximation
we use Cornwall-Jackiw-Tomboulis (CJT) formalism to include higher order
contributions. The quantum fluctuations of the pairing condensate is provided
by bosons in non-zero modes, whose interaction with fermions gives the
two-particle-irreducible (2PI) effective potential. It changes the crossover
property in the BEC regime. With the fluctuations the superfluid phase
transition becomes the first order in grand canonical ensemble. We calculate
the condensate, the critical temperature and particle abundances as
functions of crossover parameter the boson mass.Comment: The model Lagrangian is re-formulated by decomposing the complex
scalar field into its real and imaginary parts. The anomalous propagators of
the complex scalar are then included at tree level. All numerical results are
updated. ReVTex 4, 13 pages, 10 figures, PRD accepted versio
Evidence for the Collective Nature of the Reentrant Integer Quantum Hall States of the Second Landau Level
We report an unexpected sharp peak in the temperature dependence of the
magnetoresistance of the reentrant integer quantum Hall states in the second
Landau level. This peak defines the onset temperature of these states. We find
that in different spin branches the onset temperatures of the reentrant states
scale with the Coulomb energy. This scaling provides direct evidence that
Coulomb interactions play an important role in the formation of these reentrant
states evincing their collective nature
Simulation of non-hydrostatic gravity wave propagation in the upper atmosphere
The high-frequency and small horizontal scale gravity waves may be reflected
and ducted in non-hydrostatic simulations, but usually propagate vertically
in hydrostatic models. To examine gravity wave propagation, a preliminary
study has been conducted with a global ionosphere–thermosphere model (GITM),
which is a non-hydrostatic general circulation model for the upper
atmosphere. GITM has been run regionally with a horizontal resolution of
0.2° long × 0.2° lat to resolve the gravity wave
with wavelength of 250 km. A cosine wave oscillation with amplitude of
30 m s<sup>−1</sup> has been applied to the zonal wind at the low boundary, and
both high-frequency and low-frequency waves have been tested. In the
high-frequency case, the gravity wave stays below 200 km, which indicates
that the wave is reflected or ducted in propagation. The results are
consistent with the theoretical analysis from the dispersion relationship
when the wavelength is larger than the cutoff wavelength for the
non-hydrostatic situation. However, the low-frequency wave propagates to the
high altitudes during the whole simulation period, and the amplitude
increases with height. This study shows that the non-hydrostatic model
successfully reproduces the high-frequency gravity wave dissipation
- …