23,721 research outputs found
Profiles of thermal line emission from advection dominated accretion flows
Recently, Narayan & Raymond (1999) proposed that the thermal emission lines
from the hot plasma in advection dominated accretion flows (ADAFs) are
potentially observable with the next generation of X-ray observatories, with
which the physical properties of some X-ray sources can be probed. In ADAFs,
the temperature of the ion is so high that the thermal broadening of the line
is important. We calculate the profiles of thermal line emission from ADAFs, in
which both the thermal and Doppler broadening have been considered. It is found
that the double-peaked profiles are present for high inclination angles between
the axis of disk and the line of sight. The double-peaked profiles are smeared
in low inclination cases, and completely disappear while the inclination angle
is less than , where the thermal and turbulent broadening dominated
on the line profiles. We also note that the thermal line profile is affected by
the location of the transition radius of ADAF. The self-similar
height-integrated disk structure and the emissivity with power-law dependence
of radius are adopted in our calculations. The results obtained in this work
can be used as a diagnosis on the future X-ray observations of the thermal
lines. Some important physical quantities of ADAFs could be inferred from
future thermal line observations.Comment: 7 page
The angular momentum of a magnetically trapped atomic condensate
For an atomic condensate in an axially symmetric magnetic trap, the sum of
the axial components of the orbital angular momentum and the hyperfine spin is
conserved. Inside an Ioffe-Pritchard trap (IPT) whose magnetic field (B-field)
is not axially symmetric, the difference of the two becomes surprisingly
conserved. In this paper we investigate the relationship between the values of
the sum/difference angular momentums for an atomic condensate inside a magnetic
trap and the associated gauge potential induced by the adiabatic approximation.
Our result provides significant new insight into the vorticity of magnetically
trapped atomic quantum gases.Comment: 4 pages, 1 figure
Bose-stimulated scattering off a cold atom trap
The angle and temperature dependence of the photon scattering rate for
Bose-stimulated atom recoil transitions between occupied states is compared to
diffraction and incoherent Rayleigh scattering near the Bose-Einstein
transition for an optically thin trap in the limit of large particle number, N.
Each of these processes has a range of angles and temperatures for which it
dominates over the others by a divergent factor as N->oo.Comment: 18 pages (REVTeX), no figure
Non-Markovian quantum state diffusion for an open quantum system in fermionic environments
Non-Markovian quantum state diffusion (NMQSD) provides a powerful approach to
the dynamics of an open quantum system in bosonic environments. Here we develop
an NMQSD method to study the open quantum system in fermionic environments.
This problem involves anticommutative noise functions (i.e., Grassmann
variables) that are intrinsically different from the noise functions of bosonic
baths. We obtain the NMQSD equation for quantum states of the system and the
non-Markovian master equation. Moreover, we apply this NMQSD method to single
and double quantum-dot systems.Comment: 9 pages, 1 figur
Majorana fermions in s-wave noncentrosymmetric superconductor with Rashba and Dresselhaus (110) spin-orbit couplings
The asymmetric spin-orbit (SO) interactions play a crucial role in realizing
topological phases in noncentrosymmetric superconductor (NCS).We investigate
the edge states and the vortex core states in s-wave NCS with Rashba and
Dresselhaus (110) SO couplings by both numerical and analytical methods. In
particular, we demonstrate that there exists a novel semimetal phase
characterized by the flat Andreev bound states in the phase diagram of the
s-wave Dresselhaus NCS which supports the emergence of Majorana fermion (MF).
The flat dispersion implies a peak in the density of states which has a clear
experimental signature in the tunneling conductance measurements and the MFs
proposed here should be experimentally detectable
Exotic Topological States with Raman-Induced Spin-Orbit Coupling
We propose a simple experimental scheme to realize simultaneously the
one-dimensional spin-orbit coupling and the staggered spin-flip in ultracold
pseudospin- atomic Fermi gases trapped in square optical lattices. In the
absence of interspecies interactions, the system supports gapped Chern
insulators and gapless topological semimetal states. By turning on the -wave
interactions, a rich variety of gapped and gapless inhomogeneous topological
superfluids can emerge. In particular, a gapped topological Fulde-Ferrell
superfluid, in which the chiral edge states at opposite boundaries possess the
same chirality, is predicted.Comment: 11 pages, 6 figure
Cerenkov Line Emission as a Possible Mechanism of X-ray Lines in Gamma-ray Bursts
The recent discoveries of X-ray lines in the afterglows of gamma-ray bursts
(GRBs) provide significant clues to the nature of GRB progenitors and central
environments. However, the iron line interpretation by fluorescence or
recombination mechanism requires a large amount of iron material. We argue that
the very strong iron line could be attributed to an alternative mechanism:
Cerenkov line emission since relativistic electrons and dense medium exist near
GRB sites. Therefore, the broad iron lines are expected, and line intensity
will be nearly independent of the iron abundance, the medium with the
anomalously high Fe abundance is not required.Comment: 4 pages, revised version accepted for the publication in ApJ
A qubit strongly-coupled to a resonant cavity: asymmetry of the spontaneous emission spectrum beyond the rotating wave approximation
We investigate the spontaneous emission spectrum of a qubit in a lossy
resonant cavity. We use neither the rotating-wave approximation nor the Markov
approximation. The qubit-cavity coupling strength is varied from weak, to
strong, even to lower bound of the ultra-strong. For the weak-coupling case,
the spontaneous emission spectrum of the qubit is a single peak, with its
location depending on the spectral density of the qubit environment. Increasing
the qubit-cavity coupling increases the asymmetry (the positions about the
qubit energy spacing and heights of the two peaks) of the two spontaneous
emission peaks (which are related to the vacuum Rabi splitting) more.
Explicitly, for a qubit in a low-frequency intrinsic bath, the height asymmetry
of the splitting peaks becomes larger, when the qubit-cavity coupling strength
is increased. However, for a qubit in an Ohmic bath, the height asymmetry of
the spectral peaks is inverted from the same case of the low-frequency bath,
when the qubit is strongly coupled to the cavity. Increasing the qubit-cavity
coupling to the lower bound of the ultra-strong regime, the height asymmetry of
the left and right peak heights are inverted, which is consistent with the same
case of low-frequency bath, only relatively weak. Therefore, our results
explicitly show how the height asymmetry in the spontaneous emission spectrum
peaks depends not only on the qubit-cavity coupling, but also on the type of
intrinsic noise experienced by the qubit.Comment: 10pages, 5 figure
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