10,228 research outputs found
Phase slip in a superfluid Fermi gas near a Feshbach resonance
In this paper, we study the properties of a phase slip in a superfluid Fermi
gas near a Feshbach resonance. The phase slip can be generated by the phase
imprinting method. Below the superfluid transition temperature, it appears as a
dip in the density profile, and becomes more pronounced when the temperature is
lowered. Therefore the phase slip can provide a direct evidence of the
superfluid state. The condensation energy of the superfluid state can be
extracted from the density profile of the phase slip, due to the unitary
properties of the Fermi gas near the resonance. The width of the phase slip is
proportional to the square root of the difference between the transition
temperature and the temperature. The signature of the phase slip in the density
profile becomes more robust across the BCS-BEC crossover.Comment: 5 pages, 2 figures, the density profile of a phase slip under
experimental conditions was calculate
Review of Conformally Flat Approximation for Binary Neutron Star Initial Conditions
The spatially conformally flat approximation (CFA) is a viable method to
deduce initial conditions for the subsequent evolution of binary neutron stars
employing the full Einstein equations. Here we review the status of the
original formulation of the CFA for the general relativistic hydrodynamic
initial conditions of binary neutron stars. We illustrate the stability of the
conformally flat condition on the hydrodynamics by numerically evolving ~100
quasi-circular orbits. We illustrate the use of this approximation for orbiting
neutron stars in the quasi-circular orbit approximation to demonstrate the
equation of state dependence of these initial conditions and how they might
affect the emergent gravitational wave frequency as the stars approach the
innermost stable circular orbit.Comment: 22 pages, 12 figures, revised as per referee recommendation
An Efficient Bayesian Inference Framework for Coalescent-Based Nonparametric Phylodynamics
Phylodynamics focuses on the problem of reconstructing past population size
dynamics from current genetic samples taken from the population of interest.
This technique has been extensively used in many areas of biology, but is
particularly useful for studying the spread of quickly evolving infectious
diseases agents, e.g.,\ influenza virus. Phylodynamics inference uses a
coalescent model that defines a probability density for the genealogy of
randomly sampled individuals from the population. When we assume that such a
genealogy is known, the coalescent model, equipped with a Gaussian process
prior on population size trajectory, allows for nonparametric Bayesian
estimation of population size dynamics. While this approach is quite powerful,
large data sets collected during infectious disease surveillance challenge the
state-of-the-art of Bayesian phylodynamics and demand computationally more
efficient inference framework. To satisfy this demand, we provide a
computationally efficient Bayesian inference framework based on Hamiltonian
Monte Carlo for coalescent process models. Moreover, we show that by splitting
the Hamiltonian function we can further improve the efficiency of this
approach. Using several simulated and real datasets, we show that our method
provides accurate estimates of population size dynamics and is substantially
faster than alternative methods based on elliptical slice sampler and
Metropolis-adjusted Langevin algorithm
Realistic interpretation of a superposition state does not imply a mixture
Contrary to previous claims, it is shown that, for an ensemble of either
single-particle systems or multi-particle systems, the realistic interpretation
of a superposition state that mathematically describes the ensemble does not
imply that the ensemble is a mixture. Therefore it cannot be argued that the
realistic interpretation is wrong on the basis that some predictions derived
from the mixture are different from the corresponding predictions derived from
the superposition state
Phase diagram of a Bose gas near a wide Feshbach resonance
In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length exceeds a critical value depending on the atom density
, , the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, , the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure
Local orbital-angular-momentum dependent surface states with topological protection
Chiral surface states along the zigzag edge of a valley photonic crystal in
the honeycomb lattice are demonstrated. By decomposing the local fields into
orbital angular momentum (OAM) modes, we find that the chiral surface states
present OAM-dependent unidirectional propagation characteristics. Particularly,
the propagation directivities of the surface states are quantified by the local
OAM decomposition and are found to depend on the chiralities of both the source
and surface states. These findings allow for the engineering control of the
unidirectional propagation of electromagnetic energy without requiring an
ancillary cladding layer. Furthermore, we examine the propagation of the chiral
surface states against sharp bends. It turns out that although only certain
states successfully pass through the bend, the unidirectional propagation is
well maintained due to the topology of the structure.Comment: 9 pages, 6 figure
- …