5,668 research outputs found
The evolution of the magnetic inclination angle as an explanation of the long term red timing-noise of pulsars
We study the possibility that the long term red timing-noise in pulsars
originates from the evolution of the magnetic inclination angle . The
braking torque under consideration is a combination of the dipole radiation and
the current loss. We find that the evolution of can give rise to extra
cubic and fourth-order polynomial terms in the timing residuals. These two
terms are determined by the efficiency of the dipole radiation, the relative
electric-current density in the pulsar tube and . The following
observation facts can be explained with this model: a) young pulsars have
positive ; b) old pulsars can have both positive and negative
; c) the absolute values of are proportional to
; d) the absolute values of the braking indices are proportional to
the characteristic ages of pulsars. If the evolution of is purely due to
rotation kinematics, then it can not explain the pulsars with braking index
less than 3, and thus the intrinsic change of the magnetic field is needed in
this case. Comparing the model with observations, we conclude that the drift
direction of might oscillate many times during the lifetime of a pulsar.
The evolution of is not sufficient to explain the rotation behavior of
the Crab pulsar, because the observed and are inconsistent
with the values indicated from the timing residuals using this model.Comment: 5 pages, 1 figure. Accepted for publication in MNRA
Tuning Feshbach resonance in cold atomic gases with inter-channel coupling
We show that the essential properties of a Feshbach resonance in cold atomic
gases can be tuned by dressing the atomic states in different scattering
channels through inter-channel couplings. Such a scheme can be readily
implemented in the orbital Feshbach resonance of alkaline-earth-like atoms by
coupling hyperfine states in the clock-state manifolds. Using Yb atoms
as an example, we find that both the resonance position and the two-body
bound-state energy depend sensitively on the inter-channel coupling strength,
which offers control parameters in tuning the inter-atomic interactions. We
also demonstrate the dramatic impact of the dressed Feshbach resonance on
many-body processes such as the polaron to molecule transition and the BCS-BEC
crossover.Comment: 6 pages, 4 figure
Two-body bound state of ultracold Fermi atoms with two-dimensional spin-orbit coupling
In a recent experiment, a two-dimensional spin-orbit coupling (SOC) was
realized for fermions in the continuum [Nat. Phys. 12, 540 (2016)], which
represents an important step forward in the study of synthetic gauge field
using cold atoms. In the experiment, it was shown that a Raman-induced
two-dimensional SOC exists in the dressed-state basis close to a Dirac point of
the single-particle spectrum. By contrast, the short-range inter-atomic
interactions of the system are typically expressed in the hyperfine-spin basis.
The interplay between synthetic SOC and interactions can potentially lead to
interesting few- and many-body phenomena but has so far eluded theoretical
attention. Here we study in detail properties of two-body bound states of such
a system. We find that, due to the competition between SOC and interaction, the
stability region of the two-body bound state is in general reduced.
Particularly, the threshold of the lowest two-body bound state is shifted to a
positive, SOC-dependent scattering length. Furthermore, the center-of-mass
momentum of the lowest two-body bound state becomes nonzero, suggesting the
emergence of Fulde-Ferrell pairing states in a many-body setting. Our results
reveal the critical difference between the experimentally realized
two-dimensional SOC and the more symmetric Rashba or Dresselhaus SOCs in an
interacting system, and paves the way for future characterizations of
topological superfluid states in the experimentally relevant systems.Comment: 10 pages, 7 figure
Diffusion of an ellipsoid in bacterial suspensions
Active matter such as swarming bacteria and motile colloids exhibits exotic
properties different from conventional equilibrium materials. Among these
properties, the enhanced diffusion of tracer particles is generally deemed as a
hallmark of active matter. Here, rather than spherical tracers, we investigate
the diffusion of isolated ellipsoids in quasi-two-dimensional bacterial bath.
Our study reveals a nonlinear enhancement of both translational and rotational
diffusions. More importantly, we uncover an anomalous coupling between
translation and rotation that is strictly prohibited in the classic Brownian
diffusion. Combining experiments with theoretical modeling, we show that such
an anomaly arises from generic stretching flows induced by swimming bacteria.
Our work illustrates a universal organizing principle of active matter and
sheds new light on fundamental transport processes in microbiological systems.Comment: 13 pages, 4 figure
The confinement induced resonance in spin-orbit coupled cold atoms with Raman coupling
We investigate the confinement induced resonance in spin-orbit coupled cold
atoms with Raman coupling. We find that the quasi-bound levels induced by the
spin-orbit coupling and Raman coupling result in the Feshbach-type resonances.
For sufficiently large Raman coupling, the bound states in one dimension exist
only for sufficiently strong attractive interaction. Furthermore, the bound
states in quasi-one dimension exist only for sufficient large ratio of the
length scale of confinement to three dimensional s-wave scattering length. The
Raman coupling substantially changes the confinement-induced resonance
position. We give a proposal to realize confinement induced resonance by
increasing the Raman coupling strength in experiments.Comment: 5 pages, 4 figure
Detection of a Majorana-fermion zero mode by a T-shaped quantum-dot structure
Electron transport through the T-shaped quantum-dot (QD) structure is
theoretically investigated, by considering a Majorana zero mode coupled to the
terminal QD. It is found that in the double-QD case, the presence of the
Majorana zero mode can efficiently dissolve the antiresonance point in the
conductance spectrum and induce a conductance peak to appear at the same energy
position whose value is equal to . This antiresonance-resonance change
will be suitable to detect the Majorana bound states. Next in the multi-QD
case, we observe that in the zero-bias limit, the conductances are always the
same as the double-QD result, independent of the parity of the QD number. We
believe that all these results can be helpful for understanding the properties
of Majorana bound states
Radial alignment of elliptical galaxies by the tidal force of a cluster of galaxies
Unlike the random radial orientation distribution of field elliptical
galaxies, galaxies in a cluster are expected to point preferentially towards
the center of the cluster, as a result of the cluster's tidal force on its
member galaxies. In this work an analytic model is formulated to simulate this
effect. The deformation time scale of a galaxy in a cluster is usually much
shorter than the time scale of change of the tidal force; the dynamical process
of the tidal interaction within the galaxy can thus be ignored. An equilibrium
shape of a galaxy is then assumed to be the surface of equipotential, which is
the sum of the self-gravitational potential of the galaxy and the tidal
potential of the cluster at this location. We use a Monte-Carlo method to
calculate the radial orientation distribution of these galaxies, by assuming
the NFW mass profile of the cluster and the initial ellipticity of field
galaxies. The radial angles show a single peak distribution centered at zero.
The Monte-Carlo simulations also show that a shift of the reference center from
the real cluster center weakens the anisotropy of the radial angle
distribution. Therefore, the expected radial alignment cannot be revealed if
the distribution of spatial position angle is used instead of that of radial
angle. The observed radial orientations of elliptical galaxies in cluster
Abell~2744 are consistent with the simulated distribution.Comment: 8 pages, 6 figures, 2 tables. MNRAS in pres
Understanding the residual patterns of timing solutions of radio pulsars with a model of magnetic field oscillation
We explain some phenomena existing generally in the timing residuals:
amplitude and sign of the second derivative of a pulsar's spin-frequency
(), some sophisticated residual patterns, which also change with the
time span of data segments. The sample is taken from Hobbs et al.\,(2010), in
which the pulsar's spin-frequency and its first derivative have been subtracted
from the timing solution fitting. We first classify the timing residual
patterns into different types based on the sign of . Then we use the
magnetic field oscillation model developed in our group \citep{zhang12a} to fit
successfully the different kinds of timing residuals with the Markov Chain
Monte Carlo method. Finally, we simulate the spin evolution over 20 years for a
pulsar with typical parameters and analyze the data with the conventional
timing solution fitting. By choosing different segments of the simulated data,
we find that most of the observed residual patterns can be reproduced
successfully. This is the first time that the observed residual patterns are
fitted by a model and reproduced by simulations with very few parameters. From
the distribution of the different residual patterns in the diagram,
we argue that (1) a single magnetic field oscillation mode exists commonly in
all pulsars throughout their lifetimes; (2) there may be a transition period
over the lifetimes of pulsars, in which multiple magnetic field oscillation
modes exist.Comment: 21 pages, 22 figures, accepted for publication in MNRA
A "nearly parametric" solution to Selective Harmonic Elimination PWM
Selective Harmonic Elimination Pulse Width Modulation (SHEPWM) is an
important technique to solve PWM problems, which control the output voltage of
an inverter via selecting appropriate switching angles. Based on the Rational
Univariate Representation (RUR) theory for solving polynomial systems, the
paper presents an algorithm to compute a "nearly parametric" solution to a
SHEPWM problem. When the number of switching angles N is fixed, a "nearly
parametric" solution can be considered as functions of the modulation index m.
So we can adapt the amplitude of the output voltage with the same source
voltage by changing the modulation index. When m is given as a specific value,
complete solutions to the SHEPWM problem can be obtained easily using
univariate polynomial solving. Compared with other methods, m is considered as
a symbolic parameter for the first time, and this can help avoid totally
restarting when m changes. The average time for computing complete solutions
associated to 460 modulation indexes based on a "nearly parametric" solution
when N=5 is 0.0284s, so the algorithm is practical. Three groups of switching
angles associated to N=5, m=0.75 is simulated in MATLAB, and it verifies the
algorithm's correctness
BCS-BEC crossover and quantum phase transition in an ultracold Fermi gas under spin-orbit coupling
In this work, we study the BCS-BEC crossover and quantum phase transition in
a Fermi gas under Rashba spin-orbit coupling close to a Feshbach resonance. By
adopting a two-channel model, we take into account of the closed channel
molecules, and show that combined with spin-orbit coupling, a finite background
scattering in the open channel can lead to two branches of solution for both
the two-body and the many-body ground states. The branching of the two-body
bound state solution originates from the avoided crossing between bound states
in the open and the closed channels, respectively. For the many-body states, we
identify a quantum phase transition in the upper branch regardless of the sign
of the background scattering length, which is in clear contrast to the case
without spin-orbit coupling. For systems with negative background scattering
length in particular, we show that the bound state in the open channel, and
hence the quantum phase transition in the upper branch, are induced by
spin-orbit coupling. We then characterize the critical detuning of the quantum
phase transition for both positive and negative background scattering lengths,
and demonstrate the optimal parameters for the critical point to be probed
experimentally.Comment: 7 pages, 4 figure
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