453 research outputs found
Field Driven Pairing State Phase Transition in d_x^2-y^2+id_xy-Wave Superconductors
Within the framework of the Ginzburg-Landau theory for
-wave superconductors, we discuss the pairing state phase
transition in the absence of the Zeeman coupling between the Cooper pair
orbital angular momentum and the magnetic field. We find that above a
temperature , the pairing state in a magnetic field is pure
-wave. However, below , the pairing state is
-wave at low fields, and it becomes pure
-wave at higher fields. Between these pairing states there
exists a field driven phase transition . The transition field increases with
decreasing temperature. In the field-temperature phase diagram, the phase
transition line is obtained theoretically by a combined use of a variational
method and the Virial theorem. The analytical result is found to be in good
agreement with numerical simulation results of the Gingzburg-Landau equations.
The validity of the variational method is discussed. The difference to the case
with the Zeeman coupling is discussed, which may be utilized to the detection
of the Zeeman coupling.Comment: 5 pages, 2 figures, submitted to PRB Brief Repor
Numerical Simulation Of A Three Blade Marine Current Turbine
Renewable energy usually refers to those natural sources of energy which are possible to use without diminishing the resource and the current European target is to source 20% of its energy from renewable sources by 2020. The marine current turbine (MCT) is an exciting proposition for the extraction of renewable tidal and marine current power. It is gaining momentum as a viable technology and is currently the subject of much attention and research. However, the numerical predication of the power performance of a marine current turbine under a free surface is difficult due to its complex geometry, fluid-structural interactions and ever-changing free surface interface. In this paper, an immersed boundary method, first introduced by Peskin (1972) to simulate blood flow around the flexible leaflet of human heart, was used to couple the simulation of turbulent fluid flow with a solid using a three-dimensional finite volume in-house LES code, CgLes (Thomas and Williams (1997)). Extensive validation work was carried out to prove the reliability and accuracy of the coupled code which can be found in the work of Ji et al. (2012). A conservative level set method, proposed by Olsson and Kreiss (2005) was adopted to track the free surface and then validated respectively by various studies and a coupled simulation was proposed for marine current turbines operating under free surface waves. The power coefficients of a horizontal axis marine current turbine (MCT) with different rotating speeds are calculated and compared against experimental data. It is found that the method is in general agreement with published results and provides a promising potential for more extensive studies on MCT’s together with other applications
On the warp drive space-time
In this paper the problem of the quantum stability of the two-dimensional
warp drive spacetime moving with an apparent faster than light velocity is
considered. We regard as a maximum extension beyond the event horizon of that
spacetime its embedding in a three-dimensional Minkowskian space with the
topology of the corresponding Misner space. It is obtained that the interior of
the spaceship bubble becomes then a multiply connected nonchronal region with
closed timelike curves and that the most natural vacuum allows quantum
fluctuations which do not induce any divergent behaviour of the re-normalized
stress-energy tensor, even on the event (Cauchy) chronology horizon. In such a
case, the horizon encloses closed timelike curves only at scales close to the
Planck length, so that the warp drive satisfies the Ford's negative energy-time
inequality. Also found is a connection between the superluminal two-dimensional
warp drive space and two-dimensional gravitational kinks. This connection
allows us to generalize the considered Alcubierre metric to a standard,
nonstatic metric which is only describable on two different coordinate patchesComment: 7 pages, minor comment on chronology protection added, RevTex, to
appear in Phys. Rev.
Fermionic Casimir effect with helix boundary condition
In this paper, we consider the fermionic Casimir effect under a new type of
space-time topology using the concept of quotient topology. The relation
between the new topology and that in Ref. \cite{Feng,Zhai3} is something like
that between a M\"obius strip and a cylindric. We obtain the exact results of
the Casimir energy and force for the massless and massive Dirac fields in the
()-dimensional space-time. For both massless and massive cases, there is a
symmetry for the Casimir energy. To see the effect of the mass, we
compare the result with that of the massless one and we found that the Casimir
force approaches the result of the force in the massless case when the mass
tends to zero and vanishes when the mass tends to infinity.Comment: 7 pages, 4 figures, published in Eur. Phys. J.
Revisit of the Interaction between Holographic Dark Energy and Dark Matter
In this paper we investigate the possible direct, non-gravitational
interaction between holographic dark energy (HDE) and dark matter. Firstly, we
start with two simple models with the interaction terms
and , and then we move on to the general form . The cosmological constraints of the models are
obtained from the joint analysis of the present Union2.1+BAO+CMB+ data. We
find that the data slightly favor an energy flow from dark matter to dark
energy, although the original HDE model still lies in the 95.4% confidence
level (CL) region. For all models we find at the 95.4% CL. We show that
compared with the cosmic expansion, the effect of interaction on the evolution
of and is smaller, and the relative increment
(decrement) amount of the energy in the dark matter component is constrained to
be less than 9% (15%) at the 95.4% CL. By introducing the interaction, we find
that even when the big rip still can be avoided due to the existence of a
de Sitter solution at . We show that this solution can not be
accomplished in the two simple models, while for the general model such a
solution can be achieved with a large , and the big rip may be avoided
at the 95.4% CL.Comment: 26 pages, 9 figures, version accepted for publication in JCA
Nonorientable spacetime tunneling
Misner space is generalized to have the nonorientable topology of a Klein
bottle, and it is shown that in a classical spacetime with multiply connected
space slices having such a topology, closed timelike curves are formed.
Different regions on the Klein bottle surface can be distinguished which are
separated by apparent horizons fixed at particular values of the two angular
variables that eneter the metric. Around the throat of this tunnel (which we
denote a Klein bottlehole), the position of these horizons dictates an ordinary
and exotic matter distribution such that, in addition to the known diverging
lensing action of wormholes, a converging lensing action is also present at the
mouths. Associated with this matter distribution, the accelerating version of
this Klein bottlehole shows four distinct chronology horizons, each with its
own nonchronal region. A calculation of the quantum vacuum fluctuations
performed by using the regularized two-point Hadamard function shows that each
chronology horizon nests a set of polarized hypersurfaces where the
renormalized momentum-energy tensor diverges. This quantum instability can be
prevented if we take the accelerating Klein bottlehole to be a generalization
of a modified Misner space in which the period of the closed spatial direction
is time-dependent. In this case, the nonchronal regions and closed timelike
curves cannot exceed a minimum size of the order the Planck scale.Comment: 11 pages, RevTex, Accepted in Phys. Rev.
Probing interaction and spatial curvature in the holographic dark energy model
In this paper we place observational constraints on the interaction and
spatial curvature in the holographic dark energy model. We consider three kinds
of phenomenological interactions between holographic dark energy and matter,
i.e., the interaction term is proportional to the energy densities of dark
energy (), matter (), and matter plus dark energy
(). For probing the interaction and spatial curvature in
the holographic dark energy model, we use the latest observational data
including the type Ia supernovae (SNIa) Constitution data, the shift parameter
of the cosmic microwave background (CMB) given by the five-year Wilkinson
Microwave Anisotropy Probe (WMAP5) observations, and the baryon acoustic
oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS). Our
results show that the interaction and spatial curvature in the holographic dark
energy model are both rather small. Besides, it is interesting to find that
there exists significant degeneracy between the phenomenological interaction
and the spatial curvature in the holographic dark energy model.Comment: 11 pages, 5 figures; to appear in JCA
Observational constraint on generalized Chaplygin gas model
We investigate observational constraints on the generalized Chaplygin gas
(GCG) model as the unification of dark matter and dark energy from the latest
observational data: the Union SNe Ia data, the observational Hubble data, the
SDSS baryon acoustic peak and the five-year WMAP shift parameter. It is
obtained that the best fit values of the GCG model parameters with their
confidence level are ()
, ()
. Furthermore in this model, we can see that the
evolution of equation of state (EOS) for dark energy is similar to quiessence,
and its current best-fit value is with the confidence
level .Comment: 9 pages, 5 figure
Running coupling: Does the coupling between dark energy and dark matter change sign during the cosmological evolution?
In this paper we put forward a running coupling scenario for describing the
interaction between dark energy and dark matter. The dark sector interaction in
our scenario is free of the assumption that the interaction term is
proportional to the Hubble expansion rate and the energy densities of dark
sectors. We only use a time-variable coupling (with the scale factor
of the universe) to characterize the interaction . We propose a
parametrization form for the running coupling in which the
early-time coupling is given by a constant , while today the coupling is
given by another constant, . For investigating the feature of the running
coupling, we employ three dark energy models, namely, the cosmological constant
model (), the constant model (), and the time-dependent
model (). We constrain the models with the current
observational data, including the type Ia supernova, the baryon acoustic
oscillation, the cosmic microwave background, the Hubble expansion rate, and
the X-ray gas mass fraction data. The fitting results indicate that a
time-varying vacuum scenario is favored, in which the coupling crosses
the noninteracting line () during the cosmological evolution and the sign
changes from negative to positive. The crossing of the noninteracting line
happens at around , and the crossing behavior is favored at about
1 confidence level. Our work implies that we should pay more attention
to the time-varying vacuum model and seriously consider the phenomenological
construction of a sign-changeable or oscillatory interaction between dark
sectors.Comment: 8 pages, 5 figures; refs added; to appear in EPJ
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