115,960 research outputs found
Complete time-dependent treatment of a three-level system
Both unitary evolution and the effects of dissipation and decoherence for a
general three-level system are of widespread interest in quantum optics,
molecular physics, and elsewhere. A previous paper presented a technique for
solving the time-dependent operator equations involved but under certain
restrictive conditions. We now extend our results to a general three-level
system with arbitrary time-dependent Hamiltonians and Lindblad operators.
Analytical handling of the SU(3) algebra of the eight operators involved leaves
behind a set of coupled first-order differential equations for classical
functions. Solution of this set gives a complete solution of the quantum
problem, without having to invoke rotating-wave or other approximations.
Numerical illustrations are given.Comment: 1 tar.gz file containing a Tex and four eps figure files; unzip with
command gunzip RZPRA05.tar.g
Constraint on the early Universe by relic gravitational waves: From pulsar timing observations
Recent pulsar timing observations by the Parkers Pulsar Timing Array and
European Pulsar Timing Array teams obtained the constraint on the relic
gravitational waves at the frequency , which provides the
opportunity to constrain , the Hubble parameter when these waves crossed
the horizon during inflation. In this paper, we investigate this constraint by
considering the general scenario for the early Universe: we assume that the
effective (average) equation-of-state before the big bang nucleosynthesis
stage is a free parameter. In the standard hot big-bang scenario with ,
we find that the current PPTA result follows a bound H_*\leq
1.15\times10^{-1}\mpl, and the EPTA result follows H_*\leq
6.92\times10^{-2}\mpl. We also find that these bounds become much tighter in
the nonstandard scenarios with . When , the bounds become
H_*\leq5.89\times10^{-3}\mpl for the current PPTA and
H_*\leq3.39\times10^{-3}\mpl for the current EPTA. In contrast, in the
nonstandard scenario with , the bound becomes H_*\leq7.76\mpl for the
current PPTA.Comment: 8 pages, 3 figures, 1 table, PRD in pres
Accurate Modelling of Left-Handed Metamaterials Using Finite-Difference Time-Domain Method with Spatial Averaging at the Boundaries
The accuracy of finite-difference time-domain (FDTD) modelling of left-handed
metamaterials (LHMs) is dramatically improved by using an averaging technique
along the boundaries of LHM slabs. The material frequency dispersion of LHMs is
taken into account using auxiliary differential equation (ADE) based dispersive
FDTD methods. The dispersive FDTD method with averaged permittivity along the
material boundaries is implemented for a two-dimensional (2-D) transverse
electric (TE) case. A mismatch between analytical and numerical material
parameters (e.g. permittivity and permeability) introduced by the time
discretisation in FDTD is demonstrated. The expression of numerical
permittivity is formulated and it is suggested to use corrected permittivity in
FDTD simulations in order to model LHM slabs with their desired parameters. The
influence of switching time of source on the oscillation of field intensity is
analysed. It is shown that there exists an optimum value which leads to fast
convergence in simulations.Comment: 17 pages, 7 figures, submitted to Journal of Optics A Nanometa
special issu
General Design Bayesian Generalized Linear Mixed Models
Linear mixed models are able to handle an extraordinary range of
complications in regression-type analyses. Their most common use is to account
for within-subject correlation in longitudinal data analysis. They are also the
standard vehicle for smoothing spatial count data. However, when treated in
full generality, mixed models can also handle spline-type smoothing and closely
approximate kriging. This allows for nonparametric regression models (e.g.,
additive models and varying coefficient models) to be handled within the mixed
model framework. The key is to allow the random effects design matrix to have
general structure; hence our label general design. For continuous response
data, particularly when Gaussianity of the response is reasonably assumed,
computation is now quite mature and supported by the R, SAS and S-PLUS
packages. Such is not the case for binary and count responses, where
generalized linear mixed models (GLMMs) are required, but are hindered by the
presence of intractable multivariate integrals. Software known to us supports
special cases of the GLMM (e.g., PROC NLMIXED in SAS or glmmML in R) or relies
on the sometimes crude Laplace-type approximation of integrals (e.g., the SAS
macro glimmix or glmmPQL in R). This paper describes the fitting of general
design generalized linear mixed models. A Bayesian approach is taken and Markov
chain Monte Carlo (MCMC) is used for estimation and inference. In this
generalized setting, MCMC requires sampling from nonstandard distributions. In
this article, we demonstrate that the MCMC package WinBUGS facilitates sound
fitting of general design Bayesian generalized linear mixed models in practice.Comment: Published at http://dx.doi.org/10.1214/088342306000000015 in the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Electrical conductivity and thermal dilepton rate from quenched lattice QCD
We report on a continuum extrapolation of the vector current correlation
function for light valence quarks in the deconfined phase of quenched QCD. This
is achieved by performing a systematic analysis of the influence of cut-off
effects on light quark meson correlators at using clover
improved Wilson fermions. We discuss resulting constraints on the electrical
conductivity and the thermal dilepton rate in a quark gluon plasma. In addition
new results at 1.2 and 3.0 will be presented.Comment: 4 pages, 6 eps figures, to appear in the proceedings of Quark Matter
2011, 23-28 May 2011, Annecy, Franc
Liquid crystal phases of ultracold dipolar fermions on a lattice
Motivated by the search for quantum liquid crystal phases in a gas of
ultracold atoms and molecules, we study the density wave and nematic
instabilities of dipolar fermions on the two-dimensional square lattice (in the
plane) with dipoles pointing to the direction. We determine the phase
diagram using two complimentary methods, the Hatree-Fock mean field theory and
the linear response analysis of compressibility. Both give consistent results.
In addition to the staggered (, ) density wave, over a finite range
of densities and hopping parameters, the ground state of the system first
becomes nematic and then smectic, when the dipolar interaction strength is
increased. Both phases are characterized by the same broken four-fold (C)
rotational symmetry. The difference is that the nematic phase has a closed
Fermi surface but the smectic does not. The transition from the nematic to the
smectic phase is associated with a jump in the nematic order parameter. This
jump is closely related to the van Hove singularities. We derive the kinetic
equation for collective excitations in the normal isotropic phase and find that
the zero sound mode is strongly Landau damped and thus is not a well defined
excitation. Experimental implications of our results are discussed.Comment: 8 pages, 4 figures; Erratum added in the appendi
The two-dimensional frustrated Heisenberg model on the orthorhombic lattice
We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.Comment: 4 pages, 3 figures, accepted for publication in Journal of Physics:
Conference Serie
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