12,305 research outputs found
Upper critical dimension of the KPZ equation
Numerical results for the Directed Polymer model in 1+4 dimensions in various
types of disorder are presented. The results are obtained for system size
considerably larger than that considered previously. For the extreme strong
disorder case (Min-Max system), associated with the Directed Percolation model,
the expected value of the meandering exponent, zeta = 0.5 is clearly revealed,
with very week finite size effects. For the week disorder case, associated with
the KPZ equation, finite size effects are stronger, but the value of seta is
clearly seen in the vicinity of 0.57. In systems with "strong disorder" it is
expected that the system will cross over sharply from Min-Max behavior at short
chains to weak disorder behavior at long chains. This is indeed what we find.
These results indicate that 1+4 is not the Upper Critical Dimension (UCD) in
the week disorder case, and thus 4+1 does not seem to be the upper critical
dimension for the KPZ equation
Spontaneously modulated spin textures in a dipolar spinor Bose-Einstein condensate
Helical spin textures in a Rb F=1 spinor Bose-Einstein condensate are
found to decay spontaneously toward a spatially modulated structure of spin
domains. This evolution is ascribed to magnetic dipolar interactions that
energetically favor the short-wavelength domains over the long-wavelength spin
helix. This is confirmed by eliminating the dipolar interactions by a sequence
of rf pulses and observing a suppression of the formation of the short-range
domains. This study confirms the significance of magnetic dipole interactions
in degenerate Rb F=1 spinor gases
Eulerian spectral closures for isotropic turbulence using a time-ordered fluctuation-dissipation relation
Procedures for time-ordering the covariance function, as given in a previous
paper (K. Kiyani and W.D. McComb Phys. Rev. E 70, 066303 (2004)), are extended
and used to show that the response function associated at second order with the
Kraichnan-Wyld perturbation series can be determined by a local (in wavenumber)
energy balance. These time-ordering procedures also allow the two-time
formulation to be reduced to time-independent form by means of exponential
approximations and it is verified that the response equation does not have an
infra-red divergence at infinite Reynolds number. Lastly, single-time
Markovianised closure equations (stated in the previous paper above) are
derived and shown to be compatible with the Kolmogorov distribution without the
need to introduce an ad hoc constant.Comment: 12 page
A Flattened Protostellar Envelope in Absorption around L1157
Deep Spitzer IRAC images of L1157 reveal many of the details of the outflow
and the circumstellar environment of this Class 0 protostar. In IRAC band 4, 8
microns, there is a flattened structure seen in absorption against the
background emission. The structure is perpendicular to the outflow and is
extended to a diameter of 2 arcminutes. This structure is the first clear
detection of a flattened circumstellar envelope or pseudo-disk around a Class 0
protostar. Such a flattened morphology is an expected outcome for many collapse
theories that include magnetic fields or rotation. We construct an extinction
model for a power-law density profile, but we do not constrain the density
power-law index.Comment: ApJL accepte
The Lagrangian frequency spectrum as a diagnostic for magnetohydrodynamic turbulence dynamics
For the phenomenological description of magnetohydrodynamic turbulence
competing models exist, e.g. Boldyrev [Phys.Rev.Lett. \textbf{96}, 115002,
2006] and Gogoberidze [Phys.Plas. \textbf{14}, 022304, 2007], which predict the
same Eulerian inertial-range scaling of the turbulent energy spectrum although
they employ fundamentally different basic interaction mechanisms. {A relation
is found that links} the Lagrangian frequency spectrum {with} the
autocorrelation timescale of the turbulent fluctuations, ,
and the associated cascade timescale, . Thus, the
Lagrangian energy spectrum can serve to identify weak
() and strong
() interaction mechanisms providing
insight into the turbulent energy cascade. The new approach is illustrated by
results from direct numerical simulations of two- and three-dimensional
incompressible MHD turbulence.Comment: accepted for publication in PR
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Gain Modulation by Corticostriatal and Thalamostriatal Input Signals during Reward-Conditioned Behavior.
The cortex and thalamus send excitatory projections to the striatum, but little is known about how these inputs, either individually or collectively, regulate striatal dynamics during behavior. The lateral striatum receives overlapping input from the secondary motor cortex (M2), an area involved in licking, and the parafascicular thalamic nucleus (PF). Using neural recordings, together with optogenetic terminal inhibition, we examine the contribution of M2 and PF projections on medium spiny projection neuron (MSN) activity as mice performed an anticipatory licking task. Each input has a similar contribution to striatal activity. By comparing how suppressing single or multiple projections altered striatal activity, we find that cortical and thalamic input signals modulate MSN gain and that this effect is more pronounced in a temporally specific period of the task following the cue presentation. These results demonstrate that cortical and thalamic inputs synergistically regulate striatal output during reward-conditioned behavior
Spin squeezing of high-spin, spatially extended quantum fields
Investigations of spin squeezing in ensembles of quantum particles have been
limited primarily to a subspace of spin fluctuations and a single spatial mode
in high-spin and spatially extended ensembles. Here, we show that a wider range
of spin-squeezing is attainable in ensembles of high-spin atoms, characterized
by sub-quantum-limited fluctuations in several independent planes of
spin-fluctuation observables. Further, considering the quantum dynamics of an
ferromagnetic spinor Bose-Einstein condensate, we demonstrate
theoretically that a high degree of spin squeezing is attained in multiple
spatial modes of a spatially extended quantum field, and that such squeezing
can be extracted from spatially resolved measurements of magnetization and
nematicity, i.e.\ the vector and quadrupole magnetic moments, of the quantum
gas. Taking into account several experimental limitations, we predict that the
variance of the atomic magnetization and nematicity may be reduced as far as 20
dB below the standard quantum limits.Comment: 18 pages, 5 figure
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