859 research outputs found
Multifractal characterization of stochastic resonance
We use a multifractal formalism to study the effect of stochastic resonance
in a noisy bistable system driven by various input signals. To characterize the
response of a stochastic bistable system we introduce a new measure based on
the calculation of a singularity spectrum for a return time sequence. We use
wavelet transform modulus maxima method for the singularity spectrum
computations. It is shown that the degree of multifractality defined as a width
of singularity spectrum can be successfully used as a measure of complexity
both in the case of periodic and aperiodic (stochastic or chaotic) input
signals. We show that in the case of periodic driving force singularity
spectrum can change its structure qualitatively becoming monofractal in the
regime of stochastic synchronization. This fact allows us to consider the
degree of multifractality as a new measure of stochastic synchronization also.
Moreover, our calculations have shown that the effect of stochastic resonance
can be catched by this measure even from a very short return time sequence. We
use also the proposed approach to characterize the noise-enhanced dynamics of a
coupled stochastic neurons model.Comment: 10 pages, 21 EPS-figures, RevTe
Constraints on Superfluid Hydrodynamics from Equilibrium Partition Functions
Following up on recent work in the context of ordinary fluids, we study the
equilibrium partition function of a 3+1 dimensional superfluid on an arbitrary
stationary background spacetime, and with arbitrary stationary background gauge
fields, in the long wavelength expansion. We argue that this partition function
is generated by a 3 dimensional Euclidean effective action for the massless
Goldstone field. We parameterize the general form of this action at first order
in the derivative expansion. We demonstrate that the constitutive relations of
relativistic superfluid hydrodynamics are significantly constrained by the
requirement of consistency with such an effective action. At first order in the
derivative expansion we demonstrate that the resultant constraints on
constitutive relations coincide precisely with the equalities between
hydrodynamical transport coefficients recently derived from the second law of
thermodynamics.Comment: 46 page
Codimension-2 surfaces and their Hilbert spaces: low-energy clues for holography from general covariance
We argue that the holographic principle may be hinted at already from
low-energy considerations, assuming diffeomorphism invariance, quantum
mechanics and Minkowski-like causality. We consider the states of finite
spacelike hypersurfaces in a diffeomorphism-invariant QFT. A low-energy
regularization is assumed. We note a natural dependence of the Hilbert space on
a codimension-2 boundary surface. The Hilbert product is defined dynamically,
in terms of transition amplitudes which are described by a path integral. We
show that a canonical basis is incompatible with these assumptions, which opens
the possibility for a smaller Hilbert-space dimension than canonically
expected. We argue further that this dimension may decrease with surface area
at constant volume, hinting at holographic area-proportionality. We draw
comparisons with other approaches and setups, and propose an interpretation for
the non-holographic space of graviton states at asymptotically-Minkowski null
infinity.Comment: 13 pages, 9 eps figures. Added Section VI, improved presentation.
Expanded and split the Introduction into two sections. Added Section VII.
Added reference
Collective dynamics of two-mode stochastic oscillators
We study a system of two-mode stochastic oscillators coupled through their
collective output. As a function of a relevant parameter four qualitatively
distinct regimes of collective behavior are observed. In an extended region of
the parameter space the periodicity of the collective output is enhanced by the
considered coupling. This system can be used as a new model to describe
synchronization-like phenomena in systems of units with two or more oscillation
modes. The model can also explain how periodic dynamics can be generated by
coupling largely stochastic units. Similar systems could be responsible for the
emergence of rhythmic behavior in complex biological or sociological systems.Comment: 4 pages, RevTex, 5 figure
An Analytical Study of Coupled Two-State Stochastic Resonators
The two-state model of stochastic resonance is extended to a chain of coupled
two-state elements governed by the dynamics of Glauber's stochastic Ising
model. Appropriate assumptions on the model parameters turn the chain into a
prototype system of coupled stochastic resonators. In a weak-signal limit
analytical expressions are derived for the spectral power amplification and the
signal-to-noise ratio of a two-state element embedded into the chain. The
effect of the coupling between the elements on both quantities is analysed and
array-enhanced stochastic resonance is established for pure as well as noisy
periodic signals. The coupling-induced improvement of the SNR compared to an
uncoupled element is shown to be limited by a factor four which is only reached
for vanishing input noise.Comment: 29 pages, 5 figure
Anomalies in Superfluids and a Chiral Electric Effect
We analyze the chiral transport terms in relativistic superfluid
hydrodynamics. In addition to the spontaneously broken symmetry current, we
consider an arbitrary number of unbroken symmetries and extend the results of
arXiv:1105.3733. We suggest an interpretation of some of the new transport
coefficients in terms of chiral and gravitational anomalies. In particular, we
show that with unbroken gauged charges in the system, one can observe a chiral
electric conductivity - a current in a perpendicular direction to the applied
electric field. We present a motivated proposal for the value of the associated
transport coefficient, linking it to the triangle anomaly. Along the way we
present new arguments regarding the interpretation of the anomalous transport
coefficients in normal fluids. We propose a natural generalization of the
chiral transport terms to the case of an arbitrary number of spontaneously
broken symmetry currents.Comment: 30 pages; v2: Onsager-relations argument corrected, references added;
v3: fixed missing line in eq. (38
Coherence Resonance and Noise-Induced Synchronization in Globally Coupled Hodgkin-Huxley Neurons
The coherence resonance (CR) of globally coupled Hodgkin-Huxley neurons is
studied. When the neurons are set in the subthreshold regime near the firing
threshold, the additive noise induces limit cycles. The coherence of the system
is optimized by the noise. A bell-shaped curve is found for the peak height of
power spectra of the spike train, being significantly different from a
monotonic behavior for the single neuron. The coupling of the network can
enhance CR in two different ways. In particular, when the coupling is strong
enough, the synchronization of the system is induced and optimized by the
noise. This synchronization leads to a high and wide plateau in the local
measure of coherence curve. The local-noise-induced limit cycle can evolve to a
refined spatiotemporal order through the dynamical optimization among the
autonomous oscillation of an individual neuron, the coupling of the network,
and the local noise.Comment: five pages, five figure
Kelvin Waves and Internal Bores in the Marine Boundary Layer Inversion and Their Relationship to Coastally Trapped Wind Reversals
Detailed observations of a coastally trapped disturbance, or wind reversal, on 10–11 June 1994 along the
California coast provide comprehensive documentation of its structure, based on aircraft, wind profiler, radio
acoustic sounding system, and buoy measurements. Unlike the expectations from earlier studies based on limited
data, which concluded that the deepening of the marine boundary layer (MBL) was a key factor, the 1994 data
show that the perturbation was better characterized as an upward thickening of the inversion capping the MBL.
As the event propagated over a site, the reversal in the alongshore wind direction occurred first within the
inversion and then 3–4 h later at the surface. A node in the vertical structure (defined here as the altitude of
zero vertical displacement) is found just above the inversion base, with up to 200-m upward displacements of
isentropic surfaces above the node, and 70-m downward displacements below.
Although this is a single event, it is shown that the vertical structure observed is representative of most other
coastally trapped wind reversals. This is determined by comparing a composite of the 10–11 June 1994 event,
based on measurements at seven buoys, with surface pressure perturbations calculated from aircraft data. These
results are compared to the composite of many events. In each case a weak pressure trough occurred between
2.4 and 4.0 h ahead of the surface wind reversal, and the pressure rose by 0.32–0.48 mb between the trough
and the wind reversal. The pressure rise results from the cooling caused by the inversion’s upward expansion.
The propagation and structure of the event are shown to be best characterized as a mixed Kelvin wave–bore
propagating within the inversion above the MBL, with the MBL acting as a quasi-rigid lower boundary. If the
MBL is instead assumed to respond in unison with the inversion, then the theoretically predicted intrinsic phase
speeds significantly exceed the observed intrinsic phase speed. The hybrid nature of the event is indicated by
two primary characteristics: 1) the disturbance had a much shallower slope than expected for an internal bore,
while at the same time the upward perturbation within the inversion was quasi-permanent rather than sinusoidal,
which more closely resembles a bore; and 2) the predicted phase speeds for the ‘‘solitary’’ form of nonlinear
Kelvin wave and for an internal bore are both close to the observed intrinsic phase speed
Constraints on Fluid Dynamics from Equilibrium Partition Functions
We study the thermal partition function of quantum field theories on
arbitrary stationary background spacetime, and with arbitrary stationary
background gauge fields, in the long wavelength expansion. We demonstrate that
the equations of relativistic hydrodynamics are significantly constrained by
the requirement of consistency with any partition function. In examples at low
orders in the derivative expansion we demonstrate that these constraints
coincide precisely with the equalities between hydrodynamical transport
coefficients that follow from the local form of the second law of
thermodynamics. In particular we recover the results of Son and Surowka on the
chiral magnetic and chiral vorticity flows, starting from a local partition
function that manifestly reproduces the field theory anomaly, without making
any reference to an entropy current. We conjecture that the relations between
transport coefficients that follow from the second law of thermodynamics agree
to all orders in the derivative expansion with the constraints described in
this paper.Comment: Typos corrected, References adde
System size resonance in coupled noisy systems and in the Ising model
We consider an ensemble of coupled nonlinear noisy oscillators demonstrating
in the thermodynamic limit an Ising-type transition. In the ordered phase and
for finite ensembles stochastic flips of the mean field are observed with the
rate depending on the ensemble size. When a small periodic force acts on the
ensemble, the linear response of the system has a maximum at a certain system
size, similar to the stochastic resonance phenomenon. We demonstrate this
effect of system size resonance for different types of noisy oscillators and
for different ensembles -- lattices with nearest neighbors coupling and
globally coupled populations. The Ising model is also shown to demonstrate the
system size resonance.Comment: 4 page
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