32,795 research outputs found
Double Entropic Stochastic Resonance
We demonstrate the appearance of a purely entropic stochastic resonance (ESR)
occurring in a geometrically confined system, where the irregular boundaries
cause entropic barriers. The interplay between a periodic input signal, a
constant bias and intrinsic thermal noise leads to a resonant ESR-phenomenon in
which feeble signals become amplified. This new phenomenon is characterized by
the presence of two peaks in the spectral amplification at corresponding
optimal values of the noise strength. The main peak is associated with the
manifest stochastic resonance synchronization mechanism involving the
inter-well noise-activated dynamics while a second peak relates to a regime of
optimal sensitivity for intra-well dynamics. The nature of ESR, occurring when
the origin of the barrier is entropic rather than energetic, offers new
perspectives for novel investigations and potential applications. ESR by itself
presents yet another case where one constructively can harvest noise in driven
nonequilibrium systems.Comment: 6 pages, 7 figures ; Europhys. Lett., in press (2009
Ferromagnetic Domain Structure of La0.78Ca0.22MnO3 Single Crystals
The magneto-optical technique has been employed to observe spontaneous
ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The
magnetic domain topology was found to be correlated with the intrinsic twin
structure of the investigated crystals. With decreasing temperature the regular
network of ferromagnetic domains undergoes significant changes resulting in
apparent rotation of the domain walls in the temperature range of 70-150 K. The
apparent rotation of the domain walls can be understood in terms of the
Jahn-Teller deformation of the orthorhombic unit cell, accompanied by
additional twinning.Comment: 7 pages, 5 figures, to be published in PR
Demon-free quantum Brownian motors
A quantum Smoluchowski equation is put forward that consistently describes
thermal quantum states. In particular, it notably does not induce a violation
of the second law of thermodynamics. This so modified kinetic equation is
applied to study {\it analytically} directed quantum transport at strong
friction in arbitrarily shaped ratchet potentials that are driven by nonthermal
two-state noise. Depending on the mutual interplay of quantum tunneling and
quantum reflection these quantum corrections can induce both, either a sizable
enhancement or a suppression of transport. Moreover, the threshold for current
reversals becomes markedly shifted due to such quantum fluctuations.Comment: 4 pages 3 figure
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
Gain in Stochastic Resonance: Precise Numerics versus Linear Response Theory beyond the Two-Mode Approximation
In the context of the phenomenon of Stochastic Resonance (SR) we study the
correlation function, the signal-to-noise ratio (SNR) and the ratio of output
over input SNR, i.e. the gain, which is associated to the nonlinear response of
a bistable system driven by time-periodic forces and white Gaussian noise.
These quantifiers for SR are evaluated using the techniques of Linear Response
Theory (LRT) beyond the usually employed two-mode approximation scheme. We
analytically demonstrate within such an extended LRT description that the gain
can indeed not exceed unity. We implement an efficient algorithm, based on work
by Greenside and Helfand (detailed in the Appendix), to integrate the driven
Langevin equation over a wide range of parameter values. The predictions of LRT
are carefully tested against the results obtained from numerical solutions of
the corresponding Langevin equation over a wide range of parameter values. We
further present an accurate procedure to evaluate the distinct contributions of
the coherent and incoherent parts of the correlation function to the SNR and
the gain. As a main result we show for subthreshold driving that both, the
correlation function and the SNR can deviate substantially from the predictions
of LRT and yet, the gain can be either larger or smaller than unity. In
particular, we find that the gain can exceed unity in the strongly nonlinear
regime which is characterized by weak noise and very slow multifrequency
subthreshold input signals with a small duty cycle. This latter result is in
agreement with recent analogue simulation results by Gingl et al. in Refs. [18,
19].Comment: 22 pages, 5 eps figures, submitted to PR
Prompt photon hadroproduction at high energies in off-shell gluon-gluon fusion
The amplitude for production of a single photon associated with quark pair in
the fusion of two off-shell gluons is calculated. The matrix element found is
applied to the inclusive prompt photon hadroproduction at high energies in the
framework of kt-factorization QCD approach. The total and differential cross
sections are calculated in both central and forward pseudo-rapidity regions.
The conservative error analisys is performed. We used the unintegrated gluon
distributions in a proton which were obtained from the full CCFM evolution
equation as well as from the Kimber-Martin-Ryskin prescription. Theoretical
results were compared with recent experimental data taken by the D0 and CDF
collaborations at Fermilab Tevatron. Theoretical predictions for the LHC
energies are given.Comment: 32 pages, 18 figure
Absorption of Ultrashort Laser Pulses in Strongly Overdense Targets
We report on the first absorption experiments of sub-10 fs high-contrast
Ti:Sa laser pulses incident on solid targets. The very good contrast of the
laser pulse assures the formation of a very small pre-plasma and the pulse
interacts with the matter close to solid density. Experimental results indicate
that p-polarized laser pulses are absorbed up to 80 percent at 80 degrees
incidence angle. The simulation results of PSC PIC code clearly confirm the
observations and show that the collisionless absorption works efficiently in
steep density profiles
Nuclear model effects in Charged Current neutrino--nucleus quasielastic scattering
The quasielastic scattering of muon neutrinos on oxygen 16 is studied for
neutrino energies between 200 MeV and 1 GeV using a relativistic shell model.
Final state interactions are included within the distorted wave impulse
approximation, by means of a relativistic optical potential, with and without
imaginary part, and of a relativistic mean field potential. For comparison with
experimental data the inclusive charged--current quasielastic cross section for
-- scattering in the kinematical conditions of the LSND
experiment at Los Alamos is also presented and briefly discussed.Comment: 4 pages, 5 figures, two-column format. Accepted as brief report in
Phys. Rev.
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