6,371 research outputs found
Pairing symmetry of the one-band Hubbard model in the paramagnetic weak-coupling limit: a numerical RPA study
We study the spin-fluctuation-mediated superconducting pairing gap in a
weak-coupling approach to the Hubbard model for a two dimensional square
lattice in the paramagnetic state. Performing a comprehensive theoretical study
of the phase diagram as a function of filling, we find that the superconducting
gap exhibits transitions from p-wave at very low electron fillings to
d_{x^2-y^2}-wave symmetry close to half filling in agreement with previous
reports. At intermediate filling levels, different gap symmetries appear as a
consequence of the changes in the Fermi surface topology and the associated
structure of the spin susceptibility. In particular, the vicinity of a van Hove
singularity in the electronic structure close to the Fermi level has important
consequences for the gap structure in favoring the otherwise sub-dominant
triplet solution over the singlet d-wave solution. By solving the full gap
equation, we find that the energetically favorable triplet solutions are chiral
and break time reversal symmetry. Finally, we also calculate the detailed
angular gap structure of the quasi-particle spectrum, and show how
spin-fluctuation-mediated pairing leads to significant deviations from the
first harmonics both in the singlet d_{x^2-y^2} gap as well as the chiral
triplet gap solution.Comment: 11 pages 11 figure
Noise Induced Complexity: From Subthreshold Oscillations to Spiking in Coupled Excitable Systems
We study stochastic dynamics of an ensemble of N globally coupled excitable
elements. Each element is modeled by a FitzHugh-Nagumo oscillator and is
disturbed by independent Gaussian noise. In simulations of the Langevin
dynamics we characterize the collective behavior of the ensemble in terms of
its mean field and show that with the increase of noise the mean field displays
a transition from a steady equilibrium to global oscillations and then, for
sufficiently large noise, back to another equilibrium. Diverse regimes of
collective dynamics ranging from periodic subthreshold oscillations to
large-amplitude oscillations and chaos are observed in the course of this
transition. In order to understand details and mechanisms of noise-induced
dynamics we consider a thermodynamic limit of the ensemble, and
derive the cumulant expansion describing temporal evolution of the mean field
fluctuations. In the Gaussian approximation this allows us to perform the
bifurcation analysis; its results are in good agreement with dynamical
scenarios observed in the stochastic simulations of large ensembles
Numerical simulations versus theoretical predictions for a non-Gaussian noise induced escape problem in application to full counting statistics
A theoretical approach for characterizing the inïŹuence of asymmetry of noise distribution on the escape rate
of a multistable system is presented. This was carried out via the estimation of an action, which is deïŹned as
an exponential factor in the escape rate, and discussed in the context of full counting statistics paradigm. The
approach takes into account all cumulants of the noise distribution and demonstrates an excellent agreement with
the results of numerical simulations. An approximation of the third-order cumulant was shown to have limitations
on the range of dynamic stochastic system parameters. The applicability of the theoretical approaches developed
so far is discussed for an adequate characterization of the escape rate measured in experiments
Stationary and Oscillatory Spatial Patterns Induced by Global Periodic Switching
We propose a new mechanism for pattern formation based on the global
alternation of two dynamics neither of which exhibits patterns. When driven by
either one of the separate dynamics, the system goes to a spatially homogeneous
state associated with that dynamics. However, when the two dynamics are
globally alternated sufficiently rapidly, the system exhibits stationary
spatial patterns. Somewhat slower switching leads to oscillatory patterns. We
support our findings by numerical simulations and discuss the results in terms
of the symmetries of the system and the ratio of two relevant characteristic
times, the switching period and the relaxation time to a homogeneous state in
each separate dynamics.Comment: REVTEX preprint: 12 pages including 1 (B&W) + 3 (COLOR) figures (to
appear in Physical Review Letters
Enhancement of stability in randomly switching potential with metastable state
The overdamped motion of a Brownian particle in randomly switching piece-wise
metastable linear potential shows noise enhanced stability (NES): the noise
stabilizes the metastable system and the system remains in this state for a
longer time than in the absence of white noise. The mean first passage time
(MFPT) has a maximum at a finite value of white noise intensity. The analytical
expression of MFPT in terms of the white noise intensity, the parameters of the
potential barrier, and of the dichotomous noise is derived. The conditions for
the NES phenomenon and the parameter region where the effect can be observed
are obtained. The mean first passage time behaviours as a function of the mean
flipping rate of the potential for unstable and metastable initial
configurations are also analyzed. We observe the resonant activation phenomenon
for initial metastable configuration of the potential profile.Comment: 9 pages, 5 figures. In press in "European Physical Journal B
Noise delayed decay of unstable states: theory versus numerical simulations
We study the noise delayed decay of unstable nonequilibrium states in
nonlinear dynamical systems within the framework of the overdamped Brownian
motion model. We give the exact expressions for the decay times of unstable
states for polynomial potential profiles and obtain nonmonotonic behavior of
the decay times as a function of the noise intensity for the unstable
nonequilibrium states. The analytical results are compared with numerical
simulations.Comment: 9 pages, 6 figures, in press in J. Phys.
Analyzing powers Ayy, Axx, Axz and Ay in the dd->3Hen reaction at 270 MeV
The data on the tensor Ayy, Axx, Axz and vector Ay analyzing powers in the
dd->3Hen obtained at Td= 270 MeV in the angular range 0 - 110 degrees in the
c.m. are presented. The observed negative sign of the tensor analyzing powers
Ayy, Axx and Axz at small angles clearly demonstrate the sensitivity to the
ratio of the D and S wave component of the 3He wave function. However, the
one-nucleon exchange calculations by using the standard 3He wave functions have
failed to reproduce the strong variation of the tensor analyzing powers as a
function of the angle in the c.m.Comment: 8 pages, 7 figures, 4 tables. Submitted to EPJ
Fragmentation channels of relativistic Be nuclei in peripheral interactions
Nuclei of Li were accelerated at the JINR Nuclotron. After the
charge-exchange reaction involving these nuclei at an external target a second
Be beam of energy 1.23A GeV was formed. This beam was used to expose
photo-emulsion chambers. The mean free path for inelastic Be interactions
in emulsion =14.00.8 cm coincides within the errors with those
for Li and Li nuclei. More than 10% of the Be events are associated
with the peripheral interactions in which the total charge of the relativistic
fragments is equal to the charge of the Be and in which charged mesons are
not produced. An unusual ratio of the isotopes is revealed in the composition
of the doubly charged Be fragments: the number of He fragments is twice
as large as that of He fragments. In 50% of peripheral interactions, a
Be nucleus decays to two doubly charged fragments. The present paper gives
the channels of the Be fragmentation to charged fragments. In 50% of
events, the Be fragmentation proceeds only to charged fragments involving
no emission of neutrons. Of them, the He+He channel dominates, the
He+d+p and Li+pchannels constitute 10% each. Two events involving no
emission of neutrons are registered in the 3-body He+t+p and He+d+d
channels. The mean free path for the coherent dissociation of relativistic
Be nuclei to He+He is 71 m. The particular features of the
relativistic Be fragmentation in such peripheral interactions are explained
by the He+He 2-cluster structure of the Be nucleus.Comment: 10 pages, 3 figures, 3 tables, conference: Conference on Physics of
Fundamental Interactions, Moscow, Russia, 5-9 Dec 200
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