6,800 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
Coherent and Non-Coherent Double Diffractive Production of - pairs in Collisions of Heavy Ions at High Energies
The double coherent and non-coherent diffractive production of heavy quark -
antiquark pairs () in heavy ion scattering at high energies (LHC) is
considered. The total and differential cross sections of these processes with
the formation of and pairs in , and
collisions are evaluated. The contribution of the considered mechanisms is a
few per cent of the number of heavy quark - antiquark pairs obtained in the
processes of hard (QCD) scattering, and it will be taken into account in the
registration of , quarks or, for instance, in the study of the heavy
quarkonia suppression effects in Quark - Gluon Plasma, in the search for
intermediate mass Higgs bosons and so on. It is shown that the cross section of
the coherent scattering process is great enough. This makes it suitable for
studying collective effects in nuclear interactions at high energies. An
example of such effects is given: large values of the invariant mass of a pair, M_{Q \bar{Q}} \gsim 100 GeV, in association with a large
rapidity gap between diffractive jets .Comment: 22 pages, 5(.eps) figures, 3 tables, LaTe
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 influence 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 defined 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
Clustering in light nuclei in fragmentation above 1 A GeV
The relativistic invariant approach is applied to analyzing the 3.3 A GeV
Ne fragmentation in a nuclear track emulsion. New results on few-body
dissociations have been obtained from the emulsion exposures to 2.1 A GeV
N and 1.2 A GeV Be nuclei. It can be asserted that the use of the
invariant approach is an effective means of obtaining conclusions about the
behavior of systems involving a few He nuclei at a relative energy close to 1
MeV per nucleon. The first observations of fragmentation of 1.2 A GeV B
and C nuclei in emulsion are described. The presented results allow one
to justify the development of few-body aspects of nuclear astrophysics.Comment: 7 pages, 8 figures, 3 tables, Nuclear Physics in Astrophysics-2,
16-20 May, 2005 (ATOMKI), Debrecen, Hungar
Special features of the Be2He fragmentation in emulsion at an energy of 1.2~A~GeV
The results of investigations of the relativistic Be nucleus
fragmentation in emulsion which entails the production of two He fragments of
an energy of 1.2~A~GeV are presented. The results of the angular measurements
of the Be2He events are analyzed.
The BeBe+n fragmentation channel involving the Be decay from
the ground (0) and the first excited (2) states to two
particles is observed to be predominant.Comment: 10 pages, 6 figures, conference: Conference on Physics of Fundamental
Interactions, Moscow, Russia, 5-9 Dec 2005 (Author's translation
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
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