4,130 research outputs found
Shot noise and Coulomb effects on non-local electron transport in normal-superconducting-normal heterostructures
We argue that Coulomb interaction can strongly influence non-local electron
transport in normal-superconducting-normal structures and emphasize direct
relation between Coulomb effects and non-local shot noise. In the tunneling
limit non-local differential conductance is found to have an S-like shape and
can turn negative at non-zero bias. At high transmissions crossed Andreev
reflection yields positive noise cross-correlations and Coulomb anti-blockade
of non-local electron transport.Comment: 8 pages, 2 figures. Published versio
Lipschitz gradients for global optimization in a one-point-based partitioning scheme
A global optimization problem is studied where the objective function
is a multidimensional black-box function and its gradient satisfies the
Lipschitz condition over a hyperinterval with an unknown Lipschitz constant
. Different methods for solving this problem by using an a priori given
estimate of , its adaptive estimates, and adaptive estimates of local
Lipschitz constants are known in the literature. Recently, the authors have
proposed a one-dimensional algorithm working with multiple estimates of the
Lipschitz constant for (the existence of such an algorithm was a
challenge for 15 years). In this paper, a new multidimensional geometric method
evolving the ideas of this one-dimensional scheme and using an efficient
one-point-based partitioning strategy is proposed. Numerical experiments
executed on 800 multidimensional test functions demonstrate quite a promising
performance in comparison with popular DIRECT-based methods.Comment: 25 pages, 4 figures, 5 tables. arXiv admin note: text overlap with
arXiv:1103.205
Odd-Parity Negative Modes of Einstein-Yang-Mills Black Holes and Sphalerons
An analytical proof of the existence of negative modes in the odd--parity
perturbation sector is given for all known non-abelian Einstein--Yang--Mills
black holes. The significance of the normalizability condition in the
functional stability analysis is emphasized. The role of the odd--parity
negative modes in the sphaleron interpretation of the Bartnik--McKinnon
solutions is discussed.Comment: (minor typographical errors fixed, to appear in Phys.Lett.B
Correlation Functions of Sp(2n) Invariant Higher-Spin Systems
We study the general structure of correlation functions in an
Sp(2n)-invariant formulation of systems of an infinite number of higher-spin
fields. For n=4,8 and 16 these systems comprise the conformal higher-spin
fields in space-time dimensions D=4,6 and 10, respectively, while when n=2, one
deals with conventional D=3 conformal field theories of scalars and spinors. We
show that for n>2 the Sp(2n) symmetry and current conservation makes the
3-point correlators of two (rank-one or rank-two) conserved currents with a
scalar operator be that of free theory.This situation is analogous to the one
in conventional conformal field theories, where conservation of higher-spin
currents implies that the theories are free.Comment: 26 pages, some clarifications added, published versio
A hypothetical effect of the Maxwell-Proca electromagnetic stresses on galaxy rotation curves
The Maxwell-Proca electrodynamics corresponding to a finite photon mass
causes a substantial change of the Maxwell stress tensor and, under certain
circumstances, may cause the electromagnetic stresses to act effectively as
"negative pressure." The paper describes a model where this negative pressure
imitates gravitational pull and may produce forces comparable to gravity and
even become dominant. The effect is associated with the random magnetic fields
in the galactic disk with a scale exceeding the photon Compton wavelength. The
presence of a weaker regular field does not affect the forces under
consideration. The stresses act predominantly on the interstellar gas and cause
an additional force pulling the gas towards the center and towards the galactic
plane. The stars do not experience any significant direct force but get
involved in this process via a "recycling loop" where rapidly evolving massive
stars are formed from the gas undergoing galactic rotation and then lose their
masses back to the gas within a time shorter than roughly 1/6 of the rotation
period. This makes their dynamics inseparable from that of the rotating gas.
The lighter, slowly evolving stars, as soon as they are formed, lose connection
to the gas and are confined within the galaxy only gravitationally. Numerical
examples based on the parameters of our galaxy reveal both opportunities and
challenges of this model and motivate further analysis. The critical issue is
the plausibility of formation of the irregular magnetic field that would be
force free. Another challenge is developing a predictive model of the evolution
of the gaseous and stellar population of the galaxy under the aforementioned
scenario. It may be interesting to also explore possible broader cosmological
implications of the negative-pressure model.Comment: 29 pages, 1 figur
- …