8,608 research outputs found
Large-scale instability in a sheared nonhelical turbulence: formation of vortical structures
We study a large-scale instability in a sheared nonhelical turbulence that
causes generation of large-scale vorticity. Three types of the background
large-scale flows are considered, i.e., the Couette and Poiseuille flows in a
small-scale homogeneous turbulence, and the "log-linear" velocity shear in an
inhomogeneous turbulence. It is known that laminar plane Couette flow and
antisymmetric mode of laminar plane Poiseuille flow are stable with respect to
small perturbations for any Reynolds numbers. We demonstrate that in a
small-scale turbulence under certain conditions the large-scale Couette and
Poiseuille flows are unstable due to the large-scale instability. This
instability causes formation of large-scale vortical structures stretched along
the mean sheared velocity. The growth rate of the large-scale instability for
the "log-linear" velocity shear is much larger than that for the Couette and
Poiseuille background flows. We have found a turbulent analogue of the
Tollmien-Schlichting waves in a small-scale sheared turbulence. A mechanism of
excitation of turbulent Tollmien-Schlichting waves is associated with a
combined effect of the turbulent Reynolds stress-induced generation of
perturbations of the mean vorticity and the background sheared motions. These
waves can be excited even in a plane Couette flow imposed on a small-scale
turbulence when perturbations of mean velocity depend on three spatial
coordinates. The energy of these waves is supplied by the small-scale sheared
turbulence.Comment: 12 pages, 14 figures, Phys. Rev. E, in pres
Effect of Nyquist Noise on the Nyquist Dephasing Rate in 2d Electron Systems
We measure the effect of externally applied broadband Nyquist noise on the
intrinsic Nyquist dephasing rate of electrons in a two-dimensional electron gas
at low temperatures. Within the measurement error, the phase coherence time is
unaffected by the externally applied Nyquist noise, including applied noise
temperatures of up to 300 K. The amplitude of the applied Nyquist noise from
100 MHz to 10 GHz is quantitatively determined in the same experiment using a
microwave network analyzer.Comment: 5 pages, 4 figures. Author affiliation clarified; acknowledgements
modified. Replacement reason clarifie
Decoherence of interacting electrons in disordered conductors: on the relation between influence functional and diagrammatic approaches
We establish a connection between the influence functional approach of
Golubev and Zaikin (GZ) and Keldysh diagrammatic perturbation theory for
calculating the decoherence time of interacting electrons in disordered metals;
we show how the standard diagrams for the Cooperon self energy can be recovered
from GZ's influence functional . This allows us to shed
light on GZ's claim that is irrelevant for decoherence: generates
as many important self energy diagrams as ; GZ's neglect of is
permissible only at high temperatures ().Comment: 6 pages LaTeX, 2 figures, for conference proceedings: "Quantum
Transport and Quantum Coherence" -- Localisation 2002 Sophia University,
Tokyo, August 16-19, 2002, to be published in a supplement of the Journal of
Physical Society of Japa
Universal scaling of current fluctuations in disordered graphene
We analyze the full transport statistics of graphene with smooth disorder at
low dopings. First we consider the case of 1D disorder for which the
transmission probability distribution is given analytically in terms of the
graphene-specific mean free path. All current cumulants are shown to scale with
system parameters (doping, size, disorder strength and correlation length) in
an identical fashion for large enough systems. In the case of 2D disorder,
numerical evidence is given for the same kind of identical scaling of all
current cumulants, so that the ratio of any two such cumulants is universal.
Specific universal values are given for the Fano factor, which is smaller than
the pseudodiffusive value of ballistic graphene (F=1/3) both for 1D (F=0.243)
and 2D (F=0.295) disorder. On the other hand, conductivity in wide samples is
shown to grow without saturation as \sqrt{L} and Log L with system length L in
the 1D and 2D cases respectively.Comment: 9 pages, 7 figures. Published version, includes corrected figure for
Fano facto
The Problem of Scheduling for the Linear Section of a Single-Track Railway with Independent Edges Orientations
The paper is devoted to the problem of scheduling for the linear section of a single-track railway: how to organize the ow in both directions in the most efficient way. In this paper, the authors propose an algorithm for scheduling with independent edges orientations, examine the properties of this algorithm and perform the computational experiments
Electron transport and current fluctuations in short coherent conductors
Employing a real time effective action formalism we analyze electron
transport and current fluctuations in comparatively short coherent conductors
in the presence of electron-electron interactions. We demonstrate that, while
Coulomb interaction tends to suppress electron transport, it may {\it strongly
enhance} shot noise in scatterers with highly transparent conducting channels.
This effect of excess noise is governed by the Coulomb gap observed in the
current-voltage characteristics of such scatterers. We also analyze the
frequency dispersion of higher current cumulants and emphasize a direct
relation between electron-electron interaction effects and current fluctuations
in disordered mesoscopic conductors.Comment: 16 pages, 4 figure
- …