1,599 research outputs found
Parametric resonances in electrostatically interacting carbon nanotube arrays
We study, numerically and analytically, a model of a one-dimensional array of
carbon nanotube resonators in a two-terminal configuration. The system is
brought into resonance upon application of an AC-signal superimposed on a
DC-bias voltage. When the tubes in the array are close to each other,
electrostatic interactions between tubes become important for the array
dynamics. We show that both transverse and longitudinal parametric resonances
can be excited in addition to primary resonances. The intertube electrostatic
interactions couple modes in orthogonal directions and affect the mode
stability.Comment: 11 pages, 12 figures, RevTeX
On the variational homotopy perturbation method for nonlinear oscillators
In this paper we discuss a recent application of a variational homotopy
perturbation method to rather simple nonlinear oscillators . We show that the
main equations are inconsistent and for that reason the results may be of
scarce utility
Nonparallel stability of two-dimensional nonuniformly heated boundary-layer flows
An analysis is presented for the linear stability of water boundary-layer flows over nonuniformly flat plates. Included in the analysis are disturbances due to velocity, pressure, temperatures, density, and transport properties as well as variations of the liquid properties with temperature. The method of multiple scales is used to account for the nonparallelism of the mean flow. In contrast with previous analyses, the nonsimilarity of the mean flow is taken into account. No analysis agrees, even qualitatively, with the experimental data when similar profiles are used. However, both the parallel and nonparallel results qualitatively agree with the experimental results of Strazisar and Reshotko when nonsimilar profiles are used
Breakdown of Conformal Invariance at Strongly Random Critical Points
We consider the breakdown of conformal and scale invariance in random systems
with strongly random critical points. Extending previous results on
one-dimensional systems, we provide an example of a three-dimensional system
which has a strongly random critical point. The average correlation functions
of this system demonstrate a breakdown of conformal invariance, while the
typical correlation functions demonstrate a breakdown of scale invariance. The
breakdown of conformal invariance is due to the vanishing of the correlation
functions at the infinite disorder fixed point, causing the critical
correlation functions to be controlled by a dangerously irrelevant operator
describing the approach to the fixed point. We relate the computation of
average correlation functions to a problem of persistence in the RG flow.Comment: 9 page
Importance of an Astrophysical Perspective for Textbook Relativity
The importance of a teaching a clear definition of the ``observer'' in
special relativity is highlighted using a simple astrophysical example from the
exciting current research area of ``Gamma-Ray Burst'' astrophysics. The example
shows that a source moving relativistically toward a single observer at rest
exhibits a time ``contraction'' rather than a ``dilation'' because the light
travel time between the source and observer decreases with time. Astrophysical
applications of special relativity complement idealized examples with real
applications and very effectively exemplify the role of a finite light travel
time.Comment: 5 pages TeX, European Journal of Physics, in pres
Quasienergy description of the driven Jaynes-Cummings model
We analyze the driven resonantly coupled Jaynes-Cummings model in terms of a
quasienergy approach by switching to a frame rotating with the external
modulation frequency and by using the dressed atom picture. A quasienergy
surface in phase space emerges whose level spacing is governed by a rescaled
effective Planck constant. Moreover, the well-known multiphoton transitions can
be reinterpreted as resonant tunneling transitions from the local maximum of
the quasienergy surface. Most importantly, the driving defines a quasienergy
well which is nonperturbative in nature. The quantum mechanical quasienergy
state localized at its bottom is squeezed. In the Purcell limited regime, the
potential well is metastable and the effective local temperature close to its
minimum is uniquely determined by the squeezing factor. The activation occurs
in this case via dressed spin flip transitions rather than via quantum
activation as in other driven nonlinear quantum systems such as the quantum
Duffing oscillator. The local maximum is in general stable. However, in
presence of resonant coherent or dissipative tunneling transitions the system
can escape from it and a stationary state arises as a statistical mixture of
quasienergy states being localized in the two basins of attraction. This gives
rise to a resonant or an antiresonant nonlinear response of the cavity at
multiphoton transitions. The model finds direct application in recent
experiments with a driven superconducting circuit QED setup.Comment: 13 pages, 8 fi
Spatially incoherent modulational instability in a non local medium
We investigate one-dimensional transverse modulational instability in a non
local medium excited with a spatially incoherent source. Employing undoped
nematic liquid crystals in a planar pre-tilted configuration, we investigate
the role of the spectral broadening induced by incoherence in conjunction with
the spatially non local molecular reorientation. The phenomenon is modeled
using the Wigner transform.Comment: 13 pages with 4 figures included. To be published in Laser Physics
Letter
Superconducting Nanowires as Nonlinear Inductive Elements for Qubits
We report microwave transmission measurements of superconducting Fabry-Perot
resonators (SFPR), having a superconducting nanowire placed at a supercurrent
antinode. As the plasma oscillation is excited, the supercurrent is forced to
flow through the nanowire. The microwave transmission of the resonator-nanowire
device shows a nonlinear resonance behavior, significantly dependent on the
amplitude of the supercurrent oscillation. We show that such
amplitude-dependent response is due to the nonlinearity of the current-phase
relationship (CPR) of the nanowire. The results are explained within a
nonlinear oscillator model of the Duffing oscillator, in which the nanowire
acts as a purely inductive element, in the limit of low temperatures and low
amplitudes. The low quality factor sample exhibits a "crater" at the resonance
peak at higher driving power, which is due to dissipation. We observe a
hysteretic bifurcation behavior of the transmission response to frequency sweep
in a sample with a higher quality factor. The Duffing model is used to explain
the Duffing bistability diagram. We also propose a concept of a nanowire-based
qubit that relies on the current dependence of the kinetic inductance of a
superconducting nanowire.Comment: 28 pages, 7 figure
Electron-phonon relaxation and excited electron distribution in gallium nitride
We develop a theory of energy relaxation in semiconductors and insulators
highly excited by the long-acting external irradiation. We derive the equation
for the non-equilibrium distribution function of excited electrons. The
solution for this function breaks up into the sum of two contributions. The
low-energy contribution is concentrated in a narrow range near the bottom of
the conduction band. It has the typical form of a Fermi distribution with an
effective temperature and chemical potential. The effective temperature and
chemical potential in this low-energy term are determined by the intensity of
carriers' generation, the speed of electron-phonon relaxation, rates of
inter-band recombination and electron capture on the defects. In addition,
there is a substantial high-energy correction. This high-energy 'tail' covers
largely the conduction band. The shape of the high-energy 'tail' strongly
depends on the rate of electron-phonon relaxation but does not depend on the
rates of recombination and trapping. We apply the theory to the calculation of
a non-equilibrium distribution of electrons in irradiated GaN. Probabilities of
optical excitations from the valence to conduction band and electron-phonon
coupling probabilities in GaN were calculated by the density functional
perturbation theory. Our calculation of both parts of distribution function in
gallium nitride shows that when the speed of electron-phonon scattering is
comparable with the rate of recombination and trapping then the contribution of
the non-Fermi 'tail' is comparable with that of the low-energy Fermi-like
component. So the high-energy contribution can affect essentially the charge
transport in the irradiated and highly doped semiconductors.Comment: 15 pages, 6 figure
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