3,668 research outputs found
Non-axisymmetric Magnetorotational Instabilities in Cylindrical Taylor-Couette Flow
We study the stability of cylindrical Taylor-Couette flow in the presence of
azimuthal magnetic fields, and show that one obtains non-axisymmetric
magnetorotational instabilities, having azimuthal wavenumber m=1. For
Omega_o/Omega_i only slightly greater than the Rayleigh value (r_i/r_o)^2, the
critical Reynolds and Hartmann numbers are Re_c ~ 10^3 and Ha_c ~ 10^2,
independent of the magnetic Prandtl number Pm. These values are sufficiently
small that it should be possible to obtain these instabilities in the PROMISE
experimental facility.Comment: final version as accepted by Phys Rev Let
Multidimensional perfect fluid cosmology with stable compactified internal dimensions
Multidimensional cosmological models in the presence of a bare cosmological
constant and a perfect fluid are investigated under dimensional reduction to
4-dimensional effective models. Stable compactification of the internal spaces
is achieved for a special class of perfect fluids. The external space behaves
in accordance with the standard Friedmann model. Necessary restrictions on the
parameters of the models are found to ensure dynamical behavior of the external
(our) universe in agreement with observations.Comment: 11 pages, Latex2e, uses IOP packages, submitted to Class.Quant.Gra
Equation of State of Oscillating Brans-Dicke Scalar and Extra Dimensions
We consider a Brans-Dicke scalar field stabilized by a general power law
potential with power index at a finite equilibrium value. Redshifting
matter induces oscillations of the scalar field around its equilibrium due to
the scalar field coupling to the trace of the energy momentum tensor. If the
stabilizing potential is sufficiently steep these high frequency oscillations
are consistent with observational and experimental constraints for arbitrary
value of the Brans-Dicke parameter . We study analytically and
numerically the equation of state of these high frequency oscillations in terms
of the parameters and and find the corresponding evolution of the
universe scale factor. We find that the equation of state parameter can be
negative and less than -1 but it is not related to the evolution of the scale
factor in the usual way. Nevertheless, accelerating expansion is found for a
certain parameter range. Our analysis applies also to oscillations of the size
of extra dimensions (the radion field) around an equilibrium value. This
duality between self-coupled Brans-Dicke and radion dynamics is applicable for
where D is the number of extra dimensions.Comment: 10 two-column pages, RevTex4, 8 figures. Added clarifying
discussions, new references. Accepted in Phys. Rev. D (to appear
The brachistochrone problem in open quantum systems
Recently, the quantum brachistochrone problem is discussed in the literature
by using non-Hermitian Hamilton operators of different type. Here, it is
demonstrated that the passage time is tunable in realistic open quantum systems
due to the biorthogonality of the eigenfunctions of the non-Hermitian Hamilton
operator. As an example, the numerical results obtained by Bulgakov et al. for
the transmission through microwave cavities of different shape are analyzed
from the point of view of the brachistochrone problem. The passage time is
shortened in the crossover from the weak-coupling to the strong-coupling regime
where the resonance states overlap and many branch points (exceptional points)
in the complex plane exist. The effect can {\it not} be described in the
framework of standard quantum mechanics with Hermitian Hamilton operator and
consideration of matrix poles.Comment: 18 page
Topological bands in two-dimensional networks of metamaterial elements
We show that topological frequency band structures emerge in two-dimensional
electromagnetic lattices of metamaterial components without the application of
an external magnetic field. The topological nature of the band structure
manifests itself by the occurrence of exceptional points in the band structure
or by the emergence of one-way guided modes. Based on an EM network with nearly
flat frequency bands of nontrivial topology, we propose a coupled-cavity
lattice made of superconducting transmission lines and cavity QED components
which is described by the Janes-Cummings-Hubbard model and can serve as
simulator of the fractional quantum Hall effect
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