76 research outputs found
Transport Through Nanostructures with Asymmetric Coupling to the Leads
Using an approach to open quantum systems based on the effective
non-Hermitian Hamiltonian, we fully describe transport properties for a
paradigmatic model of a coherent quantum transmitter: a finite sequence of
square potential barriers. We consider the general case of asymmetric external
barriers and variable coupling strength to the environment. We demonstrate that
transport properties are very sensitive to the degree of opening of the system
and determine the parameters for maximum transmission at any given degree of
asymmetry. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian,
we show a double transition to a super-radiant regime where the transport
properties and the structure of resonances undergo a strong change. We extend
our analysis to the presence of disorder and to higher dimensions.Comment: submitted to Phys. Rev.
Internal chaos in an open quantum system: From Ericson to conductance fluctuations
The model of an open Fermi-system is used for studying the interplay of
intrinsic chaos and irreversible decay into open continuum channels. Two
versions of the model are characterized by one-body chaos coming from disorder
or by many-body chaos due to the inter-particle interactions. The continuum
coupling is described by the effective non-Hermitian Hamiltonian. Our main
interest is in specific correlations of cross sections for various channels in
dependence on the coupling strength and degree of internal chaos. The results
are generic and refer to common features of various mesoscopic objects
including conductance fluctuations and resonance nuclear reactions.Comment: 10 pages, 5 figure
Connections Between Local and Global Turbulence in Accretion Disks
We analyze a suite of global magnetohydrodynamic (MHD) accretion disk
simulations in order to determine whether scaling laws for turbulence driven by
the magnetorotational instability, discovered via local shearing box studies,
are globally robust. The simulations model geometrically-thin disks with zero
net magnetic flux and no explicit resistivity or viscosity. We show that the
local Maxwell stress is correlated with the self-generated local vertical
magnetic field in a manner that is similar to that found in local simulations.
Moreover, local patches of vertical field are strong enough to stimulate and
control the strength of angular momentum transport across much of the disk. We
demonstrate the importance of magnetic linkages (through the low-density
corona) between different regions of the disk in determining the local field,
and suggest a new convergence requirement for global simulations -- the
vertical extent of the corona must be fully captured and resolved. Finally, we
examine the temporal convergence of the average stress, and show that an
initial long-term secular drift in the local flux-stress relation dies away on
a time scale that is consistent with turbulent mixing of the initial magnetic
field.Comment: 8 Pages, 7 Figures ApJ, In Pres
Low-Frequency Oscillations in Global Simulations of Black Hole Accretion
We have identified the presence of large-scale, low-frequency dynamo cycles
in a long-duration, global, magnetohydrodynamic (MHD) simulation of black hole
accretion. Such cycles had been seen previously in local shearing box
simulations, but we discuss their evolution over 1,500 inner disk orbits of a
global pi/4 disk wedge spanning two orders of magnitude in radius and seven
scale heights in elevation above/below the disk midplane. The observed cycles
manifest themselves as oscillations in azimuthal magnetic field occupying a
region that extends into a low-density corona several scale heights above the
disk. The cycle frequencies are ten to twenty times lower than the local
orbital frequency, making them potentially interesting sources of low-frequency
variability when scaled to real astrophysical systems. Furthermore, power
spectra derived from the full time series reveal that the cycles manifest
themselves at discrete, narrow-band frequencies that often share power across
broad radial ranges. We explore possible connections between these simulated
cycles and observed low-frequency quasi-periodic oscillations (LFQPOs) in
galactic black hole binary systems, finding that dynamo cycles have the
appropriate frequencies and are located in a spatial region associated with
X-ray emission in real systems. Derived observational proxies, however, fail to
feature peaks with RMS amplitudes comparable to LFQPO observations, suggesting
that further theoretical work and more sophisticated simulations will be
required to form a complete theory of dynamo-driven LFQPOs. Nonetheless, this
work clearly illustrates that global MHD dynamos exhibit quasi-periodic
behavior on timescales much longer than those derived from test particle
considerations.Comment: Version accepted to The Astrophysical Journal, 8 pages, 7 figure
Inelastic chaotic scattering on a Bose-Einstein condensate
We devise a microscopic scattering approach to probe the excitation spectrum
of a Bose-Einstein condensate. We show that the experimentally accessible
scattering cross section exhibits universal Ericson fluctuations, with
characteristic properties rooted in the underlying classical field equations.Comment: 11 pages, 5 figure
High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we
could form spectral index, energy and magnetic field maps. We find that all but
one galaxy show evidence for non-thermal high latitude radio continuum
emission, suggesting that cosmic ray halos are common in star forming galaxies.
The high latitude emission is seen over a variety of spatial scales and in
discrete and/or smooth features. In general, the discrete features emanate from
the disk, but estimates of CR diffusion lengths suggest that diffusion alone is
insufficient to transport the particles to the high latitudes seen (> 15 kpc in
one case). Thus CRs likely diffuse through low density regions and/or are
assisted by other mechanisms (e.g. winds). We searched for correlations between
the prevalence of high latitude radio emission and a number of other
properties, including the global SFR, supernova input rate per unit star
forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg
figures (which is a radio continuum catalogue of 17 galaxies). to appear in
A. J. (around January 1999
From closed to open 1D Anderson model: Transport versus spectral statistics
Using the phenomenological expression for the level spacing distribution with
only one parameter, , covering all regimes of chaos
and complexity in a quantum system, we show that transport properties of the
one-dimensional Anderson model of finite size can be expressed in terms of this
parameter. Specifically, we demonstrate a strictly linear relation between
and the normalized localization length for the whole transition from
strongly localized to extended states. This result allows one to describe all
transport properties in the open system entirely in terms of the parameter
and strength of coupling to continuum. For non-perfect coupling, our
data show a quite unusual interplay between the degree of internal chaos
defined by , and degree of openness of the model. The results can be
experimentally tested in single-mode waveguides with either bulk or surface
disorder.Comment: 8 pages, 8 figures, fully revised version accepted for publication in
PR
How Jupiter's Unusual Magnetospheric Topology Structures Its Aurora
Jupiter's bright persistent polar aurora and Earth's dark polar region
indicate that the planets' magnetospheric topologies are very different.
High-resolution global simulations show that the reconnection rate at the
interface between the interplanetary and jovian magnetic fields is too slow to
generate a magnetically open, Earth-like polar cap on the timescale of
planetary rotation, resulting in only a small crescent-shaped region of
magnetic flux interconnected with the interplanetary magnetic field. Most of
the jovian polar cap is threaded by helical magnetic flux that closes within
the planetary interior, extends into the outer magnetosphere and piles-up near
its dawnside flank where fast differential plasma rotation pulls the field
lines sunward. This unusual magnetic topology provides new insights into
Jupiter's distinctive auroral morphology
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