919 research outputs found
Lateral current density fronts in asymmetric double-barrier resonant-tunneling structures
We present a theoretical analysis and numerical simulations of lateral
current density fronts in bistable resonant-tunneling diodes with Z-shaped
current-voltage characteristics. The bistability is due to the charge
accumulation in the quantum well of the double-barrier structure. We focus on
asymmetric structures in the regime of sequential incoherent tunneling and
study the dependence of the bistability range, the front velocity and the front
width on the structure parameters. We propose a sectional design of a structure
that is suitable for experimental observation of front propagation and discuss
potential problems of such measurements in view of our theoretical findings. We
point out the possibility to use sectional resonant-tunneling structures as
controllable three-terminal switches.Comment: to appear in J.Appl.Phy
Resonant control of stochastic spatio-temporal dynamics in a tunnel diode by multiple time delayed feedback
We study the control of noise-induced spatio-temporal current density
patterns in a semiconductor nanostructure (double barrier resonant tunnelling
diode) by multiple time-delayed feedback. We find much more pronounced resonant
features of noise-induced oscillations compared to single time feedback,
rendering the system more sensitive to variations in the delay time . The
coherence of noise-induced oscillations measured by the correlation time
exhibits sharp resonances as a function of , and can be strongly
increased by optimal choices of . Similarly, the peaks in the power
spectral density are sharpened. We provide analytical insight into the control
mechanism by relating the correlation times and mean frequencies of
noise-induced breathing oscillations to the stability properties of the
deterministic stationary current density filaments under the influence of the
control loop. Moreover, we demonstrate that the use of multiple time delays
enlarges the regime in which the deterministic dynamical properties of the
system are not changed by delay-induced bifurcations
Trust in the US-EU fruit and vegetable chain: Do US exporters understand EU importers?
Research on organizational and inter organizational trust has become an important field in management and marketing literature, as it is perceived as a pivotal aspect of business transactions. However, clarifications are still needed on the issue of whom we trust; is the person whom we are trading with trusted, or the organization, or just the product‐quality? Not only has this question not been answered within this field of research, neither have cultural differences have been described to any great extent. Additionally, if the perceived factors important to establish trusting relationships may or may not be the same on the buyers and the sellers side in international business transaction in food chains. The primary objective of this research study therefore is to identify how well US exporters understand the elements of trust that establish strong relationships with EU importers. The Analytical Hierarchy Process was used to evaluate the importance of different trust elements in interviews conducted with US exporters and EU importers of fruits and vegetables. Results are compared, providing both a picture of the important facets of trust, as well as whether the partners understand the perspectives of the other partner
On stochastic switching of bistable resonant-tunneling structures via nucleation
We estimate the critical size of the initial nucleus of the low current state
in a bistable resonant tunneling structure which is needed for this nucleus to
develop into a lateral switching front. Using the results obtained for
deterministic switching fronts, we argue that for realistic structural
parameters the critical nucleus has macroscopic dimensions and therefore is too
large to be created by stochastic electron noise.Comment: the extended version of the Comment on "Lifetime of metastable states
in resonant-tunneling structures" to appear in Phys. Rev.
Gravitational instabilities in Kerr space-times
In this paper we consider the possible existence of unstable axisymmetric
modes in Kerr space times, resulting from exponentially growing solutions of
the Teukolsky equation. We describe a transformation that casts the radial
equation that results upon separation of variables in the Teukolsky equation,
in the form of a Schr\"odinger equation, and combine the properties of the
solutions of this equations with some recent results on the asymptotic
behaviour of spin weighted spheroidal harmonics to prove the existence of an
infinite family of unstable modes. Thus we prove that the stationary region
beyond a Kerr black hole inner horizon is unstable under gravitational linear
perturbations. We also prove that Kerr space-time with angular momentum larger
than its square mass, which has a naked singularity, is unstable.Comment: 9 pages, 4 figures, comments, references and calculation details
added, asymptotic expansion typos fixe
Dust Emission from Evolved and Unevolved HII Regions in the Large Magellanic Cloud
We present a study of the dust properties of 12 classical and superbubble HII
regions in the Large Magellanic Cloud. We use infrared photometry from Spitzer
(8, 24, 70, and 160 \mum bands), obtained as part of the Surveying the Agents
of a Galaxy's Evolution (SAGE) program, along with archival spectroscopic
classifications of the ionizing stars to examine the role of stellar sources on
dust heating and processing. Our infrared observations show surprisingly little
correlation between the emission properties of the dust and the effective
temperatures or bolometric magnitudes of stars in the HII regions, suggesting
that the HII region evolutionary timescale is not on the order of the dust
processing timescale. We find that the infrared emission of superbubbles and
classical HII regions shows little differentiation between the two classes,
despite the significant differences in age and morphology. We do detect a
correlation of the 24 \mum emission from hot dust with the ratio of 70 to 160
\mum flux. This correlation can be modeled as a trend in the temperature of a
minority hot dust component, while a majority of the dust remains significantly
cooler.Comment: 15 pages, 5 figures. Accepted to Ap
Solutions to Maxwell's Equations using Spheroidal Coordinates
Analytical solutions to the wave equation in spheroidal coordinates in the
short wavelength limit are considered. The asymptotic solutions for the radial
function are significantly simplified, allowing scalar spheroidal wave
functions to be defined in a form which is directly reminiscent of the
Laguerre-Gaussian solutions to the paraxial wave equation in optics.
Expressions for the Cartesian derivatives of the scalar spheroidal wave
functions are derived, leading to a new set of vector solutions to Maxwell's
equations. The results are an ideal starting point for calculations of
corrections to the paraxial approximation
Dust emission at 8-mic and 24-mic as Diagnostics of HII Region Radiative Transfer
We use the Spitzer SAGE survey of the Magellanic Clouds to evaluate the
relationship between the 8-mic PAH emission, 24-mic hot dust emission, and HII
region radiative transfer. We confirm that in the higher-metallicity Large
Magellanic Cloud, PAH destruction is sensitive to optically thin conditions in
the nebular Lyman continuum: objects identified as optically thin candidates
based on nebular ionization structure show 6 times lower median 8-mic surface
brightness (0.18 mJy arcsec^-2) than their optically thick counterparts (1.2
mJy arcsec^-2). The 24-mic surface brightness also shows a factor of 3 offset
between the two classes of objects (0.13 vs 0.44 mJy arcsec^-2, respectively),
which is driven by the association between the very small dust grains and
higher density gas found at higher nebular optical depths. In contrast, PAH and
dust formation in the low-metallicity Small Magellanic Cloud is strongly
inhibited such that we find no variation in either 8-mic or 24-mic emission
between our optically thick and thin samples. This is attributable to extremely
low PAH and dust production together with high, corrosive UV photon fluxes in
this low-metallicity environment. The dust mass surface densities and
gas-to-dust ratios determined from dust maps using Herschel HERITAGE survey
data support this interpretation.Comment: Accepted to ApJ, May 15, 2017. 10 pages, 9 figure
The Casimir Effect in Spheroidal Geometries
We study the zero point energy of massless scalar and vector fields subject
to spheroidal boundary conditions. For massless scalar fields and small
ellipticity the zero-point energy can be found using both zeta function and
Green's function methods. The result agrees with the conjecture that the zero
point energy for a boundary remains constant under small deformations of the
boundary that preserve volume (the boundary deformation conjecture), formulated
in the case of an elliptic-cylindrical boundary. In the case of massless vector
fields, an exact solution is not possible. We show that a zonal approximation
disagrees with the result of the boundary deformation conjecture. Applying our
results to the MIT bag model, we find that the zero point energy of the bag
should stabilize the bag against deformations from a spherical shape.Comment: 24 pages, 3 figures. To appear in Phys. Rev.
- …