8,763 research outputs found
Radio Frequency Selective Addressing of Localized Particles in a Periodic Potential
We study the localization and addressability of ultra cold atoms in a
combined parabolic and periodic potential. Such a potential supports the
existence of localized stationary states and we show that using a radio
frequency field allows to selectively address the atoms in these states. This
method is used to measure the energy and momentum distribution of the atoms in
the localized states. We also discuss possible extensions of this scheme to
address and manipulate particles in single lattice sites.Comment: 4 pages, 4 figure
Ambivalence of the anisotropy of the vortex lattice in an anisotropic type-II superconductor
We present a geometry-based discussion of possible vortex configurations in
the mixed state of anisotropic type-II superconductors. It is shown that, if
energy considerations assign six nearest neighbors to each vortex, two distinct
modifications of the vortex lattice are possible. It is expected that certain
conditions lead to a first order phase transition from one modification of the
vortex lattice to the other upon varying the external magnetic field.Comment: 3 pages, 2 figure
Caustics in turbulent aerosols
Networks of caustics can occur in the distribution of particles suspended in
a randomly moving gas. These can facilitate coagulation of particles by
bringing them into close proximity, even in cases where the trajectories do not
coalesce. We show that the long-time morphology of these caustic patterns is
determined by the Lyapunov exponents lambda_1, lambda_2 of the suspended
particles, as well as the rate J at which particles encounter caustics. We
develop a theory determining the quantities J, lambda_1, lambda_2 from the
statistical properties of the gas flow, in the limit of short correlation
times.Comment: 4 pages, 3 figure
Surface, Water, and Air Biocharacterization (SWAB) Flight Experiment
The determination of risk from infectious disease during spaceflight missions is composed of several factors including both the concentration and characteristics of the microorganisms to which the crew are exposed. Thus, having a good understanding of the microbial ecology aboard spacecraft provides the necessary information to mitigate health risks to the crew. While preventive measures are taken to minimize the presence of pathogens on spacecraft, medically significant organisms have been isolated from both the Mir and International Space Station (ISS). Historically, the method for isolation and identification of microorganisms from spacecraft environmental samples depended upon their growth on culture media. Unfortunately, only a fraction of the organisms may grow on a specific culture medium, potentially omitting those microorganisms whose nutritional and physical requirements for growth are not met. To address this bias in our understanding of the ISS environment, the Surface, Water, and Air Biocharacterization (SWAB) Flight Experiment was designed to investigate and develop monitoring technology to provide better microbial characterization. For the SWAB flight experiment, we hypothesized that environmental analysis using non-culture-based technologies would reveal microorganisms, allergens, and microbial toxins not previously reported in spacecraft, allowing for a more complete health assessment. Key findings during this experiment included: a) Generally, advanced molecular techniques were able to reveal a few organisms not recovered using culture-based methods; however, there is no indication that current monitoring is "missing" any medically significant bacteria or fungi. b) Molecular techniques have tremendous potential for microbial monitoring, however, sample preparation and data analysis present challenges for spaceflight hardware. c) Analytical results indicate that some molecular techniques, such as denaturing gradient gel electrophoresis (DGGE), can be much less sensitive than culture-based methods. d) More sensitive molecular techniques, such as quantitative polymerase chain reaction (QPCR), were able to identify viral DNA from ISS environments, suggesting potential transfer of the organism between crewmembers. In addition, the hardware selected for this experiment represented advances for next-generation sample collection. The advanced nature of this collection hardware was noted, when the Sartorius MD8 Air Port air sampler from the SWAB experiment remained on board ISS at the request of JAXA investigators, who intend to use it in completion of their microbial ecology experiment
Chaotic saddles in nonlinear modulational interactions in a plasma
A nonlinear model of modulational processes in the subsonic regime involving
a linearly unstable wave and two linearly damped waves with different damping
rates in a plasma is studied numerically. We compute the maximum Lyapunov
exponent as a function of the damping rates in a two-parameter space, and
identify shrimp-shaped self-similar structures in the parameter space. By
varying the damping rate of the low-frequency wave, we construct bifurcation
diagrams and focus on a saddle-node bifurcation and an interior crisis
associated with a periodic window. We detect chaotic saddles and their stable
and unstable manifolds, and demonstrate how the connection between two chaotic
saddles via coupling unstable periodic orbits can result in a crisis-induced
intermittency. The relevance of this work for the understanding of modulational
processes observed in plasmas and fluids is discussed.Comment: Physics of Plasmas, in pres
Echoes and revival echoes in systems of anharmonically confined atoms
We study echoes and what we call 'revival echoes' for a collection of atoms
that are described by a single quantum wavefunction and are confined in a
weakly anharmonic trap. The echoes and revival echoes are induced by applying
two, successive temporally localized potential perturbations to the confining
potential, one at time , and a smaller one at time . Pulse-like
responses in the expectation value of position are predicted at $t
\approx n\tau$ ($n=2,3,...$) and are particularly evident at $t \approx 2\tau$.
A novel result of our study is the finding of 'revival echoes'. Revivals (but
not echoes) occur even if the second perturbation is absent. In particular, in
the absence of the second perturbation, the response to the first perturbation
dies away, but then reassembles, producing a response at revival times $mT_x$
($m=1,2,...$). Including the second perturbation at $t=\tau$, we find
temporally localized responses, revival echoes, both before and after $t\approx
mT_x$, e.g., at $t\approx m T_x-n \tau$ (pre-revival echoes) and at $t\approx
mT_x+n\tau$, (post-revival echoes), where $m$ and $n$ are $1,2,...$ . Depending
on the form of the perturbations, the 'principal' revival echoes at $t \approx
T_x \pm \tau$ can be much larger than the echo at $t \approx 2\tau$. We develop
a perturbative model for these phenomena, and compare its predictions to the
numerical solutions of the time-dependent Schr\"odinger Equation. The scaling
of the size of the various echoes and revival echoes as a function of the
symmetry and size of the perturbations applied at $t=0$ and $t=\tau$ is
investigated. We also study the presence of revivals and revival echoes in
higher moments of position, , , and the effect of atom-atom
interactions on these phenomena.Comment: 33 pages, 13 figures, corrected typos and added reference
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Extracting Lyapunov exponents from the echo dynamics of Bose-Einstein condensates on a lattice
We propose theoretically an experimentally realizable method to demonstrate
the Lyapunov instability and to extract the value of the largest Lyapunov
exponent for a chaotic many-particle interacting system. The proposal focuses
specifically on a lattice of coupled Bose-Einstein condensates in the classical
regime describable by the discrete Gross-Pitaevskii equation. We suggest to use
imperfect time-reversal of system's dynamics known as Loschmidt echo, which can
be realized experimentally by reversing the sign of the Hamiltonian of the
system. The routine involves tracking and then subtracting the noise of
virtually any observable quantity before and after the time-reversal. We
support the theoretical analysis by direct numerical simulations demonstrating
that the largest Lyapunov exponent can indeed be extracted from the Loschmidt
echo routine. We also discuss possible values of experimental parameters
required for implementing this proposal
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