13,998 research outputs found
Subband Engineering Even-Denominator Quantum Hall States
Proposed even-denominator fractional quantum Hall effect (FQHE) states
suggest the possibility of excitations with non-Abelian braid statistics.
Recent experiments on wide square quantum wells observe even-denominator FQHE
even under electrostatic tilt. We theoretically analyze these structures and
develop a procedure to accurately test proposed quantum Hall wavefunctions. We
find that tilted wells favor partial subband polarization to yield Abelian
even-denominator states. Our results show that tilting quantum wells
effectively engineers different interaction potentials allowing exploration of
a wide variety of even-denominator states
Multi-domain active sound control and noise shielding
This paper describes an active sound control methodology based on difference potentials. The main feature of this methodology is its ability to automatically preserve âwantedâ sound within a domain while canceling âunwantedâ noise from outside the domain. This method of preservation of the wanted sounds by active shielding control is demonstrated with various broadband and realistic sound sources such as human voice and music in multiple domains in a one-dimensional enclosure. Unlike many other conventional active control methods, the proposed approach does not require the explicit characterization of the wanted sound to be preserved. The controls are designed based on the measurements of the total field on the boundaries of the shielded domain only, which is allowed to be multiply connected. The method is tested in a variety of experimental cases. The typical attenuation of the unwanted noise is found to be about 20 dB over a large area of the shielded domain and the original wanted sound field is preserved with errors of around 1 dB and below through a broad frequency range up to 1 kHz.
© 2011 Acoustical Society of Americ
The degradation of MgB2 under ambient environment
The superconductivities of samples prepared by several procedures were found
to degrade under ambient environment. The degradation mechanism was studied by
measuring the change of surface chemical composition of dense MgB2 pellets
(prepared by hot isostatic pressure, HIPed) under atmospheric exposure using
X-ray Photoelectron Spectroscopy (XPS). Results showed that samples with poor
connectivity between grains and with smaller grain sizes degrade with time when
exposed to ambient conditions. In these samples, the Tc did not change with
time, but the superconducting transition became broader and the Meissner
fraction decreased. In contrast, our well-sintered and the HIPed samples
remained stable for several months under ambient condition. The degradation was
found to be related to surface decomposition as observed by XPS. We observed
the formation of oxidized Mg, primarily in the form of a Mg hydroxide, the
increase of C and O contents, and the reduction of B concentration in the
surface layer of MgB2 samples.Comment: 15 pages, 3 figure
Mechanisms of two-color laser-induced field-free molecular orientation
Two mechanisms of two-color (\omega + 2\omega) laser-induced field-free
molecular orientation, based on the hyperpolarizability and ionization
depletion, are explored and compared. The CO molecule is used as a
computational example. While the hyperpolarizability mechanism generates small
amounts of orientation at intensities below the ionization threshold,
ionization depletion quickly becomes the dominant mechanism as soon as ionizing
intensities are reached. Only the ionization mechanism leads to substantial
orientation (e.g. on the order of || > 0.1). For intensities typical
of laser-induced molecular alignment and orientation experiments, the two
mechanism lead to robust, characteristic timings of the field-free orientation
wave-packet revivals relative to the the alignment revivals and the revival
time. The revival timings can be used to detect the active orientation
mechanism experimentally
Photoproduction and electroproduction of charm at high energies
We estimate the differential and total cross sections for both the
photoproduction of vector D*-meson and its yield in deep inelastic scattering
at the HERA collider in the framework of model motivated by perturbative
calculations in QCD. The offered model allows us to take into account higher
twists over the transverse momentum of meson at p_T ~ m_c and to correctly
approach the dominance of -quark fragmentation at p_T >> m_c. We consider a
possibility for the hadronization of color-octet c q-bar state into the meson.
The combined contribution by the singlet and octet-color terms results in a
good agreement with the experimental data for both the photoproduction and the
production in deep inelastic scattering.Comment: 27 pages, ReVTeX, 8 eps-figures, explanations added, results
unchange
On the stability of viscous free-surface flow supported by a rotating cylinder
Using an adaptive finite-element (FE) scheme developed recently by the authors, we shed new light on the long-standing fundamental problem of the unsteady free-surface Stokes flow exterior to a circular cylinder rotating about its horizontal axis in a vertical gravitational field. For supportable loads, we observe that the steady-state is more readily attained for near-maximal fluid loads on the cylinder than for significantly sub-maximal loads. For the latter, we investigate large-time dynamics by means of a finite-difference approximation to the thin-film equations, which is also used to validate the adaptive FE simulations (applied to the full Stokes equations) for these significantly sub-maximal loads.
Conversely, by comparing results of the two methods, we assess the validity of the thin-film approximation as either the load is increased or the rotation rate of the cylinder is decreased. Results are presented on the independent effects of gravity, surface tension and initial film thickness on the decay to steady-state. Finally, new numerical simulations of load shedding are presented
Diffractive charm photoproduction at HERA ep-collider
The cross section of the -meson diffractive photoproduction at the HERA
collider has been calculated in the framework of perturbatively motivated model
for the different kinematic regions. The camparison between the different
Pomeron models has been performed.Comment: 9 pages, 3 figure
Trehalose Is A Chemical Attractant In The Establishment Of Coral Symbiosis
Coral reefs have evolved with a crucial symbiosis between photosynthetic dinoflagellates (genus Symbiodinium) and their cnidarian hosts (Scleractinians). Most coral larvae take up Symbiodinium from their environment; however, the earliest steps in this process have been elusive. Here we demonstrate that the disaccharide trehalose may be an important signal from the symbiont to potential larval hosts. Symbiodinium freshly isolated from Fungia scutaria corals constantly released trehalose (but not sucrose, maltose or glucose) into seawater, and released glycerol only in the presence of coral tissue. Spawning Fungia adults increased symbiont number in their immediate area by excreting pellets of Symbiodinium, and when these naturally discharged Symbiodinium were cultured, they also released trehalose. In Y-maze experiments, coral larvae demonstrated chemoattractant and feeding behaviors only towards a chamber with trehalose or glycerol. Concomitantly, coral larvae and adult tissue, but not symbionts, had significant trehalase enzymatic activities, suggesting the capacity to utilize trehalose. Trehalase activity was developmentally regulated in F. scutaria larvae, rising as the time for symbiont uptake occurs. Consistent with the enzymatic assays, gene finding demonstrated the presence of a trehalase enzyme in the genome of a related coral, Acropora digitifera, and a likely trehalase in the transcriptome of F. scutaria. Taken together, these data suggest that adult F. scutaria seed the reef with Symbiodinium during spawning and the exuded Symbiodinium release trehalose into the environment, which acts as a chemoattractant for F. scutaria larvae and as an initiator of feeding behavior- the first stages toward establishing the coral-Symbiodinium relationship. Because trehalose is a fixed carbon compound, this cue would accurately demonstrate to the cnidarian larvae the photosynthetic ability of the potential symbiont in the ambient environment. To our knowledge, this is the first report of a chemical cue attracting the motile coral larvae to the symbiont
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