3,266 research outputs found
Behavior of self-propelled acetone droplets in a Leidenfrost state on liquid substrates
It is demonstrated that non-coalescent droplets of acetone can be formed on
liquid substrates. The fluid flows around and in an acetone droplet hovering on
water are recorded to shed light on the mechanisms which might lead to
non-coalescence. For sufficiently low impact velocities, droplets undergo a
damped oscillation on the surface of the liquid substrate but at higher
velocities clean bounce-off occurs. Comparisons of experimentally observed
static configurations of floating droplets to predictions from a theoretical
model for a small non-wetting rigid sphere resting on a liquid substrate are
made and a tentative strategy for determining the thickness of the vapor layer
under a small droplet on a liquid is proposed. This strategy is based on the
notion of effective surface tension. The droplets show self-propulsion in
straight line trajectories in a manner which can be ascribed to a Marangoni
effect. Surprisingly, self-propelled droplets can become immersed beneath the
undisturbed water surface. This phenomenon is reasoned to be drag-inducing and
might provide a basis for refining observations in previous work
Absolute photoionization cross section measurements of the Kr I-isoelectronic sequence
Photoionization spectra have been recorded in the 4s, 4p and 3d resonance regions for the Kr Iisoelectronic sequence using both the dual laser produced plasma technique (at DCU) to produce photoabsorption spectra, and the merged ion beam and synchrotron radiation technique (at ASTRID) to measure absolute photoionization cross sections. Profile parameters are compared for the 4s â np resonances of Rb+ and Sr2+. Many new 4p " ns, md transitions are identified with the aid of Hartree-Fock calculations, and consistent quantum defects are observed for the various ns and md Rydberg series. Absolute single and double photoionization cross sections recorded in the 3d region for Rb+ and Sr2+ ions show preferential decay via double photoionization. This is only the second report where both the DLP technique and the merged beam technique have been used simultaneously to record photoionization spectra, and the advantages of both techniques (i.e. better resolution in the case of DLP and values for absolute photoionization cross sections in the case of the merged beam technique) are highlighted
Charge state of vacancy defects in Eu-doped GaN
Eu ions have been doped into GaN in order to achieve red luminescence under current injection, where coupling between the Eu ions and intrinsic defects such as vacancies are known to play an important role. However, the charge state of the vacancies and the impact it would have on the optical and magnetic properties of the Eu ions have not been explored. Through a combination of first-principle calculations and experimental results, the influence of the charge state of the defect environment surrounding the Eu ions has been investigated. We have identified two Eu centers that are related through the charge state of a local vacancy defect. These two centers were found to exhibit a mutual metastability, such that each center can be excited in one configuration and emit as the other. This metastability was found to be dependent on temperature and the wavelength of the excitation laser. Furthermore, one of these centers was found to have an effective magnetic g factor that is substantially larger than what is expected for an isolated Eu3+ ion and is explained by a change in the charge state of the defect environment around the Eu. This prediction could also offer a new explanation for the saturation magnetization previously observed in GaN : Eu and other GaN:RE systems
On the Formation of Copper Linear Atomic Suspended Chains
We report high resolution transmission electron microscopy and classical
molecular dynamics simulation results of mechanically stretching copper
nanowires conducting to linear atomic suspended chains (LACs) formation. In
contrast with some previous experimental and theoretical work in literature
that stated that the formation of LACs for copper should not exist our results
showed the existence of LAC for the [111], [110], and [100] crystallographic
directions, being thus the sequence of most probable occurence.Comment: 4 pages, 3 figure
Efficient pairing computation with theta functions
The original publication is available at www.springerlink.comInternational audienceIn this paper, we present a new approach based on theta functions to compute Weil and Tate pairings. A benefit of our method, which does not rely on the classical Miller's algorithm, is its generality since it extends to all abelian varieties the classical Weil and Tate pairing formulas. In the case of dimension and abelian varieties our algorithms lead to implementations which are efficient and naturally deterministic. We also introduce symmetric Weil and Tate pairings on Kummer varieties and explain how to compute them efficiently. We exhibit a nice algorithmic compatibility between some algebraic groups quotiented by the action of the automorphism , where the -action can be computed efficiently with a Montgomery ladder type algorithm
Weak-Coupling Theory for Multiband Superconductivity Induced by Jahn-Teller Phonons
Emergence of superconductivity in a two-band system coupled with breathing
and Jahn-Teller phonons is discussed in a weak-coupling limit. With the use of
a standard quantum mechanical procedure, the phonon-mediated attraction is
derived. From the analysis of the model including such attraction, a BCS-like
formula for a superconducting transition temperature is obtained.
When only the breathing phonon is considered, is the same as that
of the one-band model. On the other hand, when Jahn-Teller phonons are active,
is significantly enhanced by the interband attraction even within
the weak-coupling limit. Relevance of the present result to actual materials
such as iron pnictides is briefly commented.Comment: 4 pages, 3 figures
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