336 research outputs found
Quantum Markovian activated surface diffusion of interacting adsorbates
A quantum Markovian activated atom-surface diffusion model with interacting
adsorbates is proposed for the intermediate scattering function, which is shown
to be complex-valued and factorizable into a classical-like and a
quantum-mechanical factor. Applications to the diffusion of Na atoms on flat
(weakly corrugated) and corrugated-Cu(001) surfaces at different coverages and
surface temperatures are analyzed. Quantum effects are relevant to diffusion at
low surface temperatures and coverages even for relatively heavy particles,
such as Na atoms, where transport by tunneling is absent.Comment: 6 pages, 4 figure
A causal look into the quantum Talbot effect
A well-known phenomenon in both optics and quantum mechanics is the so-called
Talbot effect. This near field interference effect arises when infinitely
periodic diffracting structures or gratings are illuminated by highly coherent
light or particle beams. Typical diffraction patterns known as quantum carpets
are then observed. Here the authors provide an insightful picture of this
nonlocal phenomenon as well as its classical limit in terms of Bohmian
mechanics, also showing the causal reasons and conditions that explain its
appearance. As an illustration, theoretical results obtained from diffraction
of thermal He atoms by both N-slit arrays and weak corrugated surfaces are
analyzed and discussed. Moreover, the authors also explain in terms of what
they call the Talbot-Beeby effect how realistic interaction potentials induce
shifts and distortions in the corresponding quantum carpets.Comment: 12 pages, 6 figure
Quasi-elastic peak lineshapes in adsorbate diffusion on nearly flat surfaces at low coverages: the motional narrowing effect in Xe on Pt(111)
Quasi-elastic helium atom scattering measurements have provided clear
evidence for a two-dimensional free gas of Xe atoms on Pt(111) at low
coverages. Increasing the friction due to the surface, a gradual change of the
shape of the quasi-elastic peak is predicted and analyzed for this system in
terms of the so-called motional narrowing effect. The type of analysis
presented here for the quasi-elastic peak should be prior to any deconvolution
procedure carried out in order to better extract information from the process,
e.g. diffusion coefficients and jump distributions. Moreover, this analysis
also provides conditions for the free gas regime different than those reported
earlier.Comment: 12 pages, 4 figures (revised version
Hydrodynamic View of Wave-Packet Interference: Quantum Caves
Wave-packet interference is investigated within the complex quantum
Hamilton-Jacobi formalism using a hydrodynamic description. Quantum
interference leads to the formation of the topological structure of quantum
caves in space-time Argand plots. These caves consist of the vortical and
stagnation tubes originating from the isosurfaces of the amplitude of the wave
function and its first derivative. Complex quantum trajectories display
counterclockwise helical wrapping around the stagnation tubes and hyperbolic
deflection near the vortical tubes. The string of alternating stagnation and
vortical tubes is sufficient to generate divergent trajectories. Moreover, the
average wrapping time for trajectories and the rotational rate of the nodal
line in the complex plane can be used to define the lifetime for interference
features.Comment: 4 pages, 3 figures (major revisions with respect to the previous
version have been carried out
Corona-Australis DANCe I. Revisiting the census of stars with Gaia-DR2 data
Context. Corona-Australis is one of the nearest regions to the Sun with recent and ongoing star formation, but the current picture of its stellar (and substellar) content is not complete yet.
Aims. We take advantage of the second data release of the Gaia space mission to revisit the stellar census and search for additional members of the young stellar association in Corona-Australis.
Methods. We applied a probabilistic method to infer membership probabilities based on a multidimensional astrometric and photometric data set over a field of 128 deg(2) around the dark clouds of the region.
Results. We identify 313 high-probability candidate members to the Corona-Australis association, 262 of which had never been reported as members before. Our sample of members covers the magnitude range between G greater than or similar to 5 mag and G less than or similar to 20 mag, and it reveals the existence of two kinematically and spatially distinct subgroups. There is a distributed "off-cloud" population of stars located in the north of the dark clouds that is twice as numerous as the historically known "on-cloud" population that is concentrated around the densest cores. By comparing the location of the stars in the HR-diagram with evolutionary models, we show that these two populations are younger than 10 Myr. Based on their infrared excess emission, we identify 28 Class II and 215 Class III stars among the sources with available infrared photometry, and we conclude that the frequency of Class II stars (i.e. "disc-bearing" stars) in the on-cloud region is twice as large as compared to the off-cloud population. The distance derived for the Corona-Australis region based on this updated census is d = 149.4(-0.4)(+0.4) pc, which exceeds previous estimates by about 20 pc.
Conclusions. In this paper we provide the most complete census of stars in Corona-Australis available to date that can be confirmed with Gaia data. Furthermore, we report on the discovery of an extended and more evolved population of young stars beyond the region of the dark clouds, which was extensively surveyed in the past
Quantum Zeno and anti-Zeno effects in surface diffusion of interacting adsorbates
Surface diffusion of interacting adsorbates is here analyzed within the
context of two fundamental phenomena of quantum dynamics, namely the quantum
Zeno effect and the anti-Zeno effect. The physical implications of these
effects are introduced here in a rather simple and general manner within the
framework of non-selective measurements and for two (surface) temperature
regimes: high and very low (including zero temperature). The quantum
intermediate scattering function describing the adsorbate diffusion process is
then evaluated for flat surfaces, since it is fully analytical in this case.
Finally, a generalization to corrugated surfaces is also discussed. In this
regard, it is found that, considering a Markovian framework and high surface
temperatures, the anti-Zeno effect has already been observed, though not
recognized as such.Comment: 17 pages, 1 figur
Line Shape Broadening in Surface Diffusion of Interacting Adsorbates with Quasielastic He Atom Scattering
The experimental line shape broadening observed in adsorbate diffusion on
metal surfaces with increasing coverage is usually related to the nature of the
adsorbate-adsorbate interaction. Here we show that this broadening can also be
understood in terms of a fully stochastic model just considering two noise
sources: (i) a Gaussian white noise accounting for the surface friction, and
(ii) a shot noise replacing the physical adsorbate-adsorbate interaction
potential. Furthermore, contrary to what could be expected, for relatively weak
adsorbate-substrate interactions the opposite effect is predicted: line shapes
get narrower with increasing coverage.Comment: 4 pages, 2 figures (slightly revised version
Origin of the Electron-Phonon Interaction of Topological Semimetal Surfaces Measured with Helium Atom Scattering
He atom scattering has been demonstrated to be a sensitive probe of the
electron-phonon interaction parameter at metal and metal-overlayer
surfaces. Here it is shown that the theory linking to the thermal
attenuation of atom scattering spectra (the Debye-Waller factor), can be
applied to topological semimetal surfaces, like the quasi-one dimensional
charge-density-wave system Bi(114) and the layered pnictogen chalcogenides
Setting up tunneling conditions by means of Bohmian mechanics
Usually tunneling is established after imposing some matching conditions on
the (time-independent) wave function and its first derivative at the boundaries
of a barrier. Here an alternative scheme is proposed to determine tunneling and
estimate transmission probabilities in time-dependent problems, which takes
advantage of the trajectory picture provided by Bohmian mechanics. From this
theory a general functional expression for the transmission probability in
terms of the system initial state can be reached. This expression is used here
to analyze tunneling properties and estimate transmissions in the case of
initial Gaussian wave packets colliding with ramp-like barriers.Comment: 18 pages, 4 figure
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