11,556 research outputs found
Density functional theory modeling of vortex shedding in superfluid He-4
Formation of vortex rings around moving spherical objects in superfluid He-4
at 0 K is modeled by time-dependent density functional theory. The simulations
provide detailed information of the microscopic events that lead to vortex ring
emission through characteristic observables such as liquid current circulation,
drag force, and hydrodynamic mass. A series of simulations were performed to
determine velocity thresholds for the onset of dissipation as a function of the
sphere radius up to 1.8 nm and at external pressures of zero and 1 bar. The
threshold was observed to decrease with the sphere radius and increase with
pressure thus showing that the onset of dissipation does not involve roton
emission events (Landau critical velocity), but rather vortex emission (Feynman
critical velocity), which is also confirmed by the observed periodic response
of the hydrodynamic observables as well as visualization of the liquid current
circulation. An empirical model, which considers the ratio between the boundary
layer kinetic and vortex ring formation energies, is presented for
extrapolating the current results to larger length scales. The calculated
critical velocity value at zero pressure for a sphere that mimics an electron
bubble is in good agreement with the previous experimental observations at low
temperatures. The stability of the system against symmetry breaking was linked
to its ability to excite quantized Kelvin waves around the vortex rings during
the vortex shedding process. At high vortex ring emission rates, the downstream
dynamics showed complex vortex ring fission and reconnection events that appear
similar to those seen in previous Gross-Pitaevskii theory-based calculations,
and which mark the onset of turbulent behavior.Comment: 23 pages, 7 figure
Optical signature of the pressure-induced dimerization in the honeycomb iridate -LiIrO
We studied the effect of external pressure on the electrodynamic properties
of -LiIrO single crystals in the frequency range of the phonon
modes and the Ir - transitions. The abrupt hardening of several phonon
modes under pressure supports the onset of the dimerized phase at the critical
pressure =3.8 GPa. With increasing pressure an overall decrease in
spectral weight of the Ir - transitions is found up to . Above
, the local (on-site) - excitations gain spectral weight with
increasing pressure, which hints at a pressure-induced increase in the
octahedral distortions. The non-local (intersite) Ir - transitions show a
monotonic blue-shift and decrease in spectral weight. The changes observed for
the non-local excitations are most prominent well above , namely for
pressures 12 GPa, and only small changes occur for pressures close to
. The profile of the optical conductivity at high pressures (20 GPa)
appears to be indicative for the dimerized state in iridates.Comment: 10 pages, 6 figures; accepted for publication in Phys. Rev.
Fallstudie Axa Winterthur : intelligenter Distributionsanfrageprozess
Mitarbeitende in Vertrieb und Underwriting von zeitraubenden Routineaufgaben zu entlasten, damit sie sich auf ihre Kunden und fachliche Aspekte konzentrieren können, war die Zielsetzung des Projekts der Axa Winterthur, das in der vorliegenden Fallstudie beschreiben wird. Mit der Standardisierung des Distributionsanfrageprozesses und der Einführung einer Workflowlösung inklusive Business Rules konnte eine flexible Lösung geschaffen werden, die sich nicht nur auf andere Geschäftsbereiche übertragen lässt, sondern sich durch die zentrale Wissensbasis und auswertbare operative Daten weiter in Richtung eines entscheidungsunterstützenden und lernenden Systems entwickeln kann
Drip and Mate Operations Acting in Test Tube Systems and Tissue-like P systems
The operations drip and mate considered in (mem)brane computing resemble the
operations cut and recombination well known from DNA computing. We here
consider sets of vesicles with multisets of objects on their outside membrane
interacting by drip and mate in two different setups: in test tube systems, the
vesicles may pass from one tube to another one provided they fulfill specific
constraints; in tissue-like P systems, the vesicles are immediately passed to
specified cells after having undergone a drip or mate operation. In both
variants, computational completeness can be obtained, yet with different
constraints for the drip and mate operations
Clathrate type 2 hydrate formation in vacuo under astrophysical conditions
The properties of clathrate hydrates were used to explain the complex and poorly understood physical processes taking place within cometary nuclei and other icy solar system bodies. Most of all the experiments previously conducted used starting compositions which would yield clathrate types I hydrates. The main criterion for type I vs. type II clathrate hydrate formation is the size of the guest molecule. The stoichiometry of the two structure types is also quite different. In addition, the larger molecules which would form type II clathrate hydrates typically have lower vapor pressures. The result of these considerations is that at temperatures where we identified clathrate formation (120-130 K), it is more likely that type II clathrate hydrates will form. We also formed clathrate II hydrates of methanol by direct vapor deposition in the temperature range 125-135 K
High-pressure versus isoelectronic doping effect on the honeycomb iridate NaIrO
We study the effect of isoelectronic doping and external pressure in tuning
the ground state of the honeycomb iridate NaIrO by combining optical
spectroscopy with synchrotron x-ray diffraction measurements on single
crystals. The obtained optical conductivity of NaIrO is discussed in
terms of a Mott insulating picture versus the formation of quasimolecular
orbitals and in terms of Kitaev-interactions. With increasing Li content ,
(NaLi)IrO moves deeper into the Mott insulating regime and
there are indications that up to a doping level of 24\% the compound comes
closer to the Kitaev-limit. The optical conductivity spectrum of single
crystalline -LiIrO does not follow the trends observed for the
series up to . There are strong indications that -LiIrO
is less close to the Kitaev-limit compared to NaIrO and closer to the
quasimolecular orbital picture. Except for the pressure-induced hardening of
the phonon modes, the optical properties of NaIrO seem to be robust
against external pressure. Possible explanations of the unexpected evolution of
the optical conductivity with isolectronic doping and the drastic change
between and are given by comparing the pressure-induced changes
of lattice parameters and the optical conductivity with the corresponding
changes induced by doping.Comment: 12 pages, 6 figures, accepted for publication in Phys. Rev.
A Multivariate Training Technique with Event Reweighting
An event reweighting technique incorporated in multivariate training
algorithm has been developed and tested using the Artificial Neural Networks
(ANN) and Boosted Decision Trees (BDT). The event reweighting training are
compared to that of the conventional equal event weighting based on the ANN and
the BDT performance. The comparison is performed in the context of the physics
analysis of the ATLAS experiment at the Large Hadron Collider (LHC), which will
explore the fundamental nature of matter and the basic forces that shape our
universe. We demonstrate that the event reweighting technique provides an
unbiased method of multivariate training for event pattern recognition.Comment: 20 pages, 8 figure
Finite-distance singularities in the tearing of thin sheets
We investigate the interaction between two cracks propagating in a thin
sheet. Two different experimental geometries allow us to tear sheets by
imposing an out-of-plane shear loading. We find that two tears converge along
self-similar paths and annihilate each other. These finite-distance
singularities display geometry-dependent similarity exponents, which we
retrieve using scaling arguments based on a balance between the stretching and
the bending of the sheet close to the tips of the cracks.Comment: 4 pages, 4 figure
A q-deformation of the Coulomb problem
The algebra of observables of SO_{q}(3)-symmetric quantum mechanics is extended to include the inverse \frac{1}{R} of the radial coordinate and used to obtain eigenvalues and eigenfunctions of a \q-deformed Coulomb Hamiltonian
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