180 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
Density pattern in supercritical flow of liquid He-4
A density functional theory is used to investigate the instability arising in
superfluid He as it flows at velocity u just above the Landau critical
velocity of rotons v_c. Confirming an early theoretical prediction by one of us
[JETP Lett. 39, 511 (1984)], we find that a stationary periodic modulation of
the density occurs, with amplitude proportional to (u-v_c)^{1/2} and wave
vector equal to the roton wave vector. This density pattern is studied for
supercritical flow both in bulk helium and in a channel of nanometer
cross-section.Comment: 4 pages, 6 figures. Submitted to Phys. Rev.
DC Josephson Effect with Fermi gases in the Bose-Einstein regime
We show that the DC Josephson effect with ultracold fermionic gases in the
BEC regime of composite molecules can be described by a nonlinear Schrodinger
equation (NLSE). By comparing our results with Bogoliubov-de Gennes
calculations [Phys. Rev. Lett. 99, 040401 (2007)] we find that our superfluid
NLSE, which generalizes the Gross-Pitaevskii equation taking into account the
correct equation of state, is reliable in the BEC regime of the BCS-BEC
crossover up to the limit of very large (positive) scattering length. We also
predict that the Josephson current displays relevant beyond mean-field effects.Comment: 9 pages, 6 figures. In the new version added one figure and some
paragraph
Surface location of sodium atoms attached to He-3 nanodroplets
We have experimentally studied the electronic excitation of
Na atoms attached to He droplets by means of laser-induced fluorescence as
well as beam depletion spectroscopy. From the similarities of the spectra
(width/shift of absorption lines) with these of Na on He droplets, we
conclude that sodium atoms reside in a ``dimple'' on the droplet surface. The
experimental results are supported by Density Functional calculations at zero
temperature, which confirm the surface location of sodium on He droplets,
and provide a microscopic description of the ``dimple'' structure.Comment: 4 pages, 5 figure
Freezing of He-4 and its liquid-solid interface from Density Functional Theory
We show that, at high densities, fully variational solutions of solid-like
type can be obtained from a density functional formalism originally designed
for liquid 4He. Motivated by this finding, we propose an extension of the
method that accurately describes the solid phase and the freezing transition of
liquid 4He at zero temperature. The density profile of the interface between
liquid and the (0001) surface of the 4He crystal is also investigated, and its
surface energy evaluated. The interfacial tension is found to be in
semiquantitative agreement with experiments and with other microscopic
calculations. This opens the possibility to use unbiased DF methods to study
highly non-homogeneous systems, like 4He interacting with strongly attractive
impurities/substrates, or the nucleation of the solid phase in the metastable
liquid.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Dynamics of liquid He-4 in confined geometries from Time-Dependent Density Functional calculations
We present numerical results obtained from Time-Dependent Density Functional
calculations of the dynamics of liquid He-4 in different environments
characterized by geometrical confinement. The time-dependent density profile
and velocity field of He-4 are obtained by means of direct numerical
integration of the non-linear Schrodinger equation associated with a
phenomenological energy functional which describes accurately both the static
and dynamic properties of bulk liquid He-4. Our implementation allows for a
general solution in 3-D (i.e. no symmetries are assumed in order to simplify
the calculations). We apply our method to study the real-time dynamics of pure
and alkali-doped clusters, of a monolayer film on a weakly attractive surface
and a nano-droplet spreading on a solid surface.Comment: q 1 tex file + 9 Ps figure
Simple Model of Capillary Condensation in porous media
We employ a simple model to describe the phase behavior of 4He and Ar in a
hypothetical porous material consisting of a regular array of infinitely long,
solid, parallel cylinders. We find that high porosity geometries exhibit two
transitions: from vapor to film and from film to capillary condensed liquid. At
low porosity, the film is replaced by a ``necking'' configuration, and for a
range of intermediate porosity there are three transitions: from vapor to film,
from film to necking and from necking to a capillary condensed phase.Comment: 14 pages, 7 figure
Dynamical brittle fractures of nanocrystalline silicon using large-scale electronic structure calculations
A hybrid scheme between large-scale electronic structure calculations is
developed and applied to nanocrystalline silicon with more than 10 atoms.
Dynamical fracture processes are simulated under external loads in the [001]
direction. We shows that the fracture propagates anisotropically on the (001)
plane and reconstructed surfaces appear with asymmetric dimers. Step structures
are formed in larger systems, which is understood as the beginning of a
crossover between nanoscale and macroscale samples.Comment: 10 pages, 4 figure
Dynamical-charge neutrality at a crystal surface
For both molecules and periodic solids, the ionic dynamical charge tensors
which govern the infrared activity are known to obey a dynamical neutrality
condition. This condition enforces their sum to vanish (over the whole finite
system, or over the crystal cell, respectively). We extend this sum rule to the
non trivial case of the surface of a semiinfinite solid and show that, in the
case of a polar surface of an insulator, the surface ions cannot have the same
dynamical charges as in the bulk. The sum rule is demonstrated through
calculations for the Si-terminated SiC(001) surface.Comment: 4 pages, latex file, 1 postscript figure automatically include
(Meta-)stable reconstructions of the diamond(111) surface: interplay between diamond- and graphite-like bonding
Off-lattice Grand Canonical Monte Carlo simulations of the clean diamond
(111) surface, based on the effective many-body Brenner potential, yield the
Pandey reconstruction in agreement with \emph{ab-initio}
calculations and predict the existence of new meta-stable states, very near in
energy, with all surface atoms in three-fold graphite-like bonding. We believe
that the long-standing debate on the structural and electronic properties of
this surface could be solved by considering this type of carbon-specific
configurations.Comment: 4 pages + 4 figures, Phys. Rev. B Rapid Comm., in press (15Apr00).
For many additional details (animations, xyz files) see electronic supplement
to this paper at http://www.sci.kun.nl/tvs/carbon/meta.htm
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