150 research outputs found
Spinning superfluid helium-4 nanodroplets
We have studied spinning superfluid He nanodroplets at zero temperature
using Density Functional theory. Due to the irrotational character of the
superfluid flow, the shapes of the spinning nanodroplets are very different
from those of a viscous normal fluid drop in steady rotation. We show that when
vortices are nucleated inside the superfluid droplets, their morphology, which
evolves from axisymmetric oblate to triaxial prolate to two-lobed shapes, is in
good agreement with experiments. The presence of vortex arrays confers to the
superfluid droplets the rigid-body behavior of a normal fluid in steady
rotation, and this is the ultimate reason of the surprising good agreement
between recent experiments and the classical models used for their description.Comment: 5 pages, 3 figur
Vortex arrays in nanoscopic superfluid helium droplets
We have studied the appearance of vortex arrays in a rotating helium-4
nanodroplet at zero temperature within density functional theory. Our results
are compared with those for classical rotating fluid drops used to analyze the
shape and vorticity in recent experiments [L.F. Gomez et al., Science 345, 906
(2014)], where vortices have been directly seen in superfluid droplets for the
first time. In agreement with the experiments, we have found that the shape of
the droplet changes from pseudo-spheroid, oblate-like for a small number of
vortices to a peculiar "wheel-like" shape, delimited by nearly flat upper and
lower surfaces, when the number of vortices is large. Also in agreement with
the experiments, we have found that the droplet remains stable well above the
stability limit predicted by classical theories.Comment: 5 pages, 5 figure
The onset of nanoscale dissipation in superfluid He-4 at zero temperature: the role of vortex shedding and cavitation
Two-dimensional flow past an infinitely long cylinder of nanoscopic radius in
superfluid He-4 at zero temperature is studied by time-dependent density
functional theory. The calculations reveal two distinct critical phenomena for
the onset of dissipation: 1) vortex-antivortex pair shedding from the periphery
of the moving cylinder and 2) appearance of cavitation in the wake, which
possesses similar geometry as observed experimentally for fast moving
micrometer-scale particles in superfluid He-4. Vortex pairs with the same
circulation are occasionally emitted in the form of dimers, which constitute
the building blocks for the Benard-von Karman vortex street structure observed
in classical turbulent fluids and Bose-Einstein condensates. The cavitation
induced dissipation mechanism should be common to all superfluids that are
self-bound and have a finite surface tension, which include the recently
discovered self-bound droplets in ultracold Bose gases.Comment: 5 pages, 6 figure
Vertically coupled quantum dots in the local spin-density functional theory
We have investigated the structure of double quantum dots vertically coupled
at zero magnetic field within local spin-density functional theory. The dots
are identical and have a finite width, and the whole system is axially
symmetric. We first discuss the effect of thickness on the addition spectrum of
one single dot. Next, we describe the structure of coupled dots as a function
of the interdot distance for different electron numbers. Addition spectra,
Hund's rule and molecular-type configurations are discussed. It is shown that
self-interaction corrections to the density functional results do not play a
very important role in the calculated addition spectra.Comment: Typeset using Revtex, 14 pages and 12 Postscript figures, to be
published in Phys. Rev.
Far-infrared spectroscopy of nanoscopic InAs rings
We have employed time-dependent local-spin density theory to analyze the
far-infrared transmission spectrum of InAs self-assembled nano-rings recently
reported [A. Lorke et al, cond-mat/9908263 (1999)]. The overall agreement
between theory and experiment is good, which on the one hand confirms that the
experimental peaks indeed reflect the ring-like structure of the sample, and on
the other hand, asseses the suitability of the theoretical method to describe
such small nanostructures. The addition energies of one- and two-electron rings
are also reported and compared with the corresponding capacitance spectra.Comment: Typeset using Revtex, 7 pages and 8 Postscript figure
Wave-vector dependence of spin and density multipole excitations in quantum dots
We have employed time-dependent local-spin density functional theory to
analyze the multipole spin and charge density excitations in GaAs-AlGaAs
quantum dots. The on-plane transferred momentum degree of freedom has been
taken into account, and the wave-vector dependence of the excitations is
discussed. In agreement with previous experiments, we have found that the
energies of these modes do not depend on the transferred wave-vector, although
their intensities do. Comparison with a recent resonant Raman scattering
experiment [C. Sch\"uller et al, Phys. Rev. Lett {\bf 80}, 2673 (1998)] is
made. This allows to identify the angular momentum of several of the observed
modes as well as to reproduce their energies.Comment: 14 pages in REVTEX and 14 postscript figure
Wave-vector dependence of spin and density multipole excitations in quantum dots
We have employed time-dependent local-spin density functional theory to
analyze the multipole spin and charge density excitations in GaAs-AlGaAs
quantum dots. The on-plane transferred momentum degree of freedom has been
taken into account, and the wave-vector dependence of the excitations is
discussed. In agreement with previous experiments, we have found that the
energies of these modes do not depend on the transferred wave-vector, although
their intensities do. Comparison with a recent resonant Raman scattering
experiment [C. Sch\"uller et al, Phys. Rev. Lett {\bf 80}, 2673 (1998)] is
made. This allows to identify the angular momentum of several of the observed
modes as well as to reproduce their energies.Comment: 14 pages in REVTEX and 14 postscript figure
Vortex arrays in a rotating superfluid He-4 nanocylinder
Within Density Functional theory, we investigate stationary many-vortex
structures in a rotating He nanocylinder at zero temperature. We compute
the stability diagram and compare our results with the classical model of
vortical lines in an inviscid and incompressible fluid. Scaling the results to
millimeter-size buckets, they can be compared with experiments on vortex arrays
conducted in the past. Motivated by recent experiments that have used atomic
impurities as a means of visualizing vortices in superfluid He droplets, we
have also considered the formation of chains of xenon atoms along a vortex line
and the interaction between xenon atoms inside the same vortex and on different
neighboring vortex lines
Breakup of quantum liquid filaments into droplets
We have investigated how the Rayleigh-Plateau instability of a filament made
of a 41K-87Rb self-bound mixture may lead to an array of identical quantum
droplets, with typical breaking times which are shorter than the lifetime of
the mixture. If the filament is laterally confined -- as it happens in a
toroidal trap -- and atoms of one species are in excess with respect to the
optimal, equilibrium ratio, the droplets are immersed into a superfluid
background made by the excess species which provides global phase coherence to
the system, suggesting that the droplets array in the unbalanced system may
display supersolid character. This possibility has been investigated by
computing the non-classical translational inertia coefficient. The filament may
be a reasonable representation of a self-bound mixture subject to toroidal
confinement when the bigger circle radius of the torus is much larger than the
filament radius.Comment: 11 pages, 8 figure
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