214 research outputs found

### Density pattern in supercritical flow of liquid He-4

A density functional theory is used to investigate the instability arising in
superfluid $^4$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.

### Probing vortices in 4He nanodroplets

We present static and dynamical properties of linear vortices in 4He droplets
obtained from Density Functional calculations. By comparing the adsorption
properties of different atomic impurities embedded in pure droplets and in
droplets where a quantized vortex has been created, we suggest that Ca atoms
should be the dopant of choice to detect vortices by means of spectroscopic
experiments.Comment: Typeset using Revtex4, 4 pages and 2 Postscript file

### Anomalous low temperature specific heat of He-3 inside nanotube bundles

Helium atoms and hydrogen molecules can be strongly bound inside interstitial
channels within bundles of carbon nanotubes. An exploration of the low energy
and low temperature properties of He-3 atoms is presented here. Recent study of
the analogous He-4 system has shown that the effect of heterogeneity is to
yield a density of states N(E) that is qualitatively different from the
one-dimensional (1D) form of N(E) that would occur for an ideal set of
identical channels. In particular, the functional form of N(E) is that of a 4D
gas near the very lowest energies and a 2D gas at somewhat higher energies.
Similar behavior is found here for He-3. The resulting thermodynamic behavior
of this fermi system is computed, yielding an anomalous form of the heat
capacity and its dependence on coverage.Comment: 11 pages, 6 figure

### 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

### Alkali Atoms Attached to $^3$He Nanodroplets

We have experimentally studied the electronic $3p-> 3s$ excitation of Na
atoms attached to $^3$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 $^4$He droplets, we
conclude that sodium atoms reside in a ``dimple'' on the droplet surface and
that superfluid-related effects are negligible. The experimental results are
supported by Density Functional calculations at zero temperature, which confirm
the surface location of Na, K and Rb atoms on $^3$He droplets. In the case of
Na, the calculated shift of the excitation spectra for the two isotopes is in
good agreement with the experimental data.Comment: 6 pages, 3 figures, sent to JLT

### Shock waves in strongly interacting Fermi gas from time-dependent density functional calculations

Motivated by a recent experiment [Phys. Rev. Lett. 106, 150401 (2011)] we
simulate the collision between two clouds of cold Fermi gas at unitarity
conditions by using an extended Thomas-Fermi density functional. At variance
with the current interpretation of the experiments, where the role of viscosity
is emphasized, we find that a quantitative agreement with the experimental
observation of the dynamics of the cloud collisions is obtained within our
superfluid effective hydrodynamics approach, where density variations during
the collision are controlled by a purely dispersive quantum gradient term. We
also suggest different initial conditions where dispersive density ripples can
be detected with the available experimental spatial resolution.Comment: 5 pages, 4 figures, to be published in 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

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