206 research outputs found

### Kohn-Sham theory of rotating dipolar Fermi gas in two dimensions

A two-dimensional dipolar Fermi gas in harmonic trap under rotation is
studied by solving "ab initio" Kohn-Sham equations. The physical parameters
used match those of ultracold gas of fermionic $^{23}Na^{40}K$ molecules, a
prototype system of strongly interacting dipolar quantum matter, which has been
created very recently. We find that, as the critical rotational frequency is
approached and the system collapses into the lowest Landau level, an array of
tightly packed quantum vortices develops, in spite of the non-superfluid
character of the system. In this state the system looses axial symmetry, and
the fermionic cloud boundaries assume an almost perfect square shape. At higher
values of the filling factor the vortex lattice disappears, while the system
still exhibits square-shaped boundaries. At lower values of the filling factor
the fermions become instead localized in a "Wigner cluster" structure.Comment: 5 pages, 4 figure

### Kohn-Sham approach to Fermi gas superfluidity: the bilayer of fermionic polar molecules

By using a well established 'ab initio' theoretical approach developed in the
past to quantitatively study the superconductivity of condensed matter systems,
which is based on the Kohn-Sham Density Functional theory, I study the
superfluid properties and the BCS-BEC crossover of two parallel bi-dimensional
layers of fermionic dipolar molecules, where the pairing mechanism leading to
superfluidity is provided by the inter-layer coupling between dipoles. The
finite temperature superfluid properties of both the homogeneous system and one
were the fermions in each layer are confined by a square optical lattice are
studied at half filling conditions, and for different values of the strength of
the confining optical potential. The T=0 results for the homogeneous system are
found to be in excellent agreement with Diffusion Monte Carlo results. The
superfluid transition temperature in the BCS region is found to increase, for a
given inter-layer coupling, with the strength of the confining optical
potential. A transition occurs at sufficiently small interlayer distances,
where the fermions becomes localized within the optical lattice sites in a
square geometry with an increased effective lattice constant, forming a system
of localized composite bosons. This transition should be signalled by a sudden
drop in the superfluid fraction of the system.Comment: 10 pages, 10 figures (accepted for publication in Phys. Rev. A

### Dipolar Bose gas in highly anharmonic traps

By means of mean-field theory, we have studied the structure and excitation
spectrum of a purely dipolar Bose gas in pancake-shaped trap where the
confinement in the x-y plane is provided by a highly anharmonic potential
resulting in an almost uniform confinement in the plane. We show that the
stable condensates is characterized by marked radially structured density
profiles. The stability diagram is calculated by independently varying the
strength of the interaction and the trap geometry. By computing the Bogoliubov
excitation spectrum near the instability line we show that soft "angular"
rotons are responsible for the collapse of the system. The free expansion of
the cloud after the trap is released is also studied by means of time-dependent
calculations, showing that a prolate, cigar-shaped condensate is dynamically
stabilized during the expansion, which would otherwise collapse. Dipolar
condensates rotating with sufficiently high angular velocity show the formation
of multiply-quantized giant vortices, while the condensates acquire a
ring-shaped form.Comment: 9 pages, 10 figures. Submitted to Phys. Rev.

### Superfluid behavior of quasi-1D p-H$_2$ inside carbon nanotube

We perform ab-initio Quantum Monte Carlo simulations of para-hydrogen
(pH$_2$) at $T=0$ K confined in carbon nanotubes (CNT) of different radii. The
radial density profiles show a strong layering of the pH$_2$ molecules which
grow, with increasing number of molecules, in solid concentric cylindrical
shells and eventually a central column. The central column can be considered an
effective one-dimensional (1D) fluid whose properties are well captured by the
Tomonaga-Luttinger liquid theory. The Luttinger parameter is explicitly
computed and interestingly it shows a non-monotonic behavior with the linear
density similar to what found for pure 1D $^3$He. Remarkably, for the central
column in a (10,10) CNT, we found an ample linear density range in which the
Luttinger liquid is (i) superfluid and (ii) stable against a weak disordered
external potential, as the one expected inside realistic pores. This superfluid
behavior could be experimentally revealed in bundles of carbon nanotubes, where
deviations from classical inertial values associated with superfluid density
could be measured via torsional oscillator techniques. In summary, our results
suggest that pH$_2$ within carbon nanopores could be a practical realization of
the long sought-after, elusive superfluid phase of parahydrogen.Comment: 5 pages, 3 figures accepted as PRB rapi

### Supersolid behaviour of a dipolar Bose-Einstein condensate confined in a tube

Motivated by a recent experiment [L.Chomaz et al., Nature Physics 14, 442
(2018)], we perform numerical simulations of a dipolar Bose-Einstein Condensate
(BEC) in a tubular confinement at T=0 within Density Functional Theory, where
the beyond-mean-field correction to the ground state energy is included in the
Local Density Approximation. We study the excitation spectrum of the system by
solving the corresponding Bogoliubov-de Gennes equations. The calculated
spectrum shows a roton minimum, and the roton gap decreases by reducing the
effective scattering length. As the roton gap disappears, the system
spontaneously develops in its ground-state a periodic, linear structure formed
by denser clusters of atomic dipoles immersed in a dilute superfluid
background. This structure shows the hallmarks of a supersolid system, i.e. (i)
a finite non-classical translational inertia along the tube axis and (ii) the
appearance, besides the phonon mode, of the Nambu-Goldstone gapless mode
corresponding to phase fluctuations, and related to the spontaneous breaking of
the gauge symmetry. A further decrease in the scattering length eventually
leads to the formation of a periodic linear array of self-bound droplets.Comment: 5 pages, 4 figures (version accepted for publication in PRA Rapid
Communications

### Supersolid structure and excitation spectrum of soft-core bosons in 3D

By means of a mean-field method, we have studied the zero temperature
structure and excitation spectrum of a three-dimensional soft-core bosonic
system for a value of the interaction strength that favors a crystal structure
made of atomic nano-clusters arranged with FCC ordering. In addition to the
longitudinal and transverse phonon branches expected for a normal crystal, the
excitation spectrum shows a soft mode related to the breaking of gauge
symmetry, which signals a partial superfluid character of the solid. Additional
evidence of supersolidity is provided by the calculation of the superfluid
fraction, which shows a first-order drop, from 1 to 0.4, at the
liquid-supersolid transition and a monotonic decrease as the interaction
strength parameter is increased. The conditions for the coexistence of the
supersolid with the homogeneous superfluid are discussed, and the surface
tension of a representative solid-liquid interface is calculated.Comment: 11 pages, 11 figure

### 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

### Spinning superfluid helium-4 nanodroplets

We have studied spinning superfluid $^4$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

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