4,583 research outputs found
Path-Integral Ground-State and Superfluid Hydrodynamics of a Bosonic Gas of Hard Spheres
We study a bosonic gas of hard spheres by using of the exact zero-temperature
Path-Integral Ground-State (PIGS) Monte Carlo method and the equations of
superfluid hydrodynamics. The PIGS method is implemented to calculate for the
bulk system the energy per particle and the condensate fraction through a large
range of the gas parameter (with the number density and the
s--wave scattering length), going from the dilute gas into the solid phase. The
Maxwell construction is then adopted to determine the freezing at
and the melting at . In the liquid
phase, where the condensate fraction is finite, the equations of superfluid
hydrodynamics, based on the PIGS equation of state, are used to find other
relevant quantities as a function of the gas parameter: the chemical potential,
the pressure and the sound velocity. In addition, within the Feynman's
approximation, from the PIGS static structure factor we determine the full
excitation spectrum, which displays a maxon-roton behavior when the gas
parameter is close to the freezing value. Finally, the equations of superfluid
hydrodynamics with the PIGS equation of state are solved for bosonic system
under axially--symmetric harmonic confinement obtaining its collective
breathing modes.Comment: 7 pages, 7 figures; improved version to be published in Phys. Rev.
Superfluid behavior of quasi-1D p-H inside carbon nanotube
We perform ab-initio Quantum Monte Carlo simulations of para-hydrogen
(pH) at K confined in carbon nanotubes (CNT) of different radii. The
radial density profiles show a strong layering of the pH 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 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 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 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
Quantum Monte Carlo study of a vortex in superfluid He and search for a vortex state in the solid
We have performed a microscopic study of a straight quantized vortex line in
three dimensions in condensed He at zero temperature using the Shadow Path
Integral Ground State method and the fixed-phase approximation. We have
characterized the energy and the local density profile around the vortex axis
in superfluid He at several densities, ranging from below the equilibrium
density up to the overpressurized regime. For the Onsager-Feynman (OF) phase
our results are exact and represent a benchmark for other theories. The
inclusion of backflow correlations in the phase improves the description of the
vortex with respect to the OF phase by a large reduction of the core energy of
the topological excitation. At all densities the phase with backflow induces a
partial filling of the vortex core and this filling slightly increases with
density. The core size slightly decreases for increasing density and the
density profile has well defined density dependent oscillations whose wave
vector is closer to the wave vector of the main peak in the static density
response function rather than to the roton wave vector. Our results can be
applied to vortex rings of large radius and we find good agreement with the
experimental value of the energy as function of without any free parameter.
We have studied also He above the melting density in the solid phase using
the same functional form for the phase as in the liquid. We found that
off-diagonal properties of the solid are not qualitatively affected by the
velocity field induced by the vortex phase, both with and without backflow
correlations. Therefore we find evidence that a perfect He crystal is not a
marginally stable quantum solid in which rotation would be able to induce
off-diagonal long-range coherence.Comment: 15 pages, 8 figure
Sviluppo di tecniche di monitoraggio delle prestazioni di processi chimici controllati
La tesi proposta tratta del monitoraggio delle prestazioni dei controllori in processi chimici. Diverse sono le cause di malfunzionamento: da valvole con attrito, a regolatori sintonizzati impropriamente alla propagazione di disturbi negli impianti. Con questa tesi si vuole illustrare una metodologia per individuare le cause di mancata prestazione in modo da poterle classificare ed intraprendere le necessarie contromisure. In particolare é stato approfondito il problema della sintonizzazione dei regolatori ed è stata proposta una tecnica di identificazione basata sullo studio dei disturbi, evitando quindi ulteriori sollecitazioni agli impianti per variazioni di set-point. Inoltre è stato affrontato il problema dell’attrito sulle valvole utilizzando diverse tecniche di individuazione automatica originali e già presentetate in letteratura. Il tutto è stato organizzato in un software sviluppato in ambiente Matlab
Density functional theory of a trapped Bose gas with tunable scattering length: from weak-coupling to unitarity
We study an interacting Bose gas at T=0 under isotropic harmonic confinement
within Density Functional Theory in the Local Density approximation. The energy
density functional, which spans the whole range of positive scattering lengths
up to the unitary regime (infinite scattering length), is obtained by fitting
the recently calculated Monte Carlo bulk equation of state [Phys. Rev. A 89,
041602(R) (2014)]. We compare the density profiles of the trapped gas with
those obtained by MC calculations. We solve the time-dependent Density
Functional equation to study the effect of increasing values of the interaction
strength on the frequencies of monopole and quadrupole oscillations of the
trapped gas. We find that the monopole breathing mode shows a non-monotonous
behavior as a function of the scattering length. We also consider the damping
effect of three-body losses on such modes.Comment: 5 pages, submitted to EP
Quenched dynamics of the momentum distribution of the unitary Bose gas
We study the quenched dynamics of the momentum distribution of a unitary Bose
gas under isotropic harmonic confinement within a time-dependent density
functional approach based on our recently calculated Monte Carlo (MC) bulk
equation of state. In our calculations the inter-atomic s-wave scattering
length of the trapped bosons is suddenly increased to a very large value and
the real-time evolution of the system is studied. Prompted by the very recent
experimental data of Rb atoms at unitarity [Nature Phys. 10, 116 (2014)]
we focus on the momentum distribution as a function of time. Our results
suggest that at low momenta, a quasi-stationary momentum distribution is
reached after a long transient, contrary to what found experimentally for large
momenta which equilibrate on a time scale shorter than the one for three body
losses.Comment: 8 pages, 4 figures, submitted to a Special Issue of Few-Body Systems
"Systems on the verge of the stability
Lightweight Synchronization Algorithm with Self-Calibration for Industrial LORA Sensor Networks
Wireless sensor and actuator networks are gaining momentum in the era of
Industrial Internet of Things IIoT. The usage of the close-loop data from
sensors in the manufacturing chain is extending the common monitoring scenario
of the Wireless Sensors Networks WSN where data were just logged. In this paper
we present an accurate timing synchronization for TDMA implemented on the state
of art IoT radio, such as LoRa, that is a good solution in industrial
environments for its high robustness. Experimental results show how it is
possible to modulate the drift correction and keep the synchronization error
within the requirements
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