arXiv subjects

Franco Dalfovo

Publications and source records attributed to Franco Dalfovo.

36 records · Page 2Linked to original sources

arXiv2014

Observation of Solitonic Vortices in Bose-Einstein Condensates

We observe solitonic vortices in an atomic Bose-Einstein condensate after free expansion. Clear signatures of the nature of such defects are the twisted planar density depletion around the vortex line, observed in absorption images, and the double dislocation in the interference pattern obtained through homodyne techniques. Both methods allow us to determine the sign of the quantized circulation. Experimental observations agree with numerical simulations. These solitonic vortices are the decay product of phase defects of the BEC order parameter spontaneously created after a rapid quench across the BEC transition in a cigar-shaped harmonic trap and are shown to have a very long lifetime.

cond-mat.quant-gas↗

arXiv2014

Josephson oscillations and self-trapping of superfluid fermions in a double-well potential

We investigate the behaviour of a two-component Fermi superfluid in a double-well potential. We numerically solve the time dependent Bogoliubov-de Gennes equations and characterize the regimes of Josephson oscillations and self-trapping for different potential barriers and initial conditions. In the weak link limit the results agree with a two-mode model where the relative population and the phase difference between the two wells obey coupled nonlinear Josephson equations. A more complex dynamics is predicted for large amplitude oscillations and large tunneling.

cond-mat.quant-gas↗

arXiv2013

Spontaneous creation of Kibble-Zurek solitons in a Bose-Einstein condensate

When a system crosses a second-order phase transition on a finite timescale, spontaneous symmetry breaking can cause the development of domains with independent order parameters, which then grow and approach each other creating boundary defects. This is known as Kibble-Zurek mechanism. Originally introduced in cosmology, it applies both to classical and quantum phase transitions, in a wide variety of physical systems. Here we report on the spontaneous creation of solitons in Bose-Einstein condensates via the Kibble-Zurek mechanism. We measure the power-law dependence of defects number with the quench time, and provide a check of the Kibble-Zurek scaling with the sonic horizon. These results provide a promising test bed for the determination of critical exponents in Bose-Einstein condensates.

cond-mat.quant-gas↗

arXiv2012

Subdiffusion of nonlinear waves in quasiperiodic potentials

We study the spatio-temporal evolution of wave packets in one-dimensional quasiperiodic lattices which localize linear waves. Nonlinearity (related to two-body interactions) has destructive effect on localization, as recently observed for interacting atomic condensates [Phys. Rev. Lett. 106, 230403 (2011)]. We extend the analysis of the characteristics of the subdiffusive dynamics to large temporal and spatial scales. Our results for the second moment $m_2$ consistently reveal an asymptotic $m_2 \sim t^{1/3}$ and intermediate $m_2 \sim t^{1/2}$ laws. At variance to purely random systems [Europhys. Lett. 91, 30001 (2010)] the fractal gap structure of the linear wave spectrum strongly favors intermediate self-trapping events. Our findings give a new dimension to the theory of wave packet spreading in localizing environments.

cond-mat.dis-nn↗

arXiv2012

Localization in momentum space of ultracold atoms in incommensurate lattices

We characterize the disorder induced localization in momentum space for ultracold atoms in one-dimensional incommensurate lattices, according to the dual Aubry-André model. For low disorder the system is localized in momentum space, and the momentum distribution exhibits time-periodic oscillations of the relative intensity of its components. The behavior of these oscillations is explained by means of a simple three-mode approximation. We predict their frequency and visibility by using typical parameters of feasible experiments. Above the transition the system diffuses in momentum space, and the oscillations vanish when averaged over different realizations, offering a clear signature of the transition.

cond-mat.quant-gas↗

arXiv2012

Swallowtail Band Structure of the Superfluid Fermi Gas in an Optical Lattice

We investigate the energy band structure of the superfluid flow of ultracold dilute Fermi gases in a one-dimensional optical lattice along the BCS to BEC crossover within a mean-field approach. In each side of the crossover region, a loop structure (swallowtail) appears in the Bloch energy band of the superfluid above a critical value of the interaction strength. The width of the swallowtail is largest near unitarity. Across the critical value of the interaction strength, the profiles of density and pairing field change more drastically in the BCS side than in the BEC side. It is found that along with the appearance of the swallowtail, there exists a narrow band in the quasiparticle energy spectrum close to the chemical potential and the incompressibility of the Fermi gas consequently experiences a profound dip in the BCS side, unlike in the BEC side.

cond-mat.quant-gas↗

arXiv2011

Effects of periodic potentials on the critical velocity of superfluid Fermi gases in the BCS-BEC crossover

We study the effects of an external periodic potential on the critical velocity of a superfluid Fermi gas in the crossover between the Bardeen-Cooper-Schrieffer (BCS) phase and Bose-Einstein condensation (BEC). We numerically solve the Bogoliubov-de Gennes equations to model a three-dimensional (3D) gas of ultracold atoms in the superfluid phase flowing through a 1D optical lattice. We find that when the recoil energy is comparable to the Fermi energy, the presence of the periodic potential reduces the effect of pair-breaking excitations. This behavior is a consequence of the peculiar band structure of the quasiparticle energy spectrum in the lattice. When the lattice height is much larger than the Fermi energy, the periodic potential makes pairs of atoms to be strongly bound even in the BCS regime and pair-breaking excitations are further suppressed. We have also found that when the recoil energy is comparable to or larger than the Fermi energy, the critical velocity due to long-wavelength phonon excitations shows a non-monotonic behavior along the BCS-BEC crossover.

cond-mat.quant-gas↗

arXiv2009

Effects of interaction on the diffusion of atomic matter waves in one-dimensional quasi-periodic potentials

We study the behaviour of an ultracold atomic gas of bosons in a bichromatic lattice, where the weaker lattice is used as a source of disorder. We numerically solve a discretized mean-field equation, which generalizes the one-dimensional Aubry-Andrè model for particles in a quasi-periodic potential by including the interaction between atoms. We compare the results for commensurate and incommensurate lattices. We investigate the role of the initial shape of the wavepacket as well as the interplay between two competing effects of the interaction, namely self-trapping and delocalization. Our calculations show that, if the condensate initially occupies a single lattice site, the dynamics of the interacting gas is dominated by self-trapping in a wide range of parameters, even for weak interaction. Conversely, if the diffusion starts from a Gaussian wavepacket, self-trapping is significantly suppressed and the destruction of localization by interaction is more easily observable.

cond-mat.quant-gas↗

arXiv2008

Equation of state and effective mass of the unitary Fermi gas in a 1D periodic potential

By solving the Bogoliubov -- de Gennes equations at zero temperature, we study the effects of a one-dimensional optical lattice on the behavior of a superfluid Fermi gas at unitarity. We show that, due to the lattice, at low densities the gas becomes highly compressible and the effective mass is large, with a consequent significant reduction of the sound velocity. We discuss the role played by the lattice in the formation of molecules and the emergence of two-dimensional effects in the equation of state. Predictions for the density profiles and for the frequency of the collective oscillations in the presence of harmonic trapping are also given.

cond-mat.supr-con↗

arXiv2008

Solitons in two-dimensional Bose-Einstein condensates

The excitations of a two-dimensional (2D) Bose-Einstein condensate in the presence of a soliton are studied by solving the Kadomtsev-Petviashvili equation which is valid when the velocity of the soliton approaches the speed of sound. The excitation spectrum is found to contain states which are localized near the soliton and have a dispersion law similar to the one of the stable branch of transverse oscillations of a 1D gray soliton in a 2D condensate. By using the stabilization method we show that these localized excitations behave as resonant states coupled to the continuum of free excitations of the condensate.

cond-mat.supr-con↗

arXiv2007

Dark solitons in a superfluid Fermi gas

We investigate the behavior of dark solitons in a superfluid Fermi gas along the BCS-BEC crossover by solving the Bogoliubov - de Gennes equations and looking for real and odd solutions for the order parameter. We show that in the resonance unitary region, where the scattering length is large, the density profile of the soliton has a deep minimum, differently from what happens in the BCS regime. The superfluid gap is found to be significantly quenched by the presence of the soliton due to the occurrence of Andreev fermionic bound states localized near the nodal plane of the order parameter.

cond-mat.stat-mech↗

arXiv2004

The fate of phonons in freely expanding Bose-Einstein condensates

Phonon-like excitations can be imprinted into a trapped Bose-Einstein condensate of cold atoms using light scattering. If the condensate is suddenly let to freely expand, the initial phonons lose their collective character by transferring their energy and momentum to the motion of individual atoms. The basic mechanisms of this evaporation process are investigated by using the Gross-Pitaevskii theory and dynamically rescaled Bogoliubov equations. Different regimes of evaporation are shown to occur depending on the phonon wavelength. Distinctive signatures of the evaporated phonons are visible in the density distribution of the expanded gas, thus providing a new type of spectroscopy of Bogoliubov excitations.

cond-mat.soft↗

arXiv2003

Bogoliubov spectrum and Bragg spectroscopy of elongated Bose-Einstein condensates

The behavior of the momentum transferred to a trapped Bose-Einstein condensate by a two-photon Bragg pulse reflects the structure of the underlying Bogoliubov spectrum. In elongated condensates, axial phonons with different number of radial nodes give rise to a multibranch spectrum which can be resolved in Bragg spectroscopy, as shown by Steinhauer {\it et al.} [Phys. Rev. Lett. {\bf 90}, 060404 (2003)]. Here we present a detailed theoretical analysis of this process. We calculate the momentum transferred by numerically solving the time dependent Gross-Pitaevskii equation. In the case of a cylindrical condensate, we compare the results with those obtained by linearizing the Gross-Pitaevskii equation and using a quasiparticle projection method. This analysis shows how the axial-phonon branches affect the momentum transfer, in agreement with our previous interpretation of the observed data. We also discuss the applicability of this type of spectroscopy to typical available condensates, as well as the role of nonlinear effects.

cond-mat.soft↗

arXiv2001

Quantized vortices in mixed 3He-4He drops

Using density functional theory, we investigate the structure of mixed $^3$He$_{N_3}$-$^4$He$_{N_4}$ droplets with an embedded impurity (Xe atom or HCN molecule) which pins a quantized vortex line. We find that the dopant+vortex+$^4$He$_{N_4}$ complex, which in a previous work [F. Dalfovo {\it et al.}, Phys. Rev. Lett. {\bf 85}, 1028 (2000)] was found to be energetically stable below a critical size $N_{\rm cr}$, is robust against the addition of $^3$He. While $^3$He atoms are distributed along the vortex line and on the surface of the $^4$He drop, the impurity is mostly coated by $^4$He atoms. Results for $N_4=500$ and a number of $^3$He atoms ranging from 0 to 100 are presented, and the binding energy of the dopant to the vortex line is determined.

cond-mat↗

arXiv2000

Pinning of quantized vortices in helium drop by dopant atoms and molecules

Using a density functional method, we investigate the properties of liquid 4He droplets doped with atoms (Ne and Xe) and molecules (SF_6 and HCN). We consider the case of droplets having a quantized vortex pinned to the dopant. A liquid drop formula is proposed that accurately describes the total energy of the complex and allows one to extrapolate the density functional results to large N. For a given impurity, we find that the formation of a dopant+vortex+4He_N complex is energetically favored below a critical size N_cr. Our result support the possibility to observe quantized vortices in helium droplets by means of spectroscopic techniques.

cond-mat↗

arXiv1999

Free expansion of Bose-Einstein condensates with quantized vortices

The expansion of Bose-Einstein condensates with quantized vortices is studied by solving numerically the time-dependent Gross-Pitaevskii equation at zero temperature. For a condensate initially trapped in a spherical harmonic potential, we confirm previous results obtained by means of variational methods showing that, after releasing the trap, the vortex core expands faster than the radius of the atomic cloud. This could make the detection of vortices feasible, by observing the depletion of the density along the axis of rotation. We find that this effect is significantly enhanced in the case of anisotropic disc-shaped traps. The results obtained as a function of the anisotropy of the initial configuration are compared with the analytic solution for a noninteracting gas in 3D as well as with the scaling law predicted for an interacting gas in 2D.

cond-mat↗

arXiv1997

Density of Superfluid Helium Droplets

The classical integral cross sections of large superfluid 4He_N droplets and the number of atoms in the droplets (N=10^3-10^4) have been measured in molecular beam scattering experiments. These measurements are found to be in good agreement with the cross sections predicted from density functional calculations of the radial density distributions with a 10-90 % surface thickness of 5.7Å. By using a simple model for the density profile of the droplets a thickness of about 6-8Åis extracted directly from the data.

cond-mat↗

arXiv1994

Density Functional in Liquid Helium: Recent Applications to Droplets and Surfaces

We present a new density functional, which is the result of a natural evolution and improvement of previous density functional theories for liquid helium. We focus on the key ingredients of the theory, showing how they determine important predictions concerning inhomogeneous systems. These predictions turn out to be quite accurate even at the level of interatomic distances. We discuss the results for the free surface and droplets.

cond-mat↗