arXiv subjects

L. Salasnich

Publications and source records attributed to L. Salasnich.

At least 73 records · Page 4Linked to original sources

arXiv2015

Effective-range signatures in quasi-1D matter waves: sound velocity and solitons

We investigate ultracold and dilute bosonic atoms under strong transverse harmonic confinement by using a 1D modified Gross-Pitaevskii equation (1D MGPE), which accounts for the energy dependence of the two-body scattering amplitude within an effective-range expansion. We study sound waves and solitons of the quasi-1D system comparing 1D MGPE results with the 1D GPE ones. We point out that, when the finite-size nature of the interaction is taken into account, the speed of sound and the density profiles of both dark and bright solitons show relevant quantitative changes with respect to what predicted by the standard 1D GPE.

cond-mat.quant-gas↗

arXiv2015

Scattering length of composite bosons in the 3D BCS-BEC crossover

We study the zero-temperature grand potential of a three-dimensional superfluid made of ultracold fermionic alkali-metal atoms in the BCS-BEC crossover. In particular, we analyze the zero-point energy of both fermionic single-particle excitations and bosonic collective excitations. The bosonic elementary excitations, which are crucial to obtain a reliable equation of state in the BEC regime, are obtained with a low-momentum expansion up to the forth order of the quadratic (Gaussian) action of the fluctuating pairing field. By performing a cutoff regularization and renormalization of Gaussian fluctuations, we find that the scattering length $a_B$ of composite bosons, bound states of fermionic pairs, is given by $a_B = (2/3) a_F$, where $a_F$ is the scattering length of fermions.

cond-mat.quant-gas↗

arXiv2015

Composite bosons in the 2D BCS-BEC crossover from Gaussian fluctuations

We study Gaussian fluctuations of the zero-temperature attractive Fermi gas in the 2D BCS-BEC crossover showing that they are crucial to get a reliable equation of state in the BEC regime of composite bosons, bound states of fermionic pairs. A low-momentum expansion up to the fourth order of the quadratic action of the fluctuating pairing field gives an ultraviolent divergent contribution of the Gaussian fluctuations to the grand potential. Performing dimensional regularization we evaluate the effective coupling constant in the beyond-mean-field grand potential. Remarkably, in the BEC regime our grand potential gives exactly the Popov's equation of state of 2D interacting bosons, and allows us to identify the scattering length $a_B$ of the interaction between composite bosons as $a_B=a_F/(2^{1/2}e^{1/4})= 0.551... a_F$, with $a_F$ is the scattering length of fermions. Remarkably, the value from our analytical relationship between the two scattering lengths is in full agreement with that obtained by recent Monte Carlo calculations.

cond-mat.quant-gas↗

arXiv2015

Tunneling dynamics of bosonic Josephson junctions assisted by a cavity field

We study the interplay between the dynamics of a Bose-Einstein condensate in a double-well potential and that of an optical cavity mode. The cavity field is superimposed to the double-well potential and affects the atomic tunneling processes. The cavity field is driven by a laser red detuned from the bare cavity resonance; the dynamically changing spatial distribution of the atoms can shift the cavity in and out of resonance. At resonance the photon number is hugely enhanced and the atomic tunneling becomes amplified. The Josephson junction equations are revisited and the phase diagram is calculated. We find new solutions with finite imbalance and at the same time a lack of self-trapping solutions due to the emergence of a new separatrix resulting from enhanced tunneling.

quant-ph↗

arXiv2014

Quantum bright solitons in the Bose-Hubbard model with site-dependent repulsive interactions

We introduce a one-dimensional (1D) spatially inhomogeneous Bose-Hubbard model (BHM) with the strength of the onsite repulsive interactions growing, with the discrete coordinate $z_{j}$, as $|z_{j}|^{α}$ with $α>0$. Recently, the analysis of the mean-field (MF) counterpart of this system has demonstrated self-trapping of robust unstaggered discrete solitons, under condition $α>1$. Using the numerically implemented method of the density matrix renormalization group (DMRG), we demonstrate that, in a certain range of interaction, the BHM also self-traps, in the ground state, into a soliton-like configuration, at $α>1$, and remains weakly localized at $α<1$. An essential quantum feature is a residual density in the background surrounding the soliton-like peak in the BHM ground state, while in the MF limit the finite-density background is absent. Very strong onsite repulsion eventually destroys soliton-like states, and, for integer densities, the system enters the Mott phase with a spatially uniform density

cond-mat.quant-gas↗

arXiv2014

Pair condensation of polarized fermions in the BCS-BEC crossover

We investigate a two-component Fermi gas with unequal spin populations along the BCS-BEC crossover. By using the extended BCS equations and the concept of off-diagonal-long-range-order we derive a formula for the condensate number of Cooper pairs as a function of energy gap, average chemical potential, imbalance chemical potential and temperature. Then we study the zero-temperature condensate fraction of Cooper pairs by varying interaction strength and polarization, finding a depletion of the condensate fraction by increasing the population imbalance. We also consider explicitly the presence of an external harmonic confinement and we study, within the local-density approximation, the phase separation between superfluid and normal phase regions of the polarized fermionic cloud. In particular, we calculate both condensate density profiles and total density profiles from the inner superfluid core to the normal region passing for the interface, where a finite jump in the density is a clear manifestation of this phase-separated regime. Finally, we compare our theoretical results with the available experimental data on the condensate fraction of polarized 6Li atoms [Science 311, 492 (2006)]. These experimental data are in reasonable agreement with our predictions in a suitable range of polarizations, but only in the BCS side of the crossover up to unitarity.

cond-mat.quant-gas↗

arXiv2014

Josephson physics of spin-orbit coupled elongated Bose-Einstein condensates

We consider an ultracold bosonic binary mixture confined in a one-dimensional double-well trap. The two bosonic components are assumed to be two hyperfine internal states of the same atom. We suppose that these two components are spin-orbit coupled between each other. We employ the two-mode approximation starting from two coupled Gross-Pitaevskii equations and derive a system of ordinary differential equations governing the temporal evolution of the inter-well population imbalance of each component and that between the two bosonic species. We study the Josephson oscillations of these spin-orbit coupled Bose-Einstein condensates by analyzing the interplay between the interatomic interactions and the spin-orbit coupling and the self-trapped dynamics of the inter-species imbalance. We show that the dynamics of this latter variable is crucially determined by the relationship between the spin-orbit coupling, the tunneling energy, and the interactions.

cond-mat.quant-gas↗

arXiv2013

Superfluidity, Sound Velocity and Quasi Condensation in the 2D BCS-BEC Crossover

We study finite-temperature properties of a two-dimensional superfluid made of ultracold alkali-metal atoms in the BCS-BEC crossover. We investigate the region below the critical temperature $T_{BKT}$ of the Berezinskii-Kosterlitz-Thouless phase transition, where there is quasi-condensation, by analyzing the effects of phase and amplitude fluctuations of the order parameter. In particular, we calculate the superfluid fraction, the sound velocity and the quasi-condensate fraction as a function of the temperature and of the binding energy of fermionic pairs.

cond-mat.quant-gas↗

arXiv2013

Collective modes in the anisotropic unitary Fermi gas and the inclusion of a backflow term

We study the collective modes of the confined unitary Fermi gas under anisotropic harmonic confinement as a function of the number of atoms. We use the equations of extended superfluid hydrodynamics, which take into account a dispersive von Weizsacker-like term in the equaton of state. We also discuss the inclusion of a backflow term in the extended superfluid Lagrangian and the effects of this anomalous term on sound waves and Beliaev damping of phonons.

cond-mat.quant-gas↗

arXiv2013

Entanglement entropy and macroscopic quantum states with dipolar bosons in a triple-well potential

We study interacting dipolar atomic bosons in a triple-well potential within a ring geometry. This system is shown to be equivalent to a three-site Bose-Hubbard model. We analyze the ground state of dipolar bosons by varying the effective on-site interaction. This analysis is performed both numerically and analytically by using suitable coherent-state representations of the ground state. The latter exhibits a variety of forms ranging from the su(3) coherent state in the delocalization regime to a macroscopic cat-like state with fully localized populations, passing for a coexistence regime where the ground state displays a mixed character. We characterize the quantum correlations of the ground state from the bi-partition perspective. We calculate both numerically and analytically (within the previous coherent-state representation) the single-site entanglement entropy which, among various interesting properties, exhibits a maximum value in correspondence to the transition from the cat-like to the coexistence regime. In the latter case, we show that the ground-state mixed form corresponds, semiclassically, to an energy exhibiting two almost-degenerate minima.

cond-mat.quant-gas↗

arXiv2013

Critical velocity, vortex shedding and drag in a unitary Fermi superfluid

We study the real-time motion of a microscopic object in a cold Fermi gas at unitary conditions by using an extended Thomas-Fermi density functional approach. We find that spontaneous creation of singly quantized vortex-antivortex pairs occurs as a critical velocity is exceeded, which leads to a drag between the moving object and the Fermi gas. The resulting force is linear in the velocity for subsonic motion and becomes quadratic for supersonic motion.

cond-mat.quant-gas↗

arXiv2012

Tuning Rashba and Dresselhaus spin-orbit couplings: Effects on singlet and triplet condensation with Fermi atoms

We investigate the pair condensation of a two-spin-component Fermi gas in the presence of both Rashba and Dresselhaus spin-orbit couplings. We calculate the condensate fraction in the BCS-BEC crossover both in two and in three dimensions by taking into account singlet and triplet pairings. These quantities are studied by varying the spin-orbit interaction from the case with the only Rashba to the equal-Rashba-Dresselhaus one. We find that, by mixing the two couplings, the singlet pairing decreases while the triplet pairing is suppressed in the BCS regime and increased in the BEC regime, both in two and three dimensions. At fixed spin-orbital strength, the greatest total condensate fraction is obtained when only one coupling (only Rashba or only Dresselhaus) is present.

cond-mat.quant-gas↗

arXiv2012

Dispersive effects in the unitary Fermi gas

We investigate within density functional theory various physical properties of the zero-temperature unitary Fermi gas which critically depend on the presence of a dispersive gradient term in the equation of state. First, we consider the unitary Fermi superfluid gas confined to a semi-infinite domain and calculate analytically its density profile and surface tension. Then we study the quadrupole modes of the superfluid system under harmonic confinement finding a reliable analytical formula for the oscillation frequency, which reduces to the familiar Thomas-Fermi one in the limit of a large number of atoms. Finally, we discuss the formation and propagation of dispersive shock waves in the collision between two resonant fermionic clouds, and compare our findings with recent experimental results.

cond-mat.quant-gas↗

arXiv2012

Finite temperature effects in two-mode bosonic Josephson junctions

We analyze the effects of the temperature on a bosonic Josephson junction realized with ultracold and dilute atoms in a double-well potential. Starting from the eigenstates of the two-site Bose-Hubbard Hamiltonian, we calculate the coherence visibility and the fluctuation of the on-site occupation number and study them as functions of the temperature. We show that, contrary to naive expectations, when the boson-boson interaction is suitably chosen thermal effects can increase the coherence visibility and reduce the on-site number fluctuation.

cond-mat.quant-gas↗

arXiv2012

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.

cond-mat.quant-gas↗

arXiv2012

Quasi-one-dimensional Bose-Einstein condensates in nonlinear lattices

We consider the three-dimensional (3D) mean-field model for the Bose-Einstein condensate (BEC), with a 1D nonlinear lattice (NL), which periodically changes the sign of the nonlinearity along the axial direction, and the harmonic-oscillator trapping potential applied in the transverse plane. The lattice can be created as an optical or magnetic one, by means of available experimental techniques. The objective is to identify stable 3D solitons supported by the setting. Two methods are developed for this purpose: The variational approximation, formulated in the framework of the 3D Gross-Pitaevskii equation, and the 1D nonpolynomial Schrödinger equation (NPSE) in the axial direction, which allows one to predict the collapse in the framework of the 1D description. Results are summarized in the form of a stability region for the solitons in the plane of the NL strength and wavenumber. Both methods produce a similar form of the stability region. Unlike their counterparts supported by the NL in the 1D model with the cubic nonlinearity, kicked solitons of the NPSE cannot be set in motion, but the kick may help to stabilize them against the collapse, by causing the solitons to shed excess norm. A dynamical effect specific to the NL is found in the form of freely propagating small-amplitude wave packets emitted by perturbed solitons.

cond-mat.quant-gas↗

arXiv2011

Condensate fraction of a resonant Fermi gas with spin-orbit coupling in three and two dimensions

We study the effects of laser-induced Rashba-like spin-orbit coupling along the BCS-BEC crossover of a Feshbach resonance for a two-spin-component Fermi gas. We calculate the condensate fraction in three and two dimensions and find that this quantity characterizes the crossover better than other quantities, like the chemical potential or the pairing gap. By considering both the singlet and the triplet pairings, we calculate the condensate fraction and show that a large enough spin-orbit interaction enhances the singlet condensate fraction in the BCS side while suppressing it on the BEC side.

cond-mat.quant-gas↗

arXiv2010

Rabi-Josephson oscillations and self-trapped dynamics in atomic junctions with two bosonic species

We investigate the dynamics of two-component Bose-Einstein condensates (BECs), composed of atoms in two distinct hyperfine states, which are linearly coupled by two-photon Raman transitions. The condensate is loaded into a double-well potential (DWP). A variety of dynamical behaviors, ranging from regular Josephson oscillations, to mixed Rabi-Josephson oscillations and to regimes featuring an increasing complexity, are described in terms of a reduced Hamiltonian system with four degrees of freedoms, which are the numbers of atoms in each component in the left and right potential wells, whose canonically conjugate variables are phases of the corresponding wave functions. Using the system with the four degrees of freedom, we study the dynamics of fractional imbalances of the two bosonic components, and compare the results to direct simulations of the Gross-Pitaevskii equations (GPEs) describing the bosonic mixture. We perform this analysis when the fractional imbalance oscillates around a zero-time averaged value and in the self-trapping regime as well.

cond-mat.quant-gas↗