arXiv subjects

V. S. Bagnato

Publications and source records attributed to V. S. Bagnato.

At least 37 records · Page 2Linked to original sources

arXiv2019

A compact experimental machine for studying tunable Bose-Bose superfluid mixtures

We present a compact and versatile experimental system for producing Bose-Bose superfluid mixtures composed of sodium and potassium atoms. The compact design combines the necessary ultra-high vacuum enviroment for ultracold atom experiments with efficient atomic fluxes by using two-dimensional magneto-optical traps as independent source of atoms. We demonstrate the performance of this new machine by producing a Bose-Einstein condensate of $^{23}$Na with $\sim 1 \times 10^{6}~$atoms. The tunability of Na-K bosonic mixtures is particularly interesting for studies regarding the nucleation of vortices and quantum turbulence. In this direction, the large optical access of the science chamber along the vertical direction provides the conditions to implement high resolution optical setups for imaging and rotating the condensate with a stirring beam. We show the nucleation of a vortex lattice with up to 14 vortices in the $^{23}$Na BEC, attesting the efficiency of the experimental apparatus in studying the dynamics of vortices.

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arXiv2018

Characterization of nonequilibrium states of trapped Bose-Einstein condensates

Generation of different nonequilibrium states in trapped Bose-Einstein condensates is studied by numerically solving nonlinear Schrodinger equation. Inducing nonequilibrium states by shaking the trap, the following states are created: weak nonequilibrium, the state of vortex germs, the state of vortex rings, the state of straight vortex lines, the state of deformed vortices, vortex turbulence, grain turbulence, and wave turbulence. A characterization of nonequilibrium states is advanced by introducing effective temperature, Fresnel number, and Mach number.

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arXiv2017

Free expansion of Bose-Einstein condensates with a multicharged vortex

In this work, we analyze the free expansion of Bose-Einstein condensates containing multicharged vortices. The atomic cloud is initially confined in a three-dimensional asymmetric harmonic trap. We apply both approximate variational solutions and numerical simulations of the Gross-Pitaevskii equation. The data obtained provide a way to establish the presence as well as the multiplicity of vortices based only on the properties of the expanded cloud which can be obtained via time-of-flight measurements. In addition, several features such as the evolution of the vortex core size and the asymptotic velocity during free expansion were studied considering the atomic cloud as being released from different harmonic trap configurations.

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arXiv2017

Vinen turbulence via the decay of multicharged vortices in trapped atomic Bose-Einstein condensates

We investigate a procedure to generate turbulence in a trapped Bose-Einstein condensate which takes advantage of the decay of multicharged vortices. We show that the resulting singly-charged vortices twist around each other, intertwined in the shape of helical Kelvin waves, which collide and undergo vortex reconnections, creating a disordered vortex state. By examining the velocity statistics, the energy spectrum, the correlation functions and the temporal decay, and comparing these properties with the properties of ordinary turbulence and observations in superfluid helium, we conclude that this disordered vortex state can be identified with the `Vinen' regime of turbulence which has been discovered in the context of superfluid helium.

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arXiv2017

Phase transition of ultracold atoms immersed in a BEC vortex lattice

We investigate the quantum phases of ultracold atoms trapped in a vortex lattice using a mixture of two bosonic species (A and B), in the presence of an artificial gauge field. Heavy atoms of species B are confined in the array of vortices generated in species A, and they are described through a Bose-Hubbard model. In contrast to the optical-lattice setups, the vortex lattice has an intrinsic dynamics, given by its Tkachenko modes. Including these quantum fluctuations in the effective model for B atoms yields an extended Bose-Hubbard model, with an additional "phonon"-mediated long-range attraction. The ground-state phase diagram of this model is computed through a variational ansatz and the quantum Monte Carlo technique. When compared with the ordinary Bose-Hubbard case, the long-range interatomic attraction causes a shift and resizing of the Mott-insulator regions. Finally, we discuss the experimental feasibility of the proposed scheme, which relies on the proper choice of the atomic species and on a large control of physical parameters, like the scattering lengths and the vorticity.

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arXiv2016

Twisted unwinding of multi-charged quantum vortex and generation of turbulence in atomic Bose-Einstein condensates

We report the observation of the twisted decay of quadruply charged vortices in an atomic Bose-Einstein condensate. Supporting numerical simulations show that the singly-charged vortices, which result from the decay of a multi-charged vortex, twist around intertwined in the shape of helical Kelvin waves. Finally, we propose to apply this effect to generate an almost isotropic state of turbulence which we characterize in terms of the velocity statistics.

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arXiv2016

Isothermal compressibility determination across Bose-Einstein condensation

We apply the global thermodynamic variables approach to experimentally determine the isothermal compressibility parameter $κ_T$ of a trapped Bose gas across the phase transition. We demonstrate the behavior of $κ_T$ around the critical pressure, revealing the second order nature of the phase transition. Compressibility is the most important susceptibility to characterize the system. The use of global variables shows advantages with respect to the usual local density approximation method and can be applied to a broad range of situations.

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arXiv2016

Vortex rings and vortex ring solitons in shaken Bose-Einstein condensate

In a shaken Bose-Einstein condensate, confined in a vibrating trap, there can appear different nonlinear coherent modes. Here we concentrate on two types of such coherent modes, vortex ring solitons and vortex rings. In a cylindrical trap, vortex ring solitons can be characterized as nonlinear Hermite-Laguerre modes, whose description can be done by means of optimized perturbation theory. The energy, required for creating vortex ring solitons, is larger than that needed for forming vortex rings. This is why, at a moderate excitation energy, vortex rings appear before vortex ring solitons. The generation of vortex rings is illustrated by numerical simulations for trapped $^{87}$Rb atoms.

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arXiv2015

Controlled polarization of two-dimensional quantum turbulence in atomic Bose-Einstein condensates

We propose a scheme for generating two-dimensional turbulence in harmonically trapped atomic condensates with the novelty of controlling the polarization (net rotation) of the turbulence. Our scheme is based on an initial giant (multicharged) vortex which induces a large-scale circular flow. Two thin obstacles, created by blue-detuned laser beams, speed up the decay of the giant vortex into many singly-quantized vortices of the same circulation; at the same time, vortex-antivortex pairs are created by the decaying circular flow past the obstacles. Rotation of the obstacles against the circular flow controls the relative proportion of positive and negative vortices, from the limit of strongly anisotropic turbulence (almost all vortices having the same sign) to that of isotropic turbulence (equal number of vortices and antivortices). Using the new scheme, we numerically study quantum turbulence and report on its decay as a function of the polarization.

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arXiv2015

Strongly nonequilibrium Bose-condensed atomic systems

A trapped Bose-Einstein condensate, being strongly perturbed, exhibits several spatial structures. First, there appear quantum vortices. Increasing the amount of the injected energy leads to the formation of vortex tangles representing quantum vortex turbulence. Continuing energy injection makes the system so strongly perturbed that vortices become destroyed and there develops another kind of spatial structures with essentially heterogeneous spatial density. These structures consist of high-density droplets, or grains, surrounded by the regions of low density. The droplets are randomly distributed in space, where they can move; however they live sufficiently long time to be treated as a type of metastable creatures. Such structures have been observed in nonequilibrium trapped Bose gases of $^{87}$Rb subject to the action of an oscillatory perturbation modulating the trapping potential. Perturbing the system even stronger transforms the droplet structure into wave turbulence, where Bose condensate is destroyed. Numerical simulations are in good agreement with experimental observations.

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arXiv2015

Investigation of the momentum distribution of an excited BEC by free expansion: coupling with collective modes

We investigate the evolution of the momentum distribution of a Bose-Einstein condensate subjected to an external small oscillatory perturbation as a function of the in-trap evolution of the condensate after the external perturbation is switched-off. Besides changing its momentum distribution, we observe that the cloud distributes the input energy among its normal collective modes, displaying center-of-mass dipolar mode and quadrupolar mode. While the dipolar mode can be easily disregarded, we show that the momentum distribution is closely tied to the quadrupolar oscillation mode. This convolution hinders the actual momentum distribution.

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arXiv2015

Nonlinear dependence observed in quadrupolar collective excitation of a trapped BEC

We report the experimental observation of the collective excitations induced in a magnetically trapped 87Rb Bose-Einstein condensate. Low-lying mode excitations were studied by tracking the condensate's center-of-mass displacement, and its aspect ratio as a function of the hold time in the trap. We were able to partially control the modes onset by modulating the amplitude of the additional field gradient used to excite the BEC. The measured excitation frequencies were found to be in good agreement with the literature. We have also found that the modulation amplitude was able to change the phase of the center-of-mass oscillation. Finally, an interesting, non-linear dependence was observed on the condensate aspect ratio as a function of the perturbing amplitude which induces the quadrupolar mode.

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arXiv2015

Numerical simulation of nonequilibrium states in a trapped Bose-Einstein condensate

In this work we present numerical study of a trapped Bose-Einstein condensate perturbed by an alternating potential. The relevant physical situation has been recently realized in experiment, where the trapped condensate of $^{87}$Rb, being strongly perturbed, exhibits the set of spatial structures. Firstly, regular vortices are detected. Further, increasing either the excitation amplitude or modulation time results in the transition to quantum vortex turbulence, followed by a granular state. Numerical simulation of the nonequilibrium Bose-condensed system is based on the solution of the time-dependent 3D nonlinear Schrödinger equation within the static and dynamical algorithms. The damped gradient step and time split-step Fourier transform methods are employed. We demonstrate that computer simulations qualitatively reproduce the experimental picture, and describe well the main experimental observables.

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arXiv2015

Realization of inverse Kibble-Zurek scenario with trapped Bose gases

We show that there exists the inverse Kibble-Zurek scenario, when we start with an equilibrium system with broken symmetry and, by imposing perturbations, transform it to a strongly nonequilibrium symmetric state through the sequence of states with spontaneously arising topological defects. We demonstrate the inverse Kibble-Zurek scenario both experimentally, by perturbing the Bose-Einstein condensate of trapped $^{87}$Rb atoms, and also by accomplishing numerical simulations for the same setup as in the experiment, the experimental and numerical results being in good agreement with each other.

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arXiv2015

Bose-Einstein condensation: Twenty years after

The aim of this introductory article is two-fold. First, we aim to offer a general introduction to the theme of Bose-Einstein condensates, and briefly discuss the evolution of a number of relevant research directions during the last two decades. Second, we introduce and present the articles that appear in this Special Volume of Romanian Reports in Physics celebrating the conclusion of the second decade since the experimental creation of Bose-Einstein condensation in ultracold gases of alkali-metal atoms.

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arXiv2014

Vortex reconnections in anisotropic trapped three-dimensional Bose-Einstein condensates

Quantum vortex reconnections can be considered as a fundamental unit of interaction in complex turbulent quantum gases. Understanding the dynamics of single vortex reconnections as elementary events is an essential precursor to the explanation of the emergent properties of turbulent quantum gases. It is thought that a lone pair of quantum vortex lines will inevitably interact given a sufficiently long time. This paper investigates aspects of reconnections of quantum vortex pairs imprinted in a Bose-Einstein condensate held in an anisotropic three dimensional trap using an exact many-body treatment. In particular the impact of the interaction strength and the trap anisotropy in the reconnection time is studied. It is found that interaction strength has no effect on reconnection time over short time scales and that the trap anisotropy can cause the edge of the condensate to interfere with the reconnection process. It is also found that the initially coherent system fragments very slowly, even for relatively large interaction strength, and therefore the system likes to stay condensed during the reconnections.

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arXiv2014

Measuring The Heat Capacity in a Bose-Einstein Condensation using Global Variables

Phase transitions are well understood and generally followed by the behavior of the associated thermodynamic quantities, such as in the case of the $λ$ point superfluid transition of liquid helium, which is observed in its heat capacity. In the case of a trapped Bose-Einstein condensate (BEC), the heat capacity cannot be directly measured. In this work, we present a technique able to determine the global heat capacity from the density distribution of a weakly interacting gas trapped in an inhomogeneous potential. This approach represents an alternative to models based on local density approximation. By defining a pair of global conjugate variables, we determine the total internal energy and its temperature derivative, the heat capacity. We then apply the technique to a trapped $^{87}$Rb BEC a $λ$-type transition dependent on the atom number is observed, and the deviations from the non-interacting, ideal gas case are discussed. Finally we discuss the chances of using this method to study the heat capacity at $T \rightarrow 0$.

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arXiv2014

Formation of granular structures in trapped Bose-Einstein condensates under oscillatory excitations

We present experimental observations and numerical simulations of nonequilibrium spatial structures in a trapped Bose-Einstein condensate subject to oscillatory perturbations. In experiment, first, there appear collective excitations, followed by quantum vortices. Increasing the amount of the injected energy leads to the formation of vortex tangles representing quantum turbulence. We study what happens after the regime of quantum turbulence, with increasing further the amount of injected energy. In such a strongly nonequilibrium Bose-condensed system of trapped atoms, vortices become destroyed and there develops a new kind of spatial structure exhibiting essentially heterogeneous spatial density. The structure reminds fog consisting of high-density droplets, or grains, surrounded by the regions of low density. The grains are randomly distributed in space, where they move. They live sufficiently long time to be treated as a type of metastable objects. Such structures have been observed in nonequilibrium trapped Bose gases of $^{87}$Rb, subject to the action of alternating fields. Here we present experimental results and support them by numerical simulations. The granular, or fog structure is essentially different from the state of wave turbulence that develops after increasing further the amount of injected energy.

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