Abstract Recently, cold atoms mixtures have attracted broad interest due to their novel and exotic quantum effects with respect to single‐component systems. In this study, the focus is on massive many‐vortex states and their dynamics. Vortex configurations characterized by the same discrete rotational symmetry are investigated when confined within topologically nonequivalent geometries, and the relative stability properties at varying number of vortices and infilling mass are highlighted. It is numerically shown how massive many‐vortex systems, in a mixture of Bose–Einstein condensates, can host the bosonic tunneling of the infilling component both in a disordered way, with tunneling events involving two or more close vortices, or in an almost‐periodic way when the vortices are organized in persisting necklaces or star‐lattices. The purpose is to explore a variety of situations involving the interplay between the highly‐nonlinear vortex dynamics and the inter‐vortex atomic transfer, and so to better understand the conditions for the onset of Josephson supercurrents in rotating systems, or to reveal phenomena that can be of interest for a future application, e.g., in the context of atomtronics.
Abstract Recently, cold atoms mixtures have attracted broad interest due to their novel and exotic quantum effects with respect to single‐component systems. In this study, the focus is on massive many‐vortex states and their dynamics. Vortex configurations characterized by the same discrete rotational symmetry are investigated when confined within topologically nonequivalent geometries, and the relative stability properties at varying number of vortices and infilling mass are highlighted. It is numerically shown how massive many‐vortex systems, in a mixture of Bose–Einstein condensates, can host the bosonic tunneling of the infilling component both in a disordered way, with tunneling events involving two or more close vortices, or in an almost‐periodic way when the vortices are organized in persisting necklaces or star‐lattices. The purpose is to explore a variety of situations involving the interplay between the highly‐nonlinear vortex dynamics and the inter‐vortex atomic transfer, and so to better understand the conditions for the onset of Josephson supercurrents in rotating systems, or to reveal phenomena that can be of interest for a future application, e.g., in the context of atomtronics.
We study the mass exchange between two rotating, quantum massive vortices in a two-component Bose-Einstein condensate. The vortices, in the majority component, exhibit a filled core, where the in-filling minority component undergoes a quantum tunneling effect. Remarkably, we observe that the two-vortex system features stable Josephson oscillations, as well as all the nonlinear phenomena, including the macroscopic quantum self-trapping, that characterize a Bosonic Josephson Junction. We propose an analytical model for describing the intervortex tunneling, obtained by implementing the coherent-state representation of the two-mode Bose-Hubbard model. This allows us to give the explicit expression of the model's parameters in terms of the physical macroscopic parameters of the two-vortex system. The comparison of the dynamical scenario predicted by the model with that emerging from the Gross–Pitaevskii equations is very good for sufficiently small atom numbers, while at larger atom numbers it grows less precise, presumably due to the partial exclusion of the many-body interactions from our model. The definition of an effective self-interaction parameter allows us to include the many-body effects, thus restoring a quite good agreement with the numerical results. Interestingly, the recognition of the bosonic Josephson dynamics paves the way to the investigation of new dynamical behaviors in multivortex configurations. Published by the American Physical Society 2024
We review the topic of quantized vortices in multicomponent Bose–Einstein condensates of dilute atomic gases, with an emphasis on the two-component condensates. First, we review the fundamental structure, stability and dynamics of a single vortex state in a slowly rotating two-component condensates. To understand recent experimental results, we use the coupled Gross–Pitaevskii equations and the generalized nonlinear sigma model. An axisymmetric vortex state, which was observed by the JILA group, can be regarded as a topologically trivial skyrmion in the pseudospin representation. The internal, coherent coupling between the two components breaks the axisymmetry of the vortex state, resulting in a stable vortex molecule (a meron pair). We also mention unconventional vortex states and monopole excitations in a spin-1 Bose–Einstein condensate. Next, we discuss a rich variety of vortex states realized in rapidly rotating two-component Bose–Einstein condensates. We introduce a phase diagram with axes of rotation frequency and the intercomponent coupling strength. This phase diagram reveals unconventional vortex states such as a square lattice, a double-core lattice, vortex stripes and vortex sheets, all of which are in an experimentally accessible parameter regime. The coherent coupling leads to an effective attractive interaction between two components, providing not only a promising candidate to tune the intercomponent interaction to study the rich vortex phases but also a new regime to explore vortex states consisting of vortex molecules characterized by anisotropic vorticity. A recent experiment by the JILA group vindicated the formation of a square vortex lattice in this system.
Abstract Chapter 4 addresses the progress of studies on superfluid dynamics, mainly related to the dynamics of quantized vortices, in cold-atom BECs. First, we give a minimal description of the physics of cold atoms, e.g. cooling, trapping, control of interatomic interactions by external fields. We then turn to the argument of superfluid hydrodynamics in a scalar BEC characterized by a single-component order parameter. The properties of the vortices can be well described on the basis of the GP model, which allows a quantitative comparison between theory and experiment. We will discuss topics such as single vortex dynamics, interacting dynamics of a few vortices, vortex lattice formation in a rotating BEC, and collective properties of a vortex lattice in a rapidly rotating BEC, which have been unraveled experimentally by direct visualization of vortices in condensed gases. Next, we will discuss the properties of vortices in multicomponent BECs characterized by the multicomponent order parameters. We will first explain the vortex physics in two-component BECs as a simple case, finding that their structures and dynamics are highly nontrivial due to the presence of the intercomponent interaction; we will cover the topics of vortex structure composed of two-component order parameters, dynamics of composite vortices, exotic vortex lattices, and hydrodynamic instabilities associated with the interface dynamics between immiscible condensates. Finally, we explain the vortices in a spinor BEC. The vortex structures are extremely rich due to the complicated order parameter manifold, which is clarified by the discussion on vortices in spin-1 and spin-2 spinor BECs.
We numerically investigate vortex lattices in rotating two-component Bose-Einstein condensates in which the two components have unequal atomic masses and interact attractively with each other. For sufficiently strong attraction, the system is found to exhibit exotic ground-state structures in a harmonic trap, such as lattices having a square geometry or consisting of two-quantum vortices. The obtained states satisfy the Feynman relation, and they can be realized with current experimental techniques.
Bose-Einstein condensation is a quantum statistical phase transition which was theoretically predicted almost a hundred years ago. After years of seminal research, physicists realized the first almost ideal Bose-Einstein condensates in ultracold dilute atomic gases in 1995. Since then, the theoretical and experimental methods concerning such systems have been developing rapidly, and many fascinating phenomena have been found in these novel quantum systems.\n\nBose-Einstein condensation occurs in a system consisting of massive bosons when a single quantum state becomes macroscopically occupied as the temperature is lowered below the transition temperature. In general, condensates consisting of repulsively interacting bosons exhibit superfluidity: Particle currents can flow in the system without dissipation and viscosity. Moreover, the velocity fields of condensates have to be irrotational, which severely restricts the rotational characteristics of these systems. Apart from the center of mass motion, the system may carry angular momentum in the form of elementary excitations or so-called quantized vortices.\n\nThis Thesis is a theoretical study of subjects related to stability and dynamics of quantized vortices in dilute atomic Bose-Einstein condensates. The precession and instability of off-centered vortices in trapped condensates is investigated both in the zero-temperature limit and at finite temperatures. Dynamical stability of multiply quantized vortices and vortex clusters is studied in axisymmetric trap geometries. Splitting of energetically and dynamically unstable multiply quantized vortices into singly quantized vortices is also studied. Finally, as a separate subject, tunneling of a condensate through a potential barrier is investigated. Majority of this work relies on numerical methods for solving the Gross-Pitaevskii and Bogoliubov equations, which are of central importance in the study of dilute atomic Bose-Einstein condensates.
Abstract We study the motion of superfluid vortices with filled massive cores. Previous point-vortex models already pointed out the impact of the core mass on the vortex dynamical properties, but relied on an assumption that is questionable in many physical systems where the immiscibility condition is barely satisfied: the fact that the massive core always lays at the very bottom of the effective confining potential constituted by the hosting vortex. Here, we relax this assumption and present a new point-vortex model where quantum vortices are harmonically coupled to their massive cores. We thoroughly explore the new dynamical regimes offered by this improved model; we then show that the functional dependence of the system normal modes on the microscopic parameters can be correctly interpreted only within this new generalized framework. Our predictions are benchmarked against the numerical simulations of coupled Gross–Pitaevskii equations for a realistic mixture of atomic Bose–Einstein condensates.