arXiv subjects

C. F. Barenghi

Publications and source records attributed to C. F. Barenghi.

At least 37 records · Page 2Linked to original sources

arXiv2016

Critical velocity of a finite-temperature Bose gas

We use classical field simulations of the homogeneous Bose gas to study the breakdown of superflow due to vortex nucleation past a cylindrical obstacle at finite temperature. Thermal fluctuations modify the vortex nucleation from the obstacle, turning anti-parallel vortex lines (which would be nucleated at zero temperature) into wiggly lines, vortex rings and even vortex tangles. We find that the critical velocity for vortex nucleation decreases with increasing temperature, and scales with the speed of sound of the condensate, becoming zero at the critical temperature for condensation.

cond-mat.quant-gas↗

arXiv2015

A quantum storm in a teacup

The past decade has seen atomic Bose-Einstein condensates emerge as a promising prototype system to explore the quantum mechanical form of turbulence, buoyed by a powerful experimental toolbox to control and manipulate the fluid, and the amenity to describe the system from first-principles. This article presents an overview of quantum turbulence in atomic condensates, from its history and fundamental motivations, its characteristics and key results to date, and finally to some promising future directions.

cond-mat.quant-gas↗

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.

cond-mat.quant-gas↗

arXiv2015

Visualizing Pure Quantum Turbulence in Superfluid $^{3}$He: Andreev Reflection and its Spectral Properties

Superfluid $^3$He-B in the zero-temperature limit offers a unique means of studying quantum turbulence by the Andreev reflection of quasiparticle excitations by the vortex flow fields. We validate the experimental visualization of turbulence in $^3$He-B by showing the relation between the vortex-line density and the Andreev reflectance of the vortex tangle in the first simulations of the Andreev reflectance by a realistic 3D vortex tangle, and comparing the results with the first experimental measurements able to probe quantum turbulence on length scales smaller than the inter-vortex separation.

cond-mat.other↗

arXiv2015

Backreaction of tracer particles on vortex tangle in helium II counterflow

We report computer simulations of the interaction of seeding particles with quantized vortices and with the normal fluid flow in thermal counterflow of superfluid $^4$He. We show that if the particle concentration is too large, the vortex tangle is significantly affected, posing problems in the interpretation of visualization experiments. The main effects are an increase in vortex line density and a change in polarization of the vortex tangle, caused by the action of the Stokes drag of the viscous normal fluid on the trapped particles. We argue that in the case of large particle concentration, typically used for the particle image velocimetry technique, the tangle properties might become significantly changed. On the contrary, the particle tracking velocimetry technique that uses smaller particle concentration should not be appreciably affected.

cond-mat.other↗

arXiv2015

Local and nonlocal dynamics in superfluid turbulence

Turbulence in superfluid helium~II is a tangle of quantized vortex lines which interact via the classical Biot-Savart law. We show that vortex tangles with the same vortex line density will have different energy spectra, depending on the normal fluid which feeds energy into the superfluid component, and identify the spectral signature of two forms of superfluid turbulence: Kolmogorov tangles and Vinen tangles. By decomposing the superfluid velocity field into local and nonlocal contributions, we find that in Vinen tangles the motion of vortex lines depends mainly on the local curvature, whereas in Kolmogorov tangles the long-range vortex interaction is dominant and leads to the formation of clustering of lines, in analogy to the 'worms` of ordinary turbulence.

cond-mat.other↗

arXiv2014

Generation and Decay of Two-Dimensional Quantum Turbulence in a Trapped Bose-Einstein Condensate

In a recent experiment, Kwon et. al (arXiv:1403.4658 [cond-mat.quant-gas]) generated a disordered state of quantum vortices by translating an oblate Bose-Einstein condensate past a laser-induced obstacle and studying the subsequent decay of vortex number. Using mean-field simulations of the Gross-Pitaevskii equation, we shed light on the various stages of the observed dynamics. We find that the flow of the superfluid past the obstacle leads initially to the formation of a classical-like wake, which later becomes disordered. Following removal of the obstacle, the vortex number decays due to vortices annihilating and reaching the boundary. Our results are in excellent agreement with the experimental observations. Furthermore, we probe thermal effects through phenomenological dissipation.

cond-mat.quant-gas↗

arXiv2014

Classical-like wakes past elliptical obstacles in atomic Bose-Einstein condensates

We reinvestigate numerically the classic problem of two-dimensional superfluid flow past an obstacle. Taking the obstacle to be elongated (perpendicular to the flow), rather than the usual circular form, is shown to promote the nucleation of quantized vortices, enhance their subsequent interactions, and lead to wakes which bear striking similarity to their classical (viscous) counterparts. Then, focussing on the recent experiment of Kwon et al. (arXiv:1403.4658) in a trapped condensate, we show that an elliptical obstacle leads to a cleaner and more efficient means to generate two-dimensional quantum turbulence.

cond-mat.quant-gas↗

arXiv2014

Vortex reconnections in atomic condensates at finite temperature

The study of vortex reconnections is an essential ingredient of understanding superfluid turbulence, a phenomenon recently also reported in trapped atomic Bose-Einstein condensates. In this work we show that, despite the established dependence of vortex motion on temperature in such systems, vortex reconnections are actually temperature independent on the typical length/time scales of atomic condensates. Our work is based on a dissipative Gross-Pitaevskii equation for the condensate, coupled to a semiclassical Boltzmann equation for the thermal cloud (the Zaremba-Nikuni-Griffin formalism). Comparison to vortex reconnections in homogeneous condensates further show reconnections to be insensitive to the inhomogeneity in the background density.

cond-mat.quant-gas↗

arXiv2014

Quantum Analogs of Classical Wakes in Bose-Einstein Condensates

We show that an elliptical obstacle moving through a Bose-Einstein condensate generates wakes of quantum vortices which resemble those of classical viscous flow past a cylinder or sphere. The role of ellipticity is to facilitate the interaction of the vortices nucleated by the obstacle. Initial steady symmetric wakes lose their symmetry and form clusters of like-signed vortices, in analogy to the classical Bénard-von Kármán vortex street. Our findings, demonstrated numerically in both two and three dimensions, confirm the intuition that a sufficiently large number of quanta of circulation reproduce classical physics.

cond-mat.quant-gas↗

arXiv2014

Isotropic vortex tangles in trapped atomic Bose-Einstein condensates via laser stirring

The generation of isotropic vortex configurations in trapped atomic Bose-Einstein condensates offers a platform to elucidate quantum turbulence on mesoscopic scales. We demonstrate that a laser-induced obstacle moving in a figure-eight path within the condensate provides a simple and effective means to generate an isotropic three-dimensional vortex tangle due to its minimal net transfer of angular momentum to the condensate. Our characterisation of vortex structures and their isotropy is based on projected vortex lengths and velocity statistics obtained numerically via the Gross-Pitaevskii equation. Our methodology provides a possible experimental route for generating and characterising vortex tangles and quantum turbulence in atomic Bose-Einstein condensates.

cond-mat.quant-gas↗

arXiv2014

Coherent laminar and turbulent motion of toroidal vortex bundles

Motivated by experiments performed in superfluid helium, we study numerically the motion of toroidal bundles of vortex filaments in an inviscid fluid. We find that the evolution of these large-scale vortex structures involves the generalised leapfrogging of the constituent vortex rings. Despite three dimensional perturbations in the form of Kelvin waves and vortex reconnections, toroidal vortex bundles retain their coherence over a relatively large distance (compared to their size), in agreement with experimental observations.

physics.flu-dyn↗

arXiv2013

Quantum turbulence in atomic Bose-Einstein condensates

Weakly interacting, dilute atomic Bose-Einstein condensates (BECs) have proved to be an attractive context for the study of nonlinear dynamics and quantum effects at the macroscopic scale. Recently, atomic BECs have been used to investigate quantum turbulence both experimentally and theoretically, stimulated largely by the high degree of control which is available within these quantum gases. In this article we motivate the use of atomic BECs for the study of turbulence, discuss the characteristic regimes of turbulence which are accessible, and briefly review some selected investigations of quantum turbulence and recent results. We focus on three stages of turbulence - the generation of turbulence, its steady state and its decay - and highlight some fundamental questions regarding our understanding in each of these regimes.

cond-mat.quant-gas↗

arXiv2013

Velocity Distributions of Tracer Particles in Thermal Counterflow in Superfluid $^4$He

Quantum turbulence accompanying thermal counterflow in superfluid $^4$He was recently visualized by the Maryland group, using micron-sized tracer particles of solid hydrogen (J. Phys. Soc. Jpn. {\bf 77}, 111007 (2008)) . In order to understand the observations we formulate the coupled dynamics of fine particles and quantized vortices, in the presence of a relative motion of the normal and superfluid components. Numerical simulations based on this formulation are shown to agree reasonably well with experimental observations of the velocity distributions of the tracer particles in thermal counterflow.

cond-mat.other↗

arXiv2012

Quantum vortex reconnections

We study reconnections of quantum vortices by numerically solving the governing Gross-Pitaevskii equation. We find that the minimum distance between vortices scales differently with time before and after the vortex reconnection. We also compute vortex reconnections using the Biot-Savart law for vortex filaments of infinitesimal thickness, and find that, in this model, reconnection are time-symmetric. We argue that the likely cause of the difference between the Gross-Pitaevskii model and the Biot-Savart model is the intense rarefaction wave which is radiated away from a Gross-Pitaeveskii reconnection. Finally we compare our results to experimental observations in superfluid helium, and discuss the different length scales probed by the two models and by experiments.

cond-mat.other↗

arXiv2012

Observable Vortex Properties in Finite Temperature Bose Gases

We study the dynamics of vortices in finite temperature atomic Bose-Einstein condensates, focussing on decay rates, precession frequencies and core brightness, motivated by a recent experiment (Freilich et al. Science 329, 1182 (2010)) in which real-time dynamics of a single vortex was observed. Using the ZNG formalism based on a dissipative Gross-Pitaevskii equation for the condensate coupled to a semi-classical Boltzmann equation for the thermal cloud, we find a rapid nonlinear increase of both the decay rate and precession frequency with increasing temperatures. The increase, which is dominated by the dynamical condensate-thermal coupling is also dependent on the intrinsic thermal cloud collisional dynamics; the precession frequency also varies with the initial radial coordinate. The integrated thermal cloud density in the vortex core is for the most part independent of the position of the vortex (except when it is near the condensate edge) with its value increasing with temperature. This could potentially be used as a variant to the method of Coddington et al. (Phys. Rev. A 70, 063607 (2004)) for experimentally determining the temperature.

cond-mat.quant-gas↗

arXiv2012

Thermally- and mechanically-driven quantum turbulence in helium II

Quantum turbulence can be generated in superfluid helium either thermally (by applying a heat flux, as in thermal counterflow) or mechanically (by stirring the liquid). By modelling the superfluid vortex lines as reconnecting space curves with fixed circulation, and the driving normal fluid as a uniform flow (for thermal counterflow) and a synthetic turbulent flow (for mechanically-driven turbulence), we determine the difference between thermally-driven and mechanically-driven quantum turbulence. We find that in mechanically-driven turbulence the energy is concentrated at the large scales, the spectrum obeys Kolmogorov scaling, vortex lines have large curvature, and the presence of coherent vortex structures induces vortex reconnections at small angles. On the contrary, in thermally-driven turbulence the energy is concentrated at the mesoscales, the curvature is smaller, the vorticity field is featureless and reconnections occur at larger angles. Our results suggest a method to experimentally infer the presence of superfluid vortex bundles.

cond-mat.other↗

arXiv2012

A Dynamical Self-Consistent Finite Temperature Kinetic Theory: The ZNG Scheme

We review a self-consistent scheme for modelling trapped weakly-interacting quantum gases at temperatures where the condensate coexists with a significant thermal cloud. This method has been applied to atomic gases by Zaremba, Nikuni, and Griffin, and is often referred to as ZNG. It describes both mean-field-dominated and hydrodynamic regimes, except at very low temperatures or in the regime of large fluctuations. Condensate dynamics are described by a dissipative Gross-Pitaevskii equation (or the corresponding quantum hydrodynamic equation with a source term), while the non-condensate evolution is represented by a quantum Boltzmann equation, which additionally includes collisional processes which transfer atoms between these two subsystems. In the mean-field-dominated regime collisions are treated perturbatively and the full distribution function is needed to describe the thermal cloud, while in the hydrodynamic regime the system is parametrised in terms of a set of local variables. Applications to finite temperature induced damping of collective modes and vortices in the mean-field-dominated regime are presented.

cond-mat.quant-gas↗