Understanding Vortex Lattice Dynamics of a BEC

Peter Engels, Ian R. Coddington, Volker Schweikhard +1 more

Sálvio Jacob Bereta

There are many efforts to understand better the dipolar long-range interaction, the shell potential confinement, and the vortices in Bose-Einstein condensates (BECs). Furthermore, combining these three areas suggests that it can give rise to new phenomena. In this thesis, we studied the dynamics of vortices in a shell-trapped dipolar BEC. The long-range dipolar interaction induces an inhomogeneous density distribution on the shell, affecting the vortex velocity field. We analyzed the dynamics of one and two pairs of vortices symmetrically located in the shell to separate the vortex-vortex interaction contribution from that of the density gradient due to the inhomogeneous background. In addition to the numerical studies, a Lagrangian formalism allowed us to derive semi-analytical expressions which are in agreement with the results of the Gross-Pitaevskii equation (GPE) simulations, giving us better insights into the influence of the core in the vortex dynamics outside the validity of the point-vortex model. In the second part of this thesis, we describe an actual experimental implementation of the shell BEC. Using the Stochastic Projected GPE simulations, we determine the configuration of a finite temperature vortex lattice produced in this potential over the action of gravity. More specifically, we studied the transition from a perfectly hexagonal (triangular) lattice to the hexatic and liquid phases by detecting the vortex positions to calculate the angular and pair correlation functions of this system for different values of temperatures and trap rotation frequencies. Our results for the vortex lattice melting are consistent with the hexatic to liquid transition in the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario and can be extended to predict the melting temperature of other finite-size systems.

Guoquan Yang, Suying Zhang, Jingjing Jin

Abstract Understanding how two vortex dipoles interact with each other microscopically is helpful for understanding the dynamics of quantum turbulence in a two-dimensional Bose–Einstein condensate (BEC). We investigate the collision of two vortex dipoles propagating in opposite directions along parallel lines by numerically solving the two-dimensional Gross–Pitaevskii equation. By varying the impact parameter, we identify three general dynamical modes of the vortex dipoles in a homogeneous BEC, which are the vortex recombination mode, the encircling mode and the flyby mode. The existence of these three modes has been corroborated by numerical computations of the ordinary differential equations describing the vortex interactions. Furthermore, we show the corresponding parameter ranges for these modes.

Aaron James Sup

P. G. Kevrekidis

Volker Schweikhard, Ian R. Coddington, Peter Engels +2 more

We observe interlaced square vortex lattices in rotating dilute-gas spinor Bose-Einstein condensates (BEC). After preparing a hexagonal vortex lattice in a one-component BEC in an internal atomic state |1, we coherently transfer a fraction of the superfluid to a different state |2. The subsequent evolution of this pseudo-spin-1/2 superfluid towards a state of offset square lattices involves an intriguing interplay of phase-separation and -mixing dynamics, both macroscopically and on the length scale of the vortex cores, and a stage of vortex turbulence. The stability of the square structure is proved by its response to applied shear perturbations. An interference technique shows the spatial offset between the two vortex lattices. Vortex cores in either component are filled by fluid of the other component, such that the spin-1/2 order parameter forms a Skyrmion lattice.