Session G3: Invited Session: Vortices, Solitons, Spin Textures in Bose-Einstein Condensates

Observation of a Geometric Hall Effect in a Spinor Bose-Einstein Condensate

When a spin-carrying particle slowly moves in a spatially varying magnetic field and its spin adiabatically follows the field direction, the particle acquires a quantum-mechanical phase known as the Berry phase. This phase originates from the geometrical properties of the parameter space of the system can generate geometric forces which act like magnetic and electric forces on the spin-carrying particle. Emergent electromagnetism of this spin origin can lead to novel spin transport phenomena and recently have been studied in many areas of physics, e.g. to understand the anomalous Hall effect in magnetic materials and for spintronics applications. In this talk, I will introduce spinor Bose-Einstein condensates of neutral atoms with Skrymion spin textures [1,2] and present our experimental observation of a geometric Hall effect in the neutral atomic superfluid system [2]. When the condensate was driven in one direction to oscillate with respect to the spin texture, we observed the development of its transverse motion perpendicular to the driving direction and the effective magnetic field direction, demonstrating the existence of an effective Lorentz force in the system. Under a resonant drive, the center of mass of the condensate showed a circular motion whose direction is determined by the chirality of the spin texture. Quantized vortices were nucleated in the circulating condensate due to the anharmonicity of the trapping potential. The geometric Hall effect in our system was characterized with the vortex nucleation rate.\\[4pt] [1] Phys. Rev. Lett. 108, 035301 (2012).\\[0pt] [2] Phys. Rev. Lett. 111, 245301 (2013).   

Dirac Monopoles in a Bose-Einstein Condensate

Over eighty years ago, Dirac established a theory of magnetic monopoles consistent with both classical electrodynamics and quantum mechanics [1]. I will discuss Dirac's theory and a recent realization [2] of its essential features, including a monopole, in the context of the synthetic electric and magnetic fields supported by a spinor Bose-Einstein condensate. \\[4pt] [1] P. A. M. Dirac, Proc. Roy. Soc. A {\bf 133}, 60 (1931).\\[0pt] [2] M. W. Ray, E. Ruokokoski, S. Kandel, M. M\"ott\"onen, and D. S. Hall, Nature {\bf 505}, 657 (2014).   

Spontaneous creation of Kibble-Zurek solitons in a Bose-Einstein condensate

The Kibble-Zurek mechanism (KZM) describes the spontaneous formation of defects in systems that cross a second-order phase transition at a finite rate. The mechanism was first proposed in the context of cosmology to explain how, during the expansion of the early Universe, the rapid cooling below a critical temperature induced a cosmological phase transition resulting in the creation of domain structures. In fact, the KZM is ubiquitous in nature and regards both classical and quantum phase transitions. Experimental evidences have been observed in superfluid $^3$He, in superconducting films and rings and in ion chains. Bose-Einstein condensation in trapped dilute gases has been considered as an ideal platform for the KZM as the system is extremely clean, controllable and particularly suitable for the investigation of effects arising from the spatial inhomogeneities induced by the confinement. Quantized vortices produced in a pancake-shaped condensate by a fast quench across the transition temperature have been already observed, but their limited statistics prevented a test of the KZM scaling. The KZM has been studied across the quantum superfluid to Mott insulator transition with atomic gases trapped in optical lattices. Here we report on the observation of solitons resulting from phase defects of the order parameter, spontaneously created in an elongated Bose-Einstein condensate of sodium atoms. We show that the number of solitons in the final condensate grows according to a power-law as a function of the rate at which the transition is crossed, consistent with the expectations of the KZM, and provide the first indication of the KZM scaling with the sonic horizon. We support our observations by comparing the estimated speed of the transition front in the gas to the speed of the sonic causal horizon, showing that solitons are produced in a regime of inhomogeneous Kibble-Zurek mechanism. We will address the role of vortex-solitons in our measurements.   

Two-dimensional quantum turbulence and vortex dynamics in BECs

One of the most challenging problems in the study of turbulence is to understand how statistical flow characteristics, such as energy and velocity distributions, relate to and depend upon microscopic attributes of turbulent flows, such as vortex distributions and dynamics. Due to the quantization of circulation, prospects for tackling this problem within the rapidly developing field of two-dimensional quantum turbulence (2DQT) are now emerging. In 2DQT, a disordered distribution of quantized vortices evolves within a nominally 2D superfluid; the vortices are thus constrained to move within the superfluid without bending or tilting with respect to the plane of the superfluid. Recent experiments have demonstrated methods that permit the generation and study of 2DQT in highly oblate Bose-Einstein condensates, and new vortex detection and manipulation techniques may soon aid in obtaining a better understanding of 2DQT vortex dynamics in these systems. Additionally, analytical and numerical efforts are identifying new regimes of 2DQT vortex dynamics and corresponding statistical characteristics of the superfluid flows that should be experimentally observable in BECs. This talk will introduce the field of 2DQT, then focus on our recent progress and immediate goals in the investigation of 2DQT and vortices in highly oblate BECs.