Dynamics of Half-Quantum Vortices in a Spinor Bose-Einstein Condensate

Abstract

Since the first realization of the superfluid He-4 in the 1930s, superfluidity has been one of the central topics in the modern physics. In particular, the advent of ultra-cold atomic gas (or quantum gas) system opened new opportunities for the superfluidity researches. Unlike the liquid helium, the quantum gas is dilute. Thus, the interaction is very weak, which allows us to compare experimental results with theories directly. Furthermore, the controllability of quantum gas gives us chances to investigate superfluid dynamics in various geometry.

The present thesis deals with several interesting phenomena appear in pancake-shaped, highly oblate BECs. In particular, the main focus is on the topological defects in the BECs both with and without a spin degree of freedom which are half-quantum vortices and quantum vortices, respectively. First of all, the necessary experimental techniques is briefly reviewed. We discuss spin manipulation techniques, such as Landau-Zener sweep and microwave dressing, and various imaging techniques for density and magnetization measurement.

A half-quantum vortex (HQV) is a topological defect in the BEC with a spin degree of freedom which is called a spinor BEC. The HQVs are observed by using the magnetization-sensitive phase contrast images. When we generate quantum vortices in a spinor BEC, the quantum vortices dissociate into HQV pairs with opposite magnetization. This demonstrates that the HQVs are the fundamental excitations in a spinor BEC. Moreover, the short-range interaction arising from HQVs' core structure is identified by conducting collision experiment between two HQV pairs. We can also figure out that the HQVs are coupled to magnon excitation from decay dynamics of a turbulent spinor BEC containing a large number of HQVS.

In experiments using a BEC without a spin degree of freedom called a scalar BEC, the 2D quantum turbulence, the chaotic state of tangled vortices confined in 2D, is investigated. The 2D quantum turbulence state is prepared by moving repulsive Gaussian beam at the center of the highly oblate trap. We employed Bragg scattering to probe 2D quantum turbulence. The Bragg scattering method enables us know positions and circulation directions of the vortices simultaneously. The decay dynamics of the 2D quantum turbulence demonstrates that the there are weak pair correlations between vortices and antivortices in the turbulent BEC.

Finally, free expansion dynamics of a BEC containing quantum vortices in quasi-2D regime is studied. The vortex expansion dynamics, in this case, can be theoretically easily accessible because the expansion dynamics can be regarded as non-interacting ballistic expansion. The defocus effect on the measurement is also presented.