Exploring spin dynamics in a strongly ferromagnetic spinor Bose-Einstein condensate

Abstract

Quantum gases are well isolated from the environment and can be controlled and measured with high precision, providing an ideal platform for exploring a variety of quantum many-body phenomena. In addition, in contrast to solid systems, the time scale to equilibrium is experimentally observable, making it possible to study nonequilibrium dynamics as well as equilibrium states. We aimed to study the spin dynamics in a two-dimensional spinor Bose-Einstein condensate with a strong ferromagnetic interaction using ${}^7Li$ atoms. This thesis introduces the laser system, vacuum system, and experimental sequences to produce a Bose-Einstein condensate of ${}^7Li$ atoms. The ${}^7Li$ Bose-Einstein condensate has been prepared without the aid of the magnetic Feshbach resonance, which can tune the strength of atomic interaction, and the nonequilibrium spin dynamics were investigated. By exploring the quantum phase transition from the polar phase to the easy-plane-ferromagnetic phase and the spinor dynamics under various magnetic fields, and studying the two-dimensional breathing mode induced by flipping the spin state, we have confirmed that ${}^7Li$ spinor Bose-Einstein condensates display a strong ferromagnetic interaction. Utilizing a strongly ferromagnetic spinor Bose-Einstein condensate, we observe the universal coarsening dynamics by quenching the quadratic Zeeman shift from the polar phase to the easy-axis or isotropic ferromagnetic phase. In the scaling regime, we observe the universal dynamics where the spin correlation and structure factor at different times collapse into a single universal scaling function. Based on the symmetry of the ground state order parameter and topological defects, the two universal dynamics can be categorized into well-defined universality classes. A single magnetic domain is prepared in the easy-axis ferromagnetic phase and a spin current is imprinted along the domain wall. For the spin current above the critical value, flutter-finger patterns and skyrmions are observed as a result of quantum Kelvin-Helmholtz instability. We can identify the transverse spin winding with spin-resolved imaging and phase winding via matter wave interference. This allows us to observe the spin texture of skyrmion with integer charge as well as fractional charge.