Thesis Dissertation Defense

Abstract: Magnetic and nematic phenomena are ubiquitous in nature such as the spontaneous magnetization of magnetite and the formation of liquid crystal. Ultracold atom provides a unique platform for exploring multi-faceted quantum magnetic behavior associated with spin. The interplay between magnetic interaction and the interaction entangling spin and spatial motion (aka spin-orbit coupling) gives rise to rich phase diagrams and phase transitions between different phases, which are ideal tools for understanding universal dynamics in non-equilibrium system. Therefore in my work, we use 23Na spinor Bose-Einstein condensates (BECs) to study two different aspects of nematic and magnetic phenomena, nematic-orbit coupling and magnetic solitons.

On one hand, complex Hamiltonians can be engineered in spinor BEC to simulate condensed matter physics, such as spin-orbit coupling. Spin-orbit coupling mechansim[1], a crucial element in spin-Hall effect, couples the atomic spin to the spatial degree of freedom. As an analogy of spin-orbit coupling, a nematic-orbit coupling[2] mechanism is proposed, which couples the spin-nematic tensor to spatial motion of the atoms. To investigate the nematic-orbit coupling, we come up with an experimental proposal by using coplanar microwave arrays. And the phase diagram is carried out and two new striped phases are found theoretically. This opens a new realm for studying complex spin-nematic objects in spinor BEC.

On the other hand, spinor BEC is a non-linear magnetic system for defects to exist, such as vector solitons. A vector soliton is a type of solitary wave packet occurring in a nonlinear medium comprised of multiple components. In our experiments, a new type of soliton called magnetic solitons[3], is observed in a spinor BEC beyond the usual Manakov limit of the 1-dimensional Gross-Pitaevskii (GP) equations due to the magnetic interactions. By using a phase imprinting technique with magic wavelength, we created a pair of magnetic solitons in an antiferromagnetic spinor BEC. Besides, multiple solitons can be created by spatially modulating the pattern of the phase imprinting beam, which allows the future investigation of the bound state (Flemish strings[4]), which turns out to be a potential candidate accounting for the universal dynamics for the non-equilibrium system generated after a quench from the polar phase to the antiferromagnetic phase of 23Na BEC.

Reference:

[1] Y.-J. Lin, K. Jiménez-García, and I. B. Spielman, Spin–orbit-coupled Bose–Einstein condensates, Nature 471, 83–86 (2011)

[2] D. Lao, C. Raman, and C. A. R. Sa de Melo, Nematic-orbit coupling and Nematic density waves in spin-1 condensates, Phys. Rev. Lett. 124, 173203 (2020)

[3] ] X. Chai, D. Lao, K. Fujimoto, R.Hamazaki, M. Ueda and C. Raman, Magnetic solitons in a spin-1 Bose-Einstein condensate, Phys. Rev. Lett. 125, 030402 (2020)

[4] K. Fujimoto, R. Hamazaki, and M. Ueda, Flemish Strings of Magnetic Solitons and a Nonthermal Fixed Point in a One-Dimensional Antiferromagnetic Spin-1 Bose Gas, Phys. Rev. Lett. 122, 173001 (2019)