Dynamics of out-of-equilibrium domain walls in an ultra-cold Bose gas

Dorna Niroomand, Lydia Neary‐Zajiczek, J. M. McGuirk

J. M. McGuirk, Dorna Niroomand, Lydia Neary‐Zajiczek

Dorna Niroomand

Ultra-cold trapped atoms, with their high degree of tunability, provide ideal model systems to study physical phenomena with applications in many different fields of research. This thesis describes studies on spin transport phenomenon in a trapped ultra-cold spin-polarized 87Rb gas at temperatures above quantum degeneracy. This work is focused on the less studied regime of cross-over between classical and quantum transport. Diffusion is a fundamental dissipative process that tends to relax any system towards a state of minimum inhomogeneity. In this work we study longitudinal spin diffusion as a special case of spin transport. The system studied here consists of two anti-parallel longitudinal spin domains separated by a helical domain-wall. We report that the diffusion process manifests a significant deviation from classical diffusion due to purely quantum mechanical modifications. The two-domain spin textures are prepared using optical and microwave pulse techniques, and the dynamics of the spin structure is studied as it relaxes towards the final equilibrium state. Generally, there is a wide range of parameter space that could be studied. In this work we focused our studies on the effects related to the degree of coherence in the domainwall as well as the effective magnetic field acting on the spins. By controllably tuning these experimental parameters, we studied in detail how the spatiotemporal behaviour of the diffusion dynamics is modified. Our results show that the longitudinal spin diffusion time scales depend sensitively on the domain-wall degree of coherence. External magnetic field gradients also alter the dynamics noticeably, manifesting a significant dependence on the sign of the applied field gradients.

Tim Langen

Olha Farion, Mehdi Pourzand, J. M. McGuirk

Domain walls are carriers of spin transport whose controlled manipulation underlies a wide range of spintronic and information-processing technologies. Here we demonstrate tunable domain-wall transport in a weakly interacting nondegenerate ultracold Bose gas. We initialize a three-domain pseudo-spin-1/2 texture and observe spontaneous propagation of long-lived domain walls driven by exchange-mediated spin currents. By varying the orientation of the spin domains, we control the balance of spin currents across the walls and thereby tune their trajectories, including reversals of the initial direction of motion. Measurements reveal a crossover from an exchange-stabilized regime, in which coherent spin-exchange collisions suppress wall motion, to a diffusion-dominated regime characterized by rapid transport at thermal velocities. Numerical solutions of a quantum Boltzmann equation reproduce the observed dynamics and identify transverse phase gradients as an important control parameter governing domain-wall propagation. These results establish coherence and phase engineering as tools for programming spin transport in ultracold gases and provide a route toward controllable domain-wall dynamics in atomtronic systems.

M. van 't Woud

In this thesis, an experiment involving spin drag between two clouds of sodium atoms (one with spin -1, one with spin 0) in a Bose-Einstein condensate is described and analyzed in two different ways. These analysis methods involve both the movement of domain walls of the two clouds, and the movement of the centers of mass of the two clouds. It turns out that the movement of the domain walls is significantly different than that of the centers of mass of the clouds. Also, a couple of possible parameterizations are given to analyze the center of mass method properly. Furthermore, analytical formulas for the calculation of a magnetic cloverleaf trap (as used in the setup for Bose-Einstein condensation) are derived and the characteristics of this trap are discussed. The parameters describing the shape of the trap, which are the radial and axial trap frequencies, are calculated as a function of the current through the coils, and the change in magnetic field of the trap after applying several gradients is investigated. Finally, a series of experiments involving both the fast and slow flux of particles, used for loading the trap, from the oven to the vacuum chamber are described and analyzed. The results of the fast flux are compared with calculations involving the vapor pressure. The behavior of the magnetic field of the Zeeman coils is also discussed.