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arXiv:1803.01096 · Physical Review A

Stable spin domains in a nondegenerate ultracold gas

Sean Graham, Dorna Niroomand, Robert J. Ragan, J. M. McGuirk

Published May 8, 2018• 1 citation
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Abstract

We study the stability of two-domain spin structures in an ultracold gas of magnetically trapped $^{87}\mathrm{Rb}$ atoms above quantum degeneracy. Adding a small effective magnetic field gradient stabilizes the domains via coherent collective spin rotation effects, despite negligibly perturbing the potential energy relative to the thermal energy. We demonstrate that domain stabilization is accomplished through decoupling the dynamics of longitudinal magnetization, which remains in time-independent domains, from transverse magnetization, which undergoes a purely transverse spin wave trapped within the domain wall. We explore the effect of temperature and density on the steady-state domains, and compare our results to a hydrodynamic solution to a quantum Boltzmann equation.

DOI
10.1103/physreva.97.051603

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•Dec 9, 2019•0 citations
We study the stability of two-domain spin structures in an ultracold gas of magnetically trapped 87Rb^{87}\mathrm{Rb}87Rb atoms above quantum degeneracy. Adding a small effective magnetic field gradient stabilizes the domains via coherent collective spin rotation effects, despite negligibly perturbing the potential energy relative to the thermal energy. We demonstrate that domain stabilization is accomplished through decoupling the dynamics of longitudinal magnetization, which remains in time-independent domains, from transverse magnetization, which undergoes a purely transverse spin wave trapped within the domain wall. We explore the effect of temperature and density on the steady-state domains, and compare our results to a hydrodynamic solution to a quantum Boltzmann equation.
Longitudinal spin diffusion of two pseudospin domains is studied in a trapped ^{87}Rb sample above quantum degeneracy, and the effect of coherence in the domain wall on the dynamics of the system is investigated. Coherence in a domain wall leads to transverse-spin-mediated longitudinal spin diffusion that is slower than classical predictions, as well as altering the domains' oscillation frequency. The system also shows an instability in the longitudinal spin dynamics as longitudinal and transverse spin components couple, and a conversion of longitudinal spin to transverse spin is observed, resulting in an increase in the total amount of coherence in the system.
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.
Ultracold trapped atomic systems are ideal model systems to investigate physical phenomena in the quantum regime.In this work we studied instabilities in spin dynamics of a sample of nondegenerate trapped 87 Rb gas.This is the first experimental investigation of the so-called Castaing instability in such systems.The Castaing instability is an instability in the spin dynamics of spin-polarized systems as a result of introducing sharp spin gradients in their spin profile.We used an optical technique via the ac Stark effect to initialize arbitrary spin profiles in a sample of evaporatively cooled nondegenerate 87 Rb gas.The experimental results manifest evidence for the presence of the Castaing instability in both transverse and longitudinal spin channels.The results agree reasonably well with theoretical studies and results of a numerical solution to the spin transport equation.Some imperfection in the longitudinal spin gradient due to the spin-state preparation technique makes the signature of the instability less clear in the transverse channel.We propose another preparation technique to overcome this shortcoming in future experiments.First, I would like to thank my supervisor, Dr. Jeffrey McGuirk.This thesis would not be possible without his help and guidance.I have been fortunate to work in his group and benefit from his incredible knowledge, enthusiasm and patience.He is always supportive and willing to give advice, yet knows when to let you experience research and learn science on your own.I am grateful for having two great physicists
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.
An ultracold Bose gas of two-level atoms can be thought of as a spin-1/2 Bose gas. It supports spin-wave collective modes due to the exchange mean field. Such collective spin oscillations have been observed in recent experiments at JILA with 87Rb{}^{87}\mathrm{Rb}87Rb atoms confined in a harmonic trap. We present a theory of the spin-wave collective modes based on the moment method for trapped gases. In the collisionless and hydrodynamic limits, we derive analytic expressions for the frequencies and damping rates of modes with dipole and quadrupole symmetry. We find that the frequency for a given mode is given by a temperature-independent function of the peak density n, and falls off as 1/n.1/n.1/n. We also find that, to a very good approximation, excitations in the radial and axial directions are decoupled. We compare our model to the numerical integration of a one-dimensional version of the kinetic equation and find very good qualitative agreement. The damping rates, however, show the largest deviation for intermediate densities, where one expects Landau damping---which is unaccounted for in our moment approach---to play a significant role.
Diffusion plays a major role in describing how particles and heat move in any nonequilibrium system. The diffusion of quantum properties is not as well understood as the diffusion of classical properties, especially at low temperatures or high densities. Exploring the diffusion of the quantum property known as spin is beneficial for explaining quantum effects that arise at low temperatures or high densities, and this knowledge acquired could assist in the development of new ultra-low-power devices. This thesis examines spin diffusion at various temperatures and densities using the highly-tunable experimental platform of ultracold atoms. Around one million rubidium-87 atoms are cooled to nanokelvin temperatures to create an ultracold gas, where quantum interactions between atoms can significantly modify spin diffusion compared to classical diffusion. One-dimensional spin diffusion is observed for an initial two-domain spin profile. Remarkably, diffusion of this spin profile is slowed at temperatures above quantum degeneracy, where the ultracold gas is largely classical but with quantum collisions. We demonstrate that this slowing of spin diffusion is due to the presence of spin coherence between spin domains, and that the removal of coherence speeds up spin diffusion to classical timescales. Spin diffusion is further modified by applying a linear differential potential that can speed, slow, or stop spin diffusion of a two-domain spin profile. Differential potentials spatially alter the precession of spins, which then alters the spin-rotating quantum collisions that modify spin diffusion. For a linear differential potential with a specific sign and magnitude, stabilized spin domains in an ultracold gas are observed for 40 times longer than classical diffusion timescales. In addition to modifying spin diffusion with coherence and differential potentials, we demonstrate arbitrary control of one-dimensional spin diffusion using temporally varying differential potentials and three-domain spin profiles. These spin diffusion modifying techniques could be useful for manipulating spin in other nonequilibrium systems, and set the stage for simulating spin-based devices in ultracold atoms.