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arXiv:1501.03866 · Physical Review Letters

Longitudinal Spin Diffusion in a Nondegenerate Trapped Rb87 Gas

Dorna Niroomand, SD Graham, J. M. McGuirk

Published Aug 14, 2015• 6 citations
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Abstract

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.

DOI
10.1103/physrevlett.115.075302

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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.
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.
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Recent experiments in a mixture of two hyperfine states of trapped Bose gases show behavior analogous to a spin-12\frac{1}{2}21​ system, including transverse spin waves and other familiar Leggett-Rice-type effects. We have derived the kinetic equations applicable to these systems, including the spin dependence of interparticle interactions in the collision integral, and have solved for spin-wave frequencies and longitudinal and transverse diffusion constants in the Boltzmann limit. We find that, while the transverse and longitudinal collision times for trapped Fermi gases are identical, the Bose gas shows unusual diffusion anisotropy in both dipole and quadrupole modes. Moreover, the lack of spin isotropy in the interactions leads to the nonconservation of transverse spin, which in turn has significant effects on the hydrodynamic modes.
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.
Recent experiments in a mixture of two hyperfine states of trapped Bose gases show behavior analogous to a spin-1/2 system, including transverse spin waves and other familiar Leggett-Rice-type effects. We have derived the kinetic equations applicable to these systems, including the spin dependence of interparticle interactions in the collision integral, and have solved for spin-wave frequencies and longitudinal and transverse diffusion constants in the Boltzmann limit. We find that, while the transverse and longitudinal collision times for trapped Fermi gases are identical, the Bose gas shows diffusion anisotropy. Moreover, the lack of spin isotropy in the interactions leads to the non-conservation of transverse spin, which in turn has novel effects on the hydrodynamic modes. PACS numbers: 03.75.Mn,05.30Jp,05.60.Gg,51.10.+y,67.20.+k. In recent JILA experiments,1,2 a mixture of two hyperfine states was found to segregate by species. The theoretical explanation3−8 for this behav-ior is based on the two states playing the role of a pseudo-spin-1/2 system,