Group seminar via Zoom: Spin dynamics in a tunable Heisenberg model realized with ultracold atoms

Niklas Jepsen, MIT, USA
Group meeting via video conference (Zoom)
Tuesday, December 14, 9:00 am (MEZ)

Due to the Covid-19 pandemic, we are still holding our group seminars and journal clubs via video conference.This procedure enables us to continue our research, enhance discussions and exchange important information.

Spin dynamics has, so far, mostly been studied in systems with isotropic spin–spin interactions. Here we use ultracold atoms to realize the anisotropic Heisenberg model, with fully adjustable anisotropy of the nearest-neighbor spin–spin couplings (the XXZ model). Using spin-helix patterns we study (a) spin transport far from equilibrium, and (b) stable spin patterns far away from the ground state, which are even exact many-body eigenstates.

A longitudinal spin-helix pattern (a spatial population modulation of spin-up and spin-down) can decay only by spin transport. When spins are coupled only along two of three possible orientations (the XX model), we find ballistic behavior of spin dynamics, whereas for isotropic interactions (the XXX model), we find diffusive behavior. More generally, for positive anisotropies, the dynamics ranges from anomalous superdiffusion to subdiffusion, whereas for negative anisotropies, we observe a crossover in the time domain from ballistic to diffusive transport.

A transverse spin-helix pattern generally decays fast by local dephasing. However, we experimentally show that for one special winding angle this spin helix has a very long lifetime. This finding confirms the recent prediction of phantom Bethe states, exact many-body eigenstates carrying finite momenta yet no energy. We theoretically find analogous stable spin helices in higher dimensions and in other non-integrable systems, where they imply non-thermalizing dynamics associated with quantum many-body scars. We use phantom spin helices to directly measure the interaction anisotropy which has a major contribution from short-range off-site interactions that have not been observed before.

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