The 27th International Conference on Atomic Physics

Long-lived phantom helix states in Heisenberg quantum magnets

Jul 20, 2022, 5:00 PM

1h 30m

Hart House (Hart House)

Poster presentation Degenerate gases, many-body physics, and quantum simulation Poster session

Description

Exact solutions for quantum many-body systems are rare, but provide valuable insights for the description of universal phenomena. Recently, specific solutions of the Bethe ansatz equations for 1D anisotropic Heisenberg m​​odels were found that can carry macroscopic momentum yet no energy on top of the ferromagnetically ordered "vacuum" state, dubbed phantom Bethe states. As a consequence of these phantom Bethe states, simple spin helix states at special wave-vector becomes exact eigenstates of these systems.

With ultracold Li-7 atoms on optical lattices, we can simulate the anisotropic Heisenberg model, and tune the interaction anisotropy with Feshbach resonance. Here, we show experimentally that there exist special helical spin patterns in 1D chains which are long-lived, relaxing only very slowly in dynamics. The wave-vector of these special helices also shifts with the anisotropy parameter. These results confirming theoretical predictions.

As the wave-vector of the spin helix is determined by the anisotropy parameter, we use these phantom spin helices to directly measure the interaction anisotropy at different magnetic fields around Feshbach resonances. The measured anisotropy agrees well with the predictions based on super-exchange and Bose Hubbard model with perturbative interaction far from the Feshbach resonance, but we found discrepancies very close to the Feshbach resonance. This demonstrated the importance of higher order processes like bond charge tunneling close to the resonance, and reveals other processes not considered before (presented in detail in a separate poster).

We also generalize the theoretical description to higher dimensions and other non-integrable systems, and find analogous stable spin helices, which should show non-thermalizing dynamics associated with so-called quantum many-body scars. These results have implications for the quantum simulation of spin physics, as well as many-body dynamics.