Broad universal Feshbach resonances in the chaotic spectrum of dysprosium atoms

Abstract

We report on the observation of weakly bound dimers of bosonic dysprosium with a strong universal $s$-wave halo character, associated with broad magnetic Feshbach resonances. These states surprisingly decouple from the chaotic background of narrow resonances, persisting across many such narrow resonances. In addition they show the highest reported magnetic moment $\ensuremath{\mu}\ensuremath{\simeq}20{\ensuremath{\mu}}_{\mathrm{B}}$ of any ultracold molecule. We analyze our findings using a coupled-channel theory taking into account the short range van der Waals interaction and a correction due to the strong dipole moment of dysprosium. We are able to extract the scattering length as a function of magnetic field associated with these resonances and obtain a background scattering length ${a}_{\mathrm{bg}}=91(16)\phantom{\rule{0.16em}{0ex}}{a}_{0}$. These results offer prospects of a tunability of the interactions in dysprosium, which we illustrate by observing the saturation of three-body losses.