Martin Will, Tobias Lausch, and Michael Fleischhauer:
Phys. Rev. A 99, 062707 (2019)
🔓 arXiv:1902.07193 (2019)
We discuss the rotational cooling of diatomic molecules in a Bose-Einstein condensate (BEC) of ultracold atoms by emission of phonons with orbital angular momentum. Despite the superfluidity of the BEC there is no frictionless rotation for typical molecules since the dominant cooling occurs via emission of particle-like phonons. Only for macrodimers, whose size becomes comparable to or larger than the condensate healing length, a Landau-like, critical angular momentum exists below which phonon emission is suppressed. We find that the rotational relaxation of typical molecules is, in general, faster than the cooling of the linear motion of impurities in a BEC. This also leads to a finite lifetime of angulons, quasiparticles of rotating molecules coupled to phonons with orbital angular-momentum. We analyze the dynamics of rotational cooling for homonuclear diatomic molecules based on a quantum Boltzmann equation including single- and two-phonon scattering and discuss the effect of thermal phonons.