  In 1915, Einstein and de Haas and Barnett demonstrated that changing the
magnetization of a magnetic material results in mechanical rotation, and vice
versa. At the microscopic level, this effect governs the transfer between
electron spin and orbital angular momentum, and lattice degrees of freedom,
understanding which is key for molecular magnets, nano-magneto-mechanics,
spintronics, and ultrafast magnetism. Until now, the timescales of
electron-to-lattice angular momentum transfer remain unclear, since modeling
this process on a microscopic level requires addition of an infinite amount of
quantum angular momenta. We show that this problem can be solved by
reformulating it in terms of the recently discovered angulon quasiparticles,
which results in a rotationally invariant quantum many-body theory. In
particular, we demonstrate that non-perturbative effects take place even if the
electron--phonon coupling is weak and give rise to angular momentum transfer on
femtosecond timescales.

Mesoscale and Nanoscale Physics;Strongly Correlated Electrons