Chiral phonons with giant magnetic moment

Chiral phonons are phonons associated with the orbital motion of ions. These phonons carry finite angular momentum and exhibit many interesting magneto-phononic and phono-magnetic effects. On the basis of purely circular ionic motion, these phonons are expected to carry a magnetic moment of the order of a few nuclear magnetons. However, some recent experiments have demonstrated a phonon magnetic moment of the order of a few Bohr magnetons. This kind of giant magnetic response points towards the electronic contribution to the magnetic moment of phonons. Many diverse mechanisms have been discovered for this enhanced magnetic response of chiral phonons. The orbital-lattice coupling is one such mechanism where low-energy electronic excitations on a magnetic ion couple to phonons and impart a large magnetic moment to phonons. This mechanism has been extensively studied for rare-earth paramagnets where phonons couple to crystal electric field excitations on 4f electrons and exhibit a large phonon Zeeman effect. We propose that this mechanism can also manifest in d-electron systems where phonons can couple with spin-orbit excitations. After outlining the conditions required to observe this effect in transition-metal compounds, we present some group theoretical aspects and example systems that allow chiral phonons with a giant magnetic moment. We furthermore show that these systems should also exhibit very large phono-magnetic processes like the generation of effective magnetic fields by exciting chiral phonons. Such phonons can possibly be used for ultrafast control of quantum matter.