Scissors modes and spin-orbit locking in rare earth crystals with uniaxial symmetry

We propose a new type of experiment to observe scissors modes in crystals. These experiments are based on the assumption that in systems in which the spin-orbit coupling is much larger than the crystal field, an applied magnetic field should rotate both spin and charge density profile (spin-orbit locking). Rare earth compounds should provide examples of spin-orbit locking, since their 4f electrons have standard values of the crystal field energies one order of magnitude smaller than the spin-orbit coupling. Such standard values, however, have been questioned by a recent experiment suggesting that crystal field and spin-orbit coupling should be comparable in rare earths. Our experiments should also help to settle this issue.