Symmetry‐related motional enhancement of exciton magnetic moment

A dramatic motional enhancement of heavy‐ and light‐hole exciton magnetic moment in zinc‐blende semiconductors under a magnetic field applied parallel to the [001] direction has been put into evidence when the exciton moves along the same direction [J. J. Davies et al., Phys. Rev. Lett. 97 , 187403 (2006)]. The authors of the paper assigned the effect to a mixing between the 1S and 2P exciton states arising from the cubic term in the Luttinger Hamiltonian expansion in momentum. Such exciton states do not take into account the exact crystal structure since they are just eigenstates of the angular momentum. In addition, the Luttinger Hamiltonian does not take into account the full magnetic‐field effect since it does not include the gauge transformations under the symmetry operations of the structure under the field. By determining the exact symmetry of the exciton states, it is shown here that, under a field parallel to the [001] direction, the Zeeman Splitting value vanishes at the Γ point and, due to accidental quasi‐degeneracy in energy between dark and bright exciton states, becomes finite when the exciton moves parallel to the field. A perturbation model allows fitting experimental data and explains the exciton magnetic‐moment enhancement with kinetic energy. On the contrary, under a field parallel to the [110] direction with the exciton moving parallel to the field, no accidental degeneracy probably takes place between exciton states. As a consequence, the concept of Zeeman Splitting is not relevant since no energy level is degenerate. In addition, a possible quasi‐degeneracy between the excitons recombining with the σ + and σ − polarizations, respectively, would not allow coupling their two states, hence would not change notably the experimental results.