Hydrogenic excited states of a 2D exciton are degenerate. In the presence of a weak magnetic field, the $S$-states with a zero momentum of the center of mass get coupled to the $P$-states with finite momentum of the center of mass. This field-induced coupling leads to a strong modification of the dispersion branches of the exciton spectrum. Namely, the lower branch acquires a shape of a mexican hat with a minimum at a finite momentum. At certain magnetic field, exciton branches exhibit a linear crossing, similarly to the spectrum of a 2D electron in the presence of spin-orbit coupling. While spin is not involved, degenerate $S$ and $P$ states play the role of the spin projections. Lifting of degeneracy due to diamagnetic shifts and deviation of electron-hole attraction from purely Coulomb suppresses the linear crossing.
Download