Role of Coulomb interactions in dark-bright magnetoexciton mixing in strained quantum wells

Coupled magnetoexciton states between allowed (``bright'') and forbidden (``dark'') transitions are found in absorption spectra of strained ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum wells with increasing magnetic field up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We find large $(\ensuremath{\sim}9\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ energy splittings in the mixed states. The observed anticrossing behavior is independent of polarization, and sensitive only to the parity of the quantum confined states. Detailed experimental and theoretical investigations indicate that valence band complexity does not play a role. We find that the excitonic Coulomb interaction is a necessary condition for the anticrossing to occur, while the magnitude of the energy splitting correlates with strain. In addition, we determine the spin composition of the mixed states.