Z1,2-linear-term and polariton effects on theZ1,2

The ${Z}_{1,2}$ excitons in CuBr are investigated by magnetoreflectance and magnetoluminescence. The reflectance anomaly at the energy of the degenerate (${\ensuremath{\Gamma}}_{4},{\ensuremath{\Gamma}}_{3}$) states is explained as owing to the $k$-linear term of the ${\ensuremath{\Gamma}}_{8}$ valence band coupled with the finite exciton wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$. Based on this coupling scheme, the polariton effect on the exciton states is elucidated. The "bottleneck" for the polariton relaxation is found to be on the spin-triplet exciton state.