Z1,2-linear-term and polariton effects on theZ1,2excitons in CuBr

The $Z_{1,2}$ excitons in CuBr are investigated by magnetoreflectance and magnetoluminescence. The reflectance anomaly at the energy of the degenerate (${\Gamma }_4,{\Gamma }_3$) states is explained as owing to the $k$-linear term of the ${\Gamma }_8$ valence band coupled with the finite exciton wave vector $\overrightarrow{\mathrm{K}}$. 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.