A New Phase of Exciton Polarons Intermediate between Free and Self-Trapped Ones in the F.C.C. Lattice

Exciton polarons dressed with a cloud made of several phonons exist as a new phase in the f.c.c. lattice between the phase of self-trapped excitons dressed with much more phonons and that of free excitons essentially undressed. The exciton polaron in the new phase moves slowly resonating among localized polaron states whose phonon cloud has a symmetry of two-center type centered at each pair of nearest-neighbor sites. This phase cannot be obtained in the s.c. lattice where a pair of nearest neighbors have no common nearest neighbors. The exciton-phonon interaction modifying the transfer integral of exciton must have an intermediate strength in order for this phase to be realized. It can be ascribed to this phase that in a mixed crystal of AgBr 1- x Cl x where silver or halogen ions constitute the f.c.c. sublattice phonon replicas in the luminescence spectrum peak at a line of the severalth order for x ∼0.45 just before the self-trapped state is stabilized.