Coherent motion of high-density exciton masses at a quasi-two-dimensional interface in BiI3

Spatial correlation of a quantum coherence of weakly interacting stacking fault excitons (SFEs) excited heavily at a two-dimensional stacking fault interface in BiI3 has been studied by performing the degenerate four-wave-mixing (DFWM) measurements. It was found that the increase in the dephasing rate due to exciton–exciton scattering is suppressed above a threshold excitation density. Above such density, a propagating component of the SFE was detected on the space-resolved DFWM signals as a coherent mass of the excitons with nearly the same velocity as the group velocity of the SFE-polaritons by applying the spatially isolated two-spot excitation. In this scheme the off-diagonal element of the density matrix of the excitons induced by the exciting laser field at the one spot propagates to the other laser spot, bringing about nonlinear polarization to yield the DFWM signals. The time evolution of the space-resolved DFWM spectra evidently shows a manifestation of the macroscopic spatial expansion of a condensed quantum state of the excitons.