Coherent Motion of Excited Electronic States in Layered Crystals

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In layered crystals BiI_3, and GaSe, we have studied spatial and temporal behavior of high-density exciton masses and their nonlinear optical response in connection with coherent motion of the exciton masses in a photo-induced condensed phase such as the Bose-Einstein condensation (BEC).In BiI_3, spatial expansion of the excitons confined in a quasi tow-dimensional stacking fault interface (SFE) was examined in the space- and time-resolved photoluminescence under the spot excitation with a picosecond laser light tuned to the SFE state. The space and time evolution of the luminescence intensity was analyzed by applying a telegrapher's equation including damping. It shows that the degree of coherence increases with increasing exciton density. In order to obtain phase information of the high-density exciton masses, the degenerate four-wave-mixing (DFWM) light induced by spatially isolated two laser-light spots on the sample surface was detected in the space-resolved regime. The DFWM light … More was emitted owing to the coherent transport of the exciton mass from one to the other spot sites suggesting evidently the SFE-polariton masses being in the BEC phase. In GaSe, the exciton luminescence under heavy excitation is characterized by the recoil process due to scattering with the other excitons or carriers. The details of the dynamical scattering processes of the excitons have been made clear from space and time evolution, and spectral change of the recoil luminescence.In the theoretical work, the order formation and superfluidity have been investigated in a quasi-two dimensional Bose gas of excitons. We have calculated the elementary excitations, when the ground state is in the ordered state of the vortex-lattice, and examined the temperature dependence of the off-diagonal long range order which is the essential quantity specifying the condense state of the two-dimensional exciton system. We also calculate the melting temperature of the vortex-lattice assuming the Kosterlitz-Thouless type defect-mediated melting mechanism Less