Coherent Motion of Excited Electronic States in Layered Crystals

在层状晶体BII_3和GaS中，我们研究了高密度激子质量的时空行为及其与光致凝聚相激子质量的相干运动有关的非线性光学响应，如玻色-爱因斯坦凝聚(BEC)。在层状晶体BII_3中，我们研究了准二维层错界面(SFE)中激子在空间和时间分辨光致发光中的空间扩展。应用含阻尼的电报方程，分析了发光强度的时空演化规律。结果表明，相干度随激子密度的增加而增大。为了获得高密度激子质量的位相信息，在空间分辨区域探测到了样品表面空间隔离的两个激光光斑诱导的简并四波混频(DFWM)光。DFWM灯…更多的发射是由于激子质量从一个点到另一个点的相干输运，显然表明SFE-极化质量处于BEC相。在GASE中，强激发下的激子发光表现为与其他激子或载流子散射产生的反冲过程。从激子的时空演化、反冲发光的光谱变化等方面揭示了激子的动态散射过程。在理论工作中，我们研究了准二维玻色激子气体中的有序形成和超流性。我们计算了基态处于涡旋晶格有序态时的基元激发，并考察了描述二维激子系统凝聚态的本质量--非对角长程序与温度的关系。在假设Kosterlitz-Thouless缺陷介导的熔化机制较少的情况下，计算了涡旋晶格的熔化温度

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