Coherent Motion of Excited Electronic States in Layered Crystals

在BiI_3和GaSe层状晶体中，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（如玻色-爱因斯坦凝聚（BEC））中激子质量相干运动的非线性光学响应。在BiI_3中，利用调至SFE态的皮秒激光在光斑激发下的空间分辨和时间分辨光致发光研究了被限制在准二维层错界面（SFE）中的激子的空间扩展。应用包含阻尼的电报方程分析了发光强度的时空演变。结果表明，相干度随激子密度的增加而增加。为了获得高密度激子质量的相位信息，在空间分辨区检测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。由于激子质量从一个点到另一个点的相干输运，DFWM光被发射了更多，这明显表明sfe极化子质量处于BEC相。在GaSe中，重激发下激子发光的特征是与其他激子或载流子散射而产生的反冲过程。从时空演化、反冲发光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，研究了准二维玻色激子气体的有序形成和超流动性。我们计算了基态处于涡晶格有序态时的初等激发，并考察了非对角线长阶的温度依赖性，这是二维激子系统凝聚态的基本量。在假设Kosterlitz-Thouless型缺陷介导熔解机制的情况下，计算了涡晶格的熔解温度

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