EXCITON PHASE TRANSITION IN ATOMICALLY THIN 2D SEMICONDUCTORS

摘要：半导体中的光激发产生电子和空穴对。这些准粒子负责许多器件应用中使用的材料的光电行为，包括光伏，发光二极管和光探测器。这些电子和空穴准粒子，类似于环境中的粒子，可以在足够高的密度和低温下从非相互作用的气体转变为冷凝的液相。与稀相相比，凝聚态准粒子表现出非常不同的电子特性。然而，到目前为止，涉及电子-空穴凝聚的研究仅限于极低温度和高激光激发密度的有限材料体系。本项目主要研究高温下原子薄半导体中这些缩合电子空穴态。这项研究的教育部分包括从代表性不足的社区招募高中生到实验室。技术摘要原子薄的二维过渡金属二硫族化合物（TMDC）提供了一个显著的激子系统，其激子结合能比典型的半导体材料大一个数量级以上。这些材料可以为高温下的量子多体物理研究提供前所未有的机会。人们已经知道，在足够高的密度下，激子会凝聚成电子空穴等离子体，然后变成液体。但总的来说，气相到液相的转变发生在低温下。本文旨在研究二维TMDC材料中激子在室温及以上温度下向电子空穴液体（EHL）状态的相变。这种激子凝聚是经典气体凝聚成液滴的量子力学模拟。这项工作将对二维TMDC材料中的激子凝聚进行深入的了解。

Nontechnical Abstract: Optical excitations in semiconductors create electron and hole pairs. These quasiparticles are responsible for optoelectronic behavior of materials used in many device applications including, photovoltaics, light emitting diodes and light detectors. These electron and hole quasiparticles, similar to particles in ambient, can go through phase transitions from noninteracting gas to condensed liquid phase at high enough densities and low temperatures. The condensed quasiparticle state exhibits very different electronic properties compared to dilute phase. However so far the studies involving the electron-hole condensation are limited to very low temperatures and high laser excitation densities in limited group of material systems. This project focuses on study of these condensed electron-hole states in atomically thin semiconductors at high temperatures. The educational component of the research involves recruiting high school students specifically from underrepresented communities to the labs. Technical AbstractAtomically thin 2D transition metal dichalcogenides (TMDC) provide a remarkable excitonic system with a large exciton binding energy, more than an order of magnitude larger than typical semiconductor materials. These materials can provide unprecedented opportunities for the studies of quantum many-body physics at elevated temperatures. It has been known that excitons at high enough densities condense into electron-hole plasma and then into a liquid. But in general this gas to liquid phase transition takes place at cryo-temperatures. Here the proposed work aims to study the phase transition of excitons in 2D TMDC materials into a state of electron-hole liquid (EHL) at room temperature and above. This exciton condensation is a quantum mechanical analogue of the classical gas condensation into a liquid droplet. This work will pursue a thorough understanding of the exciton condensation in 2D TMDC materials.