激子超流体、拓扑激子绝缘态等新奇量子现象的产生与低维量子强关联体系中的激子凝聚效应密切相关。近几年在InAs/GaSb体系发现的拓扑激子BCS态引起了人们浓厚的研究兴趣，但有关拓扑量子态和激子凝聚间相互作用机理仍未清晰，更多理论预测的奇异性质尚需进一步探索。基于前期我们对弱杂化InAs/GaSb样品层间激子耦合机制的发现以及体系形成激子玻色-爱因斯坦凝聚可能性的研究，本项目拟进一步探索以下内容：1）极低温下调控出零场下热力学性质稳定的激子凝聚体，寻找激子超流的电输运证据；2）结合低温电输运和谱学测量，研究层间耦合诱导的拓扑相变与激子凝聚的内在联系，理解不同激子序参量在拓扑量子相变中的作用机制；3）研究激子凝聚体在强磁场下的演化规律。本课题的实施将帮助我们深入理解激子关联对InAs/GaSb体系拓扑电子态的影响，为将来基于量子自旋霍尔效应、激子超流等新奇物性开发拓扑量子器件提供实验基础。

The emergence of the exotic quantum phenomena, such as exciton superfluid and topological excitonic insulator, is directly related to the exciton condensation in low-dimensional strongly correlated materials. Recently, a topological excitonic insulator state with BCS pairing has been found in InAs/GaSb quantum wells, which has attracted much interest from all over the world. However, the interplay between exciton condensation and the quantum spin Hall effect is still obscure, and many other novel excitonic phenomena theoretically predicted in the material waiting to be explored further. In fact, we studied the interlayer excitonic interaction in weakly-hybridized InAs/GaSb samples in our previous measurements, and particularly we found a vanishing Hall resistive signature, which can be explained by Bose-Einstein condensation. In this project, we will explore the interlayer excitonic interaction further and focus mainly on the following three aspects: 1) We’ll make a thermodynamically stable exciton condensate at extremely low temperatures without any external magnetic fields, and experimentally probe the counterflow results to find out the electrical transport evidence of exciton superfluid; 2) Combining low-temperature electrical transport and terahertz spectroscopic measurements, we’ll explore the intrinsic connection between exciton condensate and topological quantum states in an interlayer coupling induced diagram, and understand the significance of different exciton order parameters in the topological phase transition; 3) We’ll discuss the evolution of exciton condensate with strong magnetic fields to reveal the possible quantum mechanism, like, a BEC-BCS crossover behind. Our work will point to exciting opportunities for understanding the impacts of excitonic interaction on topological quantum states in InAs/GaSb. Also, it will provide an effective experimental approach toward developing topological quantum devices based on the novel properties, such as quantum spin Hall effect and exciton superfluid.