Two-dimensional (2D) topological effects that interplay with ??electron-electron interactions have attracted lots of attention in ?the ??field of condensed matter physics. Quantum spin Hall (QSH) ?effect is such a typical topological effect. In this talk I will ?mainly present recent experimental studies of QSH effect in inverted ?InAs/GaSb quantum wells(QWs). In inverted InAs/GaSb QWs, using ?charge-neutral point (CNP) density as a tuning parameter, we observed ?two distinct QSH regimes: I), in dense limit in CNP, a ??single-particle hybridization gap opens, which hosts ??time-reversal-symmetry (TRS) QSH effect [1,2]; II), as dilute limit ??is approached, a spontaneous excitonic gap opens in the bulk, with ?the ??emergence of TRS-broken QSH effect where helical edges persist ?under high magnetic fields [1,3]. To clarify the interesting ?observation ??in regime II), we performed transport and THz ?measurements, and ??observed conclusive evidence for long-sought-after ?excitonic ??insulator(EI) [1]. The TRS-broken property of EI would be ?discussed. In the rest of my talk, I will discuss recent progresses of ??artificial graphene in nanopatterned GaAs QWs [4,5], which would ??offer exclusive tunability to 2D topological effects.

[1]. Lingjie Du et al, Nature Communications, 8, 1971 (2017)

[2]. Lingjie Du et al, Phys. Rev. Lett. 119, 056803 (2017)

[3]. Lingjie Du et al, Phys. Rev. Lett. 114, 096802 (2015)

[4]. Sheng Wang et al, Nature Nanotechnology, 13, 29 (2017)

[5]. Lingjie Du et al, Nature Communications, 9, 3299 (2018)