---

Electron-hole bilayer systems in InAs/(In)GaSb composite quantum wells (CQWs) have been extensively investigated as a host of the quantum spin Hall (QSH) insulating phase [1]. Although the QSH phase is no longer protected by time-reversal symmetry in an external magnetic field, below a critical magnetic field, helical edge channels due to band inversion remain, which evolve into counter-propagating chiral edge channels originating from electron- and hole-like Landau levels in the quantum-Hall (QH) regime [2]. In narrow-gap systems, charge puddles form in the bulk, which affect the helical edge transport [3]. Meanwhile, time-resolved transport measurements performed on chiral edge channels of QH systems have revealed the dynamics of nonequilibrium charge propagating unidirectionally as edge magnetoplasmon (EMP) modes, reflecting an influence of charge puddles in the bulk [4, 5]. Here, we investigate time-resolved helical edge transport of EMPs in the QH regime of electron-hole bilayer systems in band-inverted InAs/InGaSb CQWs. Figure 1 shows the schematic of the device, comprising a band-inverted InAs/InGaSb CQW and experimental setup for time-resolved transport measurements of pulsed EMPs [5]. Figures 2(a) and 2(b) show color-scale plots of EMP waveforms as a function of a perpendicular magnetic field B measured at the net filling factors net (= e  h) = 2 and 0 for propagation lengths L = 5 and 20 m, respectively. Pulsed EMPs appear only for one field direction at net = 2, indicating the chiral edge transport without counter-propagating modes, while pulsed EMPs measured at net = 0 show the symmetry for the positive and negative B, indicating the helical edge transport. Figure 3 shows waveforms meausred at the net = 0 of B = 9 T for various L. The waveforms broaden along the time axis as L increases. The observed dispersive transport can be understood by considering charge transfer between counter-propagating channels [6]. Our measurement scheme will pave the way for investigations of dynamical properties of topological edge states.

---

Hiroshi Kamata (SFB1238/Kolloquium), NTT Basic Research Labs, Atsugi (Japan)
Seminar Room I. Physik
Contact: Erwann Bocquillon