Photoexcitation of electron wave packets in quantum spin Hall edge states: effects of chiral anomaly from a localised electric pulse

TL;DR

Applying a localized electric pulse to quantum spin Hall edges can generate and control electron wave packets through intra-branch transitions, revealing effects of the chiral anomaly and enabling pulse-based wave packet shaping.

Contribution

This work demonstrates that localized electric pulses can photoexcite and manipulate helical edge states without bulk involvement, highlighting the role of the chiral anomaly in wave packet dynamics.

Findings

Wave packets propagate without dispersion after pulse ends.

Space profile depends linearly on pulse parameters, independent of temperature and chemical potential.

Momentum distribution shows non-linear dependence on pulse amplitude and temperature.

Abstract

We show that, when a spatially localised electric pulse is applied at the edge of a quantum spin Hall system, electron wavepackets of the helical states can be photoexcited by purely intra-branch electrical transitions, without invoking the bulk states or the magnetic Zeeman coupling. In particular, as long as the electric pulse remains applied, the photoexcited densities lose their character of right- and left-movers, whereas after the ending of the pulse they propagate in opposite directions without dispersion, i.e. maintaining their space profile unaltered. Notably we find that, while the momentum distribution of the photoexcited wave packets depends on the temperature $T$ and the chemical potential $\mu$ of the initial equilibrium state and displays a non-linear behavior on the amplitude of the applied pulse, in the mesoscopic regime the space profile of the wave packets is independent of $T$ and $\mu$. Instead, it depends purely on the applied electric pulse, in a linear manner, as a signature of the chiral anomaly characterising massless Dirac electrons. We also discuss how the photoexcited wave packets can be tailored with the electric pulse parameters, for both low and finite frequencies.