Interplay between Rashba interaction and electromagnetic field in the edge states of a 2D topological insulator

Abstract

The effects of Rashba interaction and electromagnetic field on the edge states of a two-dimensional topological insulator are investigated in a non-perturbative way. We show that the electron dynamics is equivalent to a problem of massless Dirac fermions propagating with an inhomogeneous velocity, enhanced by the Rashba profile with respect to the bare Fermi value $v_F$. Despite the inelastic and time-reversal breaking processes induced by the electromagnetic field, no backscattering occurs without interaction. The photoexcited electron densities are explicitly obtained in terms of the electric field and the Rashba interaction, and are shown to fulfil generalised chiral anomaly equations. The case of a Gaussian electromagnetic pulse is analysed in detail. When the photoexcitation occurs far from the Rashba region, the latter effectively acts as a "superluminal gate" boosting the photoexcited wavepacket outside the light-cone determined by $v_F$. In contrast, for an electric pulse overlapping the Rashba region the emerging wavepackets are squeezed in a manner that depends on the overlap area. The electron-electron interaction effects are also discussed, for both intra-spin and inter-spin density-density coupling. The results suggest that Rashba interaction, often considered as an unwanted disorder effect, may be exploited to tailor the shape and the propagation time of photoexcited spin-polarised wave packets.