Photogalvanic probing of helical edge channels in 2D HgTe topological insulators

TL;DR

This study demonstrates a circular photogalvanic current in helical edge channels of HgTe 2D topological insulators induced by terahertz radiation, revealing insights into edge electron photoionization and ballistic transport.

Contribution

It provides the first observation and microscopic explanation of photogalvanic effects in helical edge states of 2D topological insulators.

Findings

Photogalvanic current reverses with radiation polarization.

Current peaks near the charge neutrality point.

Conductance approaches 2e^2/h at CNP, indicating ballistic edge transport.

Abstract

We report on the observation of a circular photogalvanic current excited by terahertz (THz) laser radiation in helical edge channels of HgTe-based 2D topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from right-handed to left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a 7 $\mu$m distance at CNP approaches 2e2/h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with the experimental data.