Abstract-Edge photocurrent driven by terahertz electric field in bilayer graphene

S. Candussio, M. V. Durnev, S. A. Tarasenko, J. Yin, J. Keil, Y. Yang, S. -K. Son, A. Mishchenko, H. Plank, V. V. Bel’kov, S. Slizovskiy, V. Fal’ko, and S. D. Ganichev

https://journals.aps.org/prb/accepted/2a075O28F8a1563cb8704928a7c656be3f612e62c

We report on the observation of edge electric currents excited in bi-layer graphene by terahertz laser radiation. We show that the current generation belongs to the class of second order in electric field phenomena and is controlled by the orientation of the THz electric field polarization plane. Additionally, applying a small magnetic field normal to the graphene plane leads to a phase shift in the polarization dependence. With increasing the magnetic field strength, the current starts to exhibit $1/B$-magnetooscillations with a period consistent with that of the Shubnikov-de Haas effect and amplitude by an order of magnitude larger as compared to the current at zero magnetic field measured under the same conditions. The microscopic theory developed shows that the current is formed in the edges vicinity limited by the mean-free path of carriers and the screening length of the high-frequency electric field. The current originates from the alignment of the free carrier momenta and dynamic accumulation of charge at the edges, where the $P$-symmetry is naturally broken. The observed magnetooscillations of the photocurrent are attributed to the formation of Landau levels.

Abstract-Photogalvanic probing of helical edge channels in two-dimensional HgTe topological insulators

K.-M. Dantscher, D. A. Kozlov, M. T. Scherr, S. Gebert, J. Bärenfänger, M. V. Durnev, S. A. Tarasenko, V. V. Bel'kov, N. N. Mikhailov, S. A. Dvoretsky, Z. D. Kvon, J. Ziegler, D. Weiss, and S. D. Ganichev

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.201103

We report on the observation of a circular photogalvanic current excited by terahertz laser radiation in helical edge channels of two-dimensional (2D) HgTe topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from a right-handed to a 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 $\approx 3\mu m$ distance at CNP approaches $2e^2/h$, 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 experimental data.

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