Terahertz magneto-photocurrents in the topological insulator Bi$_2$Se$_3$ probe its topological surface states

TL;DR

This study investigates ultrafast magneto-photocurrents in topological insulator Bi2Se3 films, revealing how magnetic fields and indium doping influence surface states and spin-charge conversion, with implications for contact-free spin probing.

Contribution

It demonstrates the measurement of terahertz magneto-photocurrents in topological insulators and links photocurrent behavior to the quenching of topological surface states by indium doping.

Findings

Photocurrent scales linearly with external magnetic field and is perpendicular to it.

Indium doping above 4% causes an abrupt reduction in photocurrent.

Photocurrent dynamics are explained by spin-polarized electrons propagating and converting spin into charge current.

Abstract

We study ultrafast magneto-photocurrents in a three-dimensional topological insulator. For this purpose, we excite (In$_r$Bi$_{1-r}$)$_2$Se$_3$ thin films with a femtosecond laser pulse in the presence of an external magnetic field $B_{\text{ext}}$ up to 0.3 T parallel to the film plane. The resulting in-plane photocurrent is measured by detecting the emitted terahertz (THz) electromagnetic pulse. It scales linearly with $B_{\text{ext}}$ and is perpendicular to $B_{\text{ext}}$. Strikingly, for $r\ge$4%, we observe an abrupt photocurrent reduction, which is strongly correlated with the Indium-induced quenching of the topological surface states. The rise time, decay time and amplitude of the THz magneto-photocurrent can consistently be explained by a scenario in which optically excited spin-polarized electrons propagate toward the film surface where the accumulated spin is converted into an in-plane charge current due to spin-velocity locking. Our results are highly relevant for contact-free probing of spin-charge conversion in systems with paramagnetic rather than spontaneous magnetic order.