Laser, Magnet, and Wireless Current express
A thin film of a bismuth-selenium compound sits on a substrate. A laser pulse lasting a millionth of a billionth of a second strikes it. Nearby, a magnet is switched on, with a field weaker than that of a fridge magnet. And at that moment, an electric current spontaneously appears in the film, perpendicular to the magnetic field.
This current doesn't flow through wires: its birth is detected via a terahertz wave emitted from the sample. The stronger the magnetic field, the larger the current. This is how materials with topological surface states behave: on their surface, electrons move like on a highway, where the direction of motion is strictly linked to spin — the electron's intrinsic rotation.
When the laser pulse knocks electrons out of the film's depth, they fly to the surface. Those with spin 'up' accumulate at one edge, those with spin 'down' at the other. Due to the rigid spin-motion coupling, this charge separation turns into a current along the surface. The magnetic field controls how the spin rotates along the way, so the current grows with the field.
But adding just four percent indium to the material made the current drop sharply. Indium destroys those surface highways: electrons no longer stick to the surface, and the spin pumping vanishes. This confirms that the current is born on the surface, not in the film's bulk.
This approach (light, magnet, wave measurement) allows peering into spin-charge conversion without ever touching the sample. For future electronics, where information is carried not only by charge but also by spin, it's like an X-ray for materials: you can see where spin is lost and where it works properly.
Original abstract
as written by the authors arXiv:2601.01030
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.
The terahertz radiation emitted by the sample sits between infrared light and microwaves. It passes through paper, clothing, and plastic, but not through metal — which is why it's used in airport security scanners.