Controlling spinning electrons

Researchers from Linköping University have demonstrated how to combine a commonly used semiconductor with a topological insulator, in a groundbreaking discovery of how to control and transfer spinning electrons. Their work has been published in the journal Nature Communications.

Just as the Earth spins around its own axis, so does an electron, in a clockwise or counter-clockwise direction. ‘Spintronics’ is the name used to describe technologies that exploit both the spin and the charge of the electron. Current applications are limited, with the technology mainly used in computer hard drives. Spintronics does, however, promise great advantages over conventional electronics, including lower power consumption and higher speed.

Meanwhile, researchers recently discovered an exotic phase of matter known as a ‘topological insulator’, which is an insulator on the inside but a conductor on the outside. One of the most striking properties of topological insulators is that an electron must travel in a specific direction along the surface of the material, determined by its spin direction. This property is known as ‘spin-momentum locking’.

“The surface of a topological insulator is like a well-organised divided highway for electrons, where electrons having one spin direction travel in one direction, while electrons with the opposite spin direction travel in the opposite direction,” said PhD student Yuqing Huang, a co-author on the new study. “They can travel fast in their designated directions without colliding and without losing energy.”

These properties make topological insulators promising for spintronic applications. However, one key question is how to generate and manipulate the surface spin current in topological insulators. Now, Huang and his fellow researchers have taken the first step towards transferring spin-oriented electrons between a topological insulator and a conventional semiconductor.

The team generated electrons with the same spin in gallium arsenide (GaAs), a semiconductor commonly used in electronics. To achieve this, they used circularly polarised light in which the electric field rotates either clockwise or counter-clockwise when seen in the direction of travel of the light. The spin-polarised electrons could then be transferred from GaAs to a topological insulator, to generate a directional electric current on the surface.

The researchers found they could control the orientation of spin of the electrons and the direction and the strength of the electric current in the topological insulator bismuth telluride (Bi₂Te₃) — flexibility that has not been available before. This was accomplished without applying an external electric voltage, demonstrating the potential of efficient conversion from light energy to electricity.

The findings are significant for the design of novel spintronic devices that exploit the interaction of matter with light — a technology known as ‘opto-spintronics’. According to study leader Professor Weimin Chen, the combination of the optical properties of GaAs with the electrical properties of the topological insulator “has given us new ideas for designing opto-spintronic devices that can be used for efficient and robust information storage, exchange, processing and read-out in future information technology”.

Image caption: The researchers have taken the first step towards transferring spin-oriented electrons between a topological insulator (orange layer) and a conventional semiconductor (blue layer).