Topological insulator maintains quantum spin Hall effect at higher temperatures

Topological insulators could form the basis for revolutionary electronic components. However, as they generally only function at very low temperatures, their practical application has been severely limited to date. Researchers at the University of Würzburg have now developed a topological insulator that also works at higher temperatures. Their results are published in Science Advances.

A topological insulator can be imagined as a material that is a perfect insulator on the inside—it does not conduct electricity there. At its edges, however, it behaves like an almost lossless “electron highway.” Electrons can move along these paths with almost no loss.

To deepen the analogy: these highways have separate lanes for electrons with different “spins”—a kind of intrinsic angular momentum. Electrons with “spin-up” move in one direction, electrons with “spin-down” in the opposite direction. This strict traffic regulation prevents collisions and thus energy losses. The phenomenon behind this is known as the quantum spin Hall effect (QSHE)—an effect that was also first experimentally proven at the University of Würzburg.

A quantum well structure with three layers

The main advantage of this property lies in the possibility of loss-free and spin-polarized transport of electrons, which could form the basis for revolutionary future electronic components. Although this effect has enormous potential, its practical application has faced considerable challenges to date, mainly because topological insulators usually only exhibit their coveted properties at extremely low temperatures—just above absolute zero, which is around -273°C.

A research team at the University of Würzburg, in collaboration with scientists from the University of Montpellier and the École Normale Supérieure in Paris, has now developed a topological insulator that exhibits the desired effect even at significantly higher temperatures: around -213°C, as experiments have shown. A team led by Professor Sven Höfling, Chair of Technical Physics, was responsible for this achievement; Fabian Hartmann and Manuel Meyer are joint first authors.

“We developed and tested a new material system for our experiments: a special quantum well structure consisting of three layers,” explains Höfling. Indium arsenide (InAs) forms the two outer layers of the three-layer structure. GaInSb, an alloy of gallium (Ga), indium (In), and antimony (Sb), forms the middle layer. According to the physicists, this specially developed three-layer structure offers decisive advantages over previous approaches.

A promising candidate for technological applications

“The problem with the materials used to date is often that their band-gap energy is too low,” says Hartmann. Band gap can be thought of as a kind of “energy barrier” that electrons must overcome in order to make the interior of the material conductive. A larger band gap therefore means a more robust barrier that prevents the interior from becoming conductive even at higher temperatures and disrupting the loss-free edge channels.

In fact, the use of a GaInSb alloy increases the band-gap energy of the material. At the same time, the addition of a third InAs layer creates a symmetrical structure that significantly improves the size and robustness of the band-gap energy.

“Our system is a promising candidate for technological applications because it combines three key advantages,” says Meyer. First, it can be manufactured in large quantities and on large scales. Second, the results are reliable and repeatable. And third, the material is compatible with existing silicon-chip technology.

In summary, the physicists believe that these results pave the way for the development of topological electronics. This could also work at less extreme temperatures and be seamlessly integrated into established semiconductor technology, opening the door to a new generation of energy-efficient and powerful devices.