A donut is not a breakfast roll. These are two very clearly distinguishable objects: one has a hole, the other does not. In mathematics, the two shapes are said to be topologically different – one cannot be transformed into the other by small, continuous deformations. This means the difference between them is robust to perturbations – even if you knead and bend the bun it still doesn't look like a donut.

Such topological properties also play an important role in material science, albeit in a somewhat more abstract way. If a material property can be explained topologically, then it is also robust to disturbances: a change in environmental conditions does not make it disappear.

Now, however, a research team has found a way to switch the topological properties of a quantum material on and off. In paper in *Nature Communications*, the researchers report that even though certain material states are stable against disturbances over a wide range of parameters, at a certain magnetic field they can still be switched off completely. This makes topological material properties manipulable for the first time.

In physics, the ‘topological properties’ of a material have nothing to do with its geometric shape – it is not about crystal samples that are donut-shaped or spherical. Rather, the term ‘topological properties’ refers to the complex interaction of the many electrons in the material.

This interaction can be represented mathematically in very specific ways. It is often useful not to think about the position of electrons in a material, but rather their momentum – or in other words, about their position in an abstract ‘momentum space’. In such mathematical spaces, certain properties of a material can be studied. These properties can be distinguished from each other according to topological criteria – similar to the donut and bun.

"Finding such topological properties is an exciting thing in itself; in 2016, the Nobel Prize in Physics was awarded for the discoveries of such states," says Silke Bühler-Paschen from the Institute of Solid State Physics at the Vienna University of Technology (TU Wien) in Austria. "But we have now been able to show something completely new: we have succeeded for the first time in manipulating and even switching off such topological states."

The researchers did this with a special material made of cerium, bismuth and palladium. Bühler-Paschen's research group had already made several spectacular discoveries in previous years using this topological material. For example, they were able to demonstrate exotic topological behavior by precisely measuring the material’s electrical and thermal properties.

This exotic behavior arises from the fact the electric charge in this material moves in a peculiar way. In an ordinary electrically conductive material, current flows as a result of individual electrons moving through the material. In this topological material, however, the situation is quite different. Here, the interaction of many charge carriers creates very special ‘quasiparticles’ – a collective excitation of the charge carriers that can propagate through the material. This is similar to the way sound propagates through air as a density wave without individual air particles having to move from the sound source to the sound receiver.

These collective excitations move very slowly in this material; in a sense, they do not move past each other very well. This means the topological properties of the material in momentum space have particularly strong consequences.

"Our measurements show that these electrical and thermal properties are indeed robust, as one would expect from topological material properties," says Bühler-Paschen. Small impurities or external disturbances do not bring about a dramatic change. "But surprisingly, we found out: with an external magnetic field, you can control these topological properties. You can even make them disappear completely at a certain point. So we have stable, robust properties that you can selectively turn on and off."

This control is made possible by the internal structure of the excitations responsible for charge transport. These excitations not only carry electric charge but also a magnetic moment – and this makes it possible to switch them on and off using a magnetic field.

"If you apply an ever-stronger external magnetic field, you can imagine these charge carriers to be pushed closer and closer together until they meet and annihilate each other – similar to a matter particle and an antimatter particle if you let them collide," says Bühler-Paschen.

The experiments were conducted at TU Wien, but for some additional measurements the team was able to use high-field laboratories in Nijmegen (Netherlands) and at Los Alamos National Laboratory. Theoretical support was provided by researchers at Rice University.

"This newly discovered controllability makes the topological materials that have already attracted so much attention in physics even more interesting," says Bühler-Paschen. Possibly, these switchable topological states could be used for sensor or switching technology.

It is precisely because the excitations in the material are so slow, and therefore have a very low energy, that they are so interesting. This means the excitations will couple to radiation in the microwave range, which is particularly important for many technical applications. Entirely new, more exotic applications in electronics, including quantum computers, are also conceivable.

*This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.*