Topological materials become switchable

A donut is not a breakfast sandwich. These are two very distinct objects: one has a hole and the other doesn’t. In mathematics, the two forms are described as topologically different – they cannot be converted into one another by small, continuous deformations. Therefore, the difference between them is robust to disturbances: 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 materials science, albeit in a somewhat more abstract form. If a material property can be explained topologically, then it is also robust to disturbances: a change in the environmental conditions does not make it disappear. A research team has now succeeded for the first time in switching such a topological property in a targeted manner: Certain material states are stable to disturbances over a wide range of parameters, but can be switched off completely at a certain magnetic field. This means that topological material properties can be manipulated for the first time.

Geometry in abstract spaces

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

This interaction can be represented mathematically in a very specific way. It often makes sense not to think about the position of the electrons, but about their momentum – or to put it another way: about their position in an abstract “momentum space”. In such mathematical spaces, certain properties of the material can be examined, which can be distinguished from one another according to topological criteria – like donuts and bread rolls.

“Finding such topological properties is in itself an exciting thing, for the discovery of such states the Nobel Prize for Physics was awarded in 2016,” says Prof. Silke Bühler-Paschen from the Institute for Solid State Physics at the TU Vienna. “But now we have been able to show something completely new: for the first time, we have managed to manipulate and even switch off such topological states.”

Extreme topological effects on slow charge carriers

A special material made of cerium, bismuth and palladium was used for this. Using this material, Bühler-Paschen’s research group had already made several spectacular discoveries in recent years. For example, they were able to demonstrate exotic topological behavior in this material by precisely measuring its electrical or thermal properties.

This behavior results from the peculiar motion of the electric charge in this material. In an ordinary electrically conductive material, current simply flows through individual electrons moving through the material. With this particular material, however, things are different.

The interaction of many charge carriers creates very special “quasi-particles” here – a collective excitation of the charge carriers, which can propagate through the material, similar to how sound can propagate through air as a density wave, without individual air particles having to move from the sound source to the sound receiver.

These excitations move very slowly in this material. In a way, they don’t get along very well. And that means that the topological properties of the material in the momentum space have a particularly strong effect.

Turn off topological properties

“Our measurements show that these electrical and thermal properties are actually 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 that these topological properties can be controlled with an external magnetic field. You can even make them disappear entirely at a certain point. So we have stable, robust properties that you can switch on and off in a targeted manner.”

This control is made possible by the internal structure of the excitations that are responsible for charge transport: They not only carry electric charge, but also a magnetic moment – and this makes it possible to switch them through a magnetic field.

“If you apply an ever stronger external magnetic field, you can imagine that these charge carriers are pushed ever closer together until they collide and destroy each other – similar to a matter particle and an antimatter particle if you let them collide,” says Silke Bühler Paschen .

Worldwide search for exciting applications

The experiments were carried out at TU Wien (Vienna), but for some additional measurements the team was able to use high-field laboratories in Nijmegen (Netherlands) and Los Alamos National Laboratory (USA). Theoretical support came from Rice University (USA).

Silke Bühler-Paschen is convinced that this newly discovered controllability makes the topological materials, which are already receiving so much attention in physics, even more interesting.

The switchable topological states could possibly be used for sensor or switching technology. Precisely because the excitations in the material are so slow and therefore very low in energy, they are particularly interesting: the excitations couple to radiation in the microwave range, which is particularly important for many technical applications. Completely new, more exotic applications in electronics up to and including quantum computers are also conceivable.

The study was published in nature communication.