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Topic 2: Astonishing Bombardment of Electrons With Ions

When you toss a coin, you put it into a state of higher energy until it falls down again, it is said. It can then assume two different states—heads or tails, as is well known. Regardless of which state the coin was in before, both variants are equally likely after the toss, provided the coin is not unbalanced but ideally shaped. So much for the prelude! A team from the Vienna University of Technology has now analyzed a quantum system in which there are also two equal basic states: If you add energy to it by bombarding it with ions, you can change this state.

Surprisingly, however, this system behaves quite differently to the coin, as it switches every time. It reliably ends up in the opposite state after the ion bombardment. For the experiment, the equipment from TU Wien was transported to DESY (Deutsches Elektronen-Synchrotron) in Hamburg. The crystals studied came from the Christian-Albrechts-Universität zu Kiel, which was also involved in the experiments at DESY.

There Are Two Possible Surface Configurations

"We are investigating a very special material, namely tantalum disulphide, the quantum material 1T-TaS2," says Richard Wilhelm from the Institute of Applied Physics at TU Wien. The particles in this material behave very differently to what we are used to from everyday objects, he explains. This is because the tantalum disulphide electrons are strongly correlated with each other. This means that they cannot be viewed independently of each other. Wilhelm: "When we shoot ions at this material, these ions don't just interact with one electron that they hit there, they interact with the entire electron collective at once, so to speak."

Another interesting aspect of tantalum disulphide is that the electrons can be arranged in two different ways. Just like the coin, which can lie on the table with the heads or tails side up, both of these variants have the same energy. "The electrons create a hexagonal star pattern on the surface of the material," explains Anna Niggas, the first author of the new paper. However, this electron pattern can be rotated in two different directions, similar to a rotary switch that can be switched back and forth between two positions.

Ion Bombardment Actually Messes Everything Up

The research group at TU Wien has also been investigating for many years how materials behave when bombarded with high-energy ions. To see what happens to tantalum disulphide in particular in this situation, the TU Vienna's ion systems were transported to DESY in Hamburg (Germany)—a large-scale research facility that can generate extremely intense radiation that can be used to study the electrons of the material. "When our fast, highly charged ions hit the tantalum disulphide surface, the electron system is thrown massively out of balance," says Wilhelm. However, some electrons are knocked out, while others move to higher energy states. Electrons that are located further inside the material are also massively affected, it is said.

After this turbulent phase, the system returns to one of the two possible ground states with low energy. Now you might think that the probability of each of the two possible states is 50 percent—just like a tossed coin. But surprisingly, this is not the case! The ion bombardment actually switches the state of the material. The tantalum disulphide surface is then in a different state than before. This surprising property has to do with the complex quantum correlations of the electrons. In the turbulent intermediate phase, directly after the ion impact, a new order initially begins to form selectively on the material surface. However, the coupling between the electrons on the surface and the electrons inside the material is completely different than before due to the ion impact. This means that in this case, the opposite electron configuration is energetically better suited to the electrons inside the material. Quantum effects therefore ensure that the result is not completely random, but that there is even a predictable switchover.