When you toss a coin, you put it into a higher-energy state until it falls back down again. It can then end up in one of two possible states: heads or tails. No matter which state the coin was in before, after the toss both outcomes are equally likely.
A team at TU Wien has analyzed a quantum system that also has two equivalent ground states. By supplying energy through ion bombardment, this state can be changed. Remarkably, however, the system behaves very differently from a coin toss: it switches every single time. After ion impact, it reliably ends up in the opposite state. For the experiment, the ion-beam equipment of TU Wien was transported to DESY in Hamburg. The crystals studied were provided by Christian-Albrechts-Universität zu Kiel (CAU), which also participated in the experiments at DESY.
Two possible surface configurations
“We are investigating a very special material, tantalum disulfide, 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 from objects in our everyday experience. The electrons are strongly correlated, meaning they cannot be considered independently. When we fire ions at this material, these ions do not merely interact with a single electron they happen to hit—they effectively interact with the entire collective of electrons at once.”
Tantalum disulfide is also fascinating because its electrons can arrange themselves in two different ways. Just like a coin lying on a table can show either heads or tails, both configurations have the same energy. “The electrons form a hexagonal star-shaped pattern at the surface of the material,” explains Anna Niggas, first author of the new study. “This electronic pattern can be rotated in two different directions—similar to a rotary switch that can be toggled between two positions.”
For many years, the research group at TU Wien has been studying how materials behave under bombardment with energetic ions. To observe what happens to tantalum disulfide under these conditions, the TU Wien ion-beam setups were transported to DESY in Hamburg—a large-scale research facility capable of generating extremely intense radiation that allows detailed investigation of a material’s electronic structure.
A single ion impact throws everything off balance
“When our fast, highly charged ions strike the surface of tantalum disulfide, the electronic system is driven far out of equilibrium,” says Richard Wilhelm. “Some electrons are ejected, others are promoted to higher energy states, and even electrons deeper inside the material are strongly affected.”
After this turbulent phase, the system relaxes back into one of the two possible low-energy ground states. “One might expect that the two states would occur with equal probability—just like heads or tails after a coin toss,” says Anna Niggas. “But surprisingly, that is not what happens. The ion bombardment switches the state of the material. Afterward, the tantalum disulfide surface always ends up in the opposite state from the one it had before.”
This remarkable behavior is a consequence of the complex quantum correlations among the electrons. In the turbulent intermediate phase immediately after ion impact, a new order begins to form locally at the surface. “However, the coupling between the surface electrons and the electrons in the interior of the material is fundamentally altered by the ion impact,” explains Richard Wilhelm. “As a result, the electronic configuration opposite to the original one becomes energetically more compatible with the electrons inside the material. Quantum effects therefore ensure that the outcome is not completely random, as one might naively expect, but instead leads to a predictable switching process.”
Nano Letters
Randomized controlled/clinical trial
Not applicable
Chirality Switching in 1T-TaS2 by Highly Charged Ion Irradiation
6-Feb-2026