A new state in a quantum material

In the mysterious world of quantum materials, things don't always behave as we expect. These materials have unique properties governed by the rules of quantum mechanics, which often means that they can perform tasks in ways traditional materials cannot – like conducting electricity without loss – or having magnetic properties that may prove useful in advanced technologies.

Magnons in quantum materials

Some quantum materials feature tiny magnetic waves called magnons running through them, which behave in puzzling ways. Understanding magnons helps us unlock secrets of how magnets work at a microscopic level, which is crucial for the next generation of electronics and computers.

Scientists have been studying how these magnons act under strong magnetic fields, and they thought they knew what to expect – until now. In a new study, researchers led by Henrik Rønnow and Frédéric Mila at EPFL have unveiled a new, unexpected behavior in the quantum material strontium copper borate, SrCu₂(BO₃)₂. The study challenges our current understanding of quantum physics but also hints at exciting possibilities for future technologies.

The only game in town

But why this material? The specifics are quite technical, but SrCu₂(BO₃)₂ is important in the field of quantum materials because it is the only known real-world example of the “Shastry-Sutherland model”, a theoretical framework for understanding structures where the arrangement and interactions of atoms prevent them from settling into a simple, ordered state.

These structures are known as “highly frustrated lattices” and often give the quantum material complex, unusual behaviors and properties. So, the unique structure of SrCu₂(BO₃)₂ makes it an ideal candidate for studying complex quantum phenomena and transitions.

Neutron scattering and massive magnetic fields

To study the magnons in SrCu₂(BO₃)₂, the scientists used a technique called neutron scattering. Essentially, they fired neutrons at the material, and measured their deflections off it. Neutron scattering is particularly effective in studying magnetic materials, since neutrons, being neutral in charge can decipher magnetism without being disturbed by the charge of electrons and nuclei in the material.

This work was carried out at the high-field neutron scattering facility at Helmholtz-Zentrum Berlin, which was capable of probing fields up to 25.9 Tesla, making this an unprecedented level of magnetic field study, that allowed the scientists to observe the magnons’ behavior directly.

They then combined the data with “cylinder matrix-product-states” calculations, a powerful computational method that helped confirm the experimental observations from the neutron scattering and understand the material's two-dimensional quantum behaviors.

It takes two to tango

The unique approach revealed something surprising: instead of behaving as single, independent unities – as expected – the material’s magnons were pairing up, forming “bound states” – like pairing up to dance instead of going solo.

This unusual pairing leads to a new, unexpected quantum state that has implications for the material's properties: the “spin-nematic phase”. Think of it like magnets on a fridge: normally, they point north or south (that’s the spin), but this new phase is not about the direction to which they point, but rather how they align with each other, creating a unique pattern.

This is an exciting discovery. It reveals a behavior in magnetic materials never seen before. This uncovering of a hidden rule in quantum physics might lead us to new ways to use magnetic materials for quantum technologies that we haven't even thought of yet.

Other contributors

• Helmholtz-Zentrum Berlin für Materialien und Energie
• Rutherford Appleton Laboratory
• Comprehensive Research Organization for Science and Society (Japan)
• Paul Scherrer Institute
• Carnegie Mellon University in Qatar
• Kyoto University
• Tohoku University