New route to quantum spin liquid materials discovered

A new route to materials with complex disordered magnetic properties at the quantum level has been produced by scientists for the first time. The material, based on a framework of ruthenium, fulfills the requirements of the Kitaev quantum spin liquid state—an elusive phenomenon that scientists have been trying to understand for decades.

Published in Nature Communications, the study by scientists at the University of Birmingham offers an important step towards achieving and controlling quantum materials with sought-after new properties that do not follow classical laws of physics.

Crucially, the materials provide a route to magnetic properties which behave differently from conventional “ferromagnets,” ordered around two poles. Ferromagnets—which include the familiar bar magnets found on fridges or noticeboards—contain electrons which interact with each other, each functioning as a tiny magnet to attract and repel, so that they all point in the same direction, giving the magnet its force.

Quantum spin liquid materials have magnetic properties that don’t behave in that way. Instead of the well-ordered characteristics of ferromagnets, these materials are disordered and the electrons within them connect magnetically via a process called quantum entanglement.

Although quantum spin liquids exist in theory and have been modeled by scientists, it has not previously been possible to produce them experimentally or to find them in nature.

In the new study, researchers describe the properties of a novel ruthenium-based material that opens up new pathways for exploring these states of matter.

Lead researcher Dr. Lucy Clark explains, “This work is a really important step in understanding how we can engineer new materials that allow us to explore quantum states of matter. It opens up a large family of materials that have so far been underexplored and which could yield important clues about how we can engineer new magnetic properties for use in quantum applications.”

While there are a number of naturally occurring copper minerals and mineral crystal systems in which scientists believe quantum spin liquid state might exist, these have not been proved due to the additional structural complexities found in nature.

The complexity of quantum spin liquids poses difficulties for theorists too, because modeling results in many competing magnetic interactions, which are extremely difficult to untangle, causing disagreement among physicists.

A model produced by the theoretical physicist Alexei Kitaev in 2009 was able to demonstrate some foundational principles for quantum spin liquids. However, the magnetic interactions it described required an environment that scientists have been unable to produce experimentally without the materials reverting to a conventionally ordered magnetic state.

It is thought this behavior is connected to the densely packed crystal structures of candidate materials. Because the ions are packed so closely together, they are able to interact directly with each other, resulting in them reverting to magnetic order.

Using specialist instruments at the UK’s ISIS Neutron and Muon Source, and Diamond Light Source, the Birmingham-based team were able to show that a new material with an open framework structure can tune the interactions between the ruthenium metal ions, providing a new route to the Kitaev quantum spin liquid state.

Importantly, the magnetic interactions produced within these more open structures are weaker than they might otherwise be, giving scientists more scope to tune their precise behaviors.

“While this work has not led to a perfect Kitaev material, it has demonstrated a useful bridge between theory in this field and experimentation, and opened up fruitful new areas for research,” added Dr. Clark.