PROVIDENCE, R.I. [Brown University] — Quantum spin liquids are difficult to explain and even harder to understand.
To start, they have nothing to do with everyday liquids, like water or juice, but everything to do with special magnets and how they spin. In regular magnets, when the temperature drops, the spin of the electrons essentially freezes and forms a solid piece of matter. In quantum spin liquids, however, the spin of electrons doesn’t freeze — instead the electrons stay in a constant state of flux, as they would in a free-flowing liquid.
Quantum spin liquids are one of the most entangled quantum states conceived to date, and their properties are key in applications that scientists say could catapult quantum technologies. Despite a 50-year search for them and multiple theories pointing to their existence, no one has ever seen definitive evidence of this state of matter. In fact, researchers may never see that evidence because of the difficulty of directly measuring quantum entanglement, a phenomenon Albert Einstein famously termed “spooky action at a distance.” This is where two atoms become linked and able to exchange information no matter how far apart they are.
The mystery around quantum spin liquids has led to major questions about this exotic material in condensed matter physics that have to this point gone unanswered. But in a new paper in Nature Communications, a team of Brown University-led physicists begins to shed light on one of the most important questions, and does so by introducing a new phase of matter.
It all comes down to disorder.
Kemp Plumb, an assistant professor of physics at Brown and senior author of the new study, explains that “all materials on some level have disorder” and that disorder has to do with the number of microscopic ways components of a system can be arranged. An ordered system, like a solid crystal, has very few ways to rearrange it, for instance, while a disordered system, like a gas, has no real structure to it.
In quantum spin liquids, disorder introduces discrepancies that essentially butt heads with the theory behind the liquids. One prevailing explanation was that when disorder is introduced, the material ceases to be a quantum spin liquid and instead is simply a magnet that’s in a state of disorder. “So, the big question was whether the quantum spin liquid state survives in the presence of disorder and if it does survive, how?” Plumb said.
The researchers addressed the question by using some of the brightest X-rays in the world to analyze magnetic waves in the compound they studied for tell-tale signatures that it’s a quantum spin liquid. The measurements showed that not only does the material not magnetically order (or freeze) at low temperatures, but that the disorder that’s present in the system doesn’t mimic or destroy the quantum liquid state.
It does significantly alter it, they found.
“The quantum liquid state sort of survives,” Plumb said. “It doesn't do what you would expect a normal magnet to do where it just freezes. It stays in this dynamic state, but it's like a de-correlated version of that dynamic state. Our interpretation right now is the quantum spin liquid is broken up into little puddles throughout the material.”