Brown University researchers have made great strides in understanding the complex state of matter known as quantum spin liquids. In contrast to standard magnets, which solidify as their temperature decreases, quantum spin liquids remain in a state of fluctuation. A recent study, focusing on the compound H3LiIr2O6, provides insight into the role of disorder in these materials. They found that the quantum liquid state was not imitated or destroyed by disorder, but was significantly changed. This research brings hope for quantum technology, especially in the field of quantum computing.
A study led by Brown University scientists begins to address a long-standing question in condensed matter physics: whether disorder mimics or destroys the quantum liquid state in a prominent compound. Quantum spin liquids are difficult to explain and even harder to understand.
First, quantum spin liquids have nothing to do with everyday liquids like water or juice, but rather with special magnets and how they spin. In a normal magnet, when the temperature is lowered, the spins of the electrons essentially freeze, forming a solid mass. However, in quantum spin liquids, the electrons' spins do not freeze - instead, the electrons remain in a constant flow, as in a free-flowing liquid.
Quantum spin liquids are among the most entangled quantum states ever conceived, and their properties are key to applications that scientists believe could advance the development of quantum technology. Despite 50 years of exploration of quantum spin liquids 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 such evidence because it is so difficult to directly measure quantum entanglement, a phenomenon that Einstein famously dubbed "spooky action at a distance." This phenomenon was called "spooky action at a distance" by Einstein, that is, two atoms are linked together and can exchange information no matter how far apart they are.
The role of disorder in quantum spin liquids
The mystery of quantum spin liquids raises major questions about this exotic material in condensed matter physics that remain unanswered to this day. But in a new paper published in Nature Communications, a team of physicists led by Brown University sets out to unravel one of the most important questions, and does so by introducing a new phase of matter. It all comes down to disorder.
"All materials are disordered to some degree," explained Kemp-Plumb, an assistant professor of physics at Brown University and senior author of the new study, and disorder is related to the number of microscopic ways in which the components of a system are arranged. For example, ordered systems (such as solid crystals) have few ways to rearrange themselves, while disordered systems (such as gases) have no real structure.
In quantum spin liquids, the difference brought about by disorder essentially goes against the theory behind the liquid. One popular explanation is that when disorder is introduced, the material is no longer a quantum spin liquid but simply a magnet in a disordered state. "So the big question is whether quantum spin liquid states can survive disorder, and if so, how?" Plum said.
To solve this problem, the researchers used the world's brightest X-rays to analyze magnetic waves in the compounds they studied, looking for clues of quantum spin liquids. The measurements show that not only does the material not become magnetically ordered (or freeze) at low temperatures, but the disordered states present in the system do not mimic or disrupt the quantum liquid state.
They found that disorder does significantly change this state.
"The quantum liquid state is viable," Plum said. "It doesn't freeze like a normal magnet. It maintains this dynamic state, but it's like a decorrelated version of the dynamic state. Our explanation now is that the quantum spin liquid is broken up into little puddles throughout the material."
Impact and future research
The results basically show that the material they studied is one of the leading candidates for quantum spin liquids, and it does look close to quantum spin liquids, but it has one more ingredient. Researchers believe that this is a disordered quantum spin liquid, a new stage of disordered matter.
"One thing that could happen in this material is that it turns into a disordered version of the non-quantum spin liquid state, but our measurements would tell us that. Instead, our measurements show that it's a very different state," Plum said.
These results deepen our understanding of how disorder affects quantum systems and how to interpret it, which is important for exploring the applications of these materials in quantum computing.
The work is part of a long-standing study of exotic magnetic states in the Plum Lab at Brown University. The study focused on the compound H3LiIr2O6, a material believed to be the best prototype of a special type of quantum spin liquid known as a Kitaev spin liquid. While H3LiIr2O6 is known not to freeze at low temperatures, it is notoriously difficult to produce in the lab, and disorder is known to exist in H3LiIr2O6, which obscures whether H3LiIr2O6 is actually a spin liquid.
Brown University researchers, working with collaborators at Boston College, synthesized the material and then irradiated it with high-energy light using a powerful X-ray system at Argonne National Laboratory in Illinois. Light excites magnetism in compounds, and measuring it from the waves it creates is a workaround for measuring entanglement because it provides a way to see how light affects the entire system.
The researchers hope to continue to expand on this work by improving the method, the materials themselves, and studying different materials.
"The biggest thing going forward is what we've been doing, which is continue to search the really vast space of materials that the periodic table gives us," Plum said. "Now we have a much better understanding of how different combinations of elements affect interactions or create different kinds of disorder that affect spin liquids. We have a lot more guidance, and that's really important because this is really a very broad area to explore."