Physicists have long theorized the existence of a quantum spin liquid, a unique state of matter where magnetic particles remain in a constantly fluctuating, entangled state, even at absolute zero temperature. In this state, the particles do not settle into an orderly pattern, and their behavior is governed by complex quantum rules. This leads to emergent properties that resemble fundamental interactions of light and matter. However, experimentally confirming the existence of quantum spin liquids and exploring their properties has proven to be a major challenge.
An international team of researchers from Switzerland, France, Canada, the U.S., and Rice University recently published a paper in Nature Physics in which they present evidence of a quantum spin liquid in a material called pyrochlore cerium stannate. The breakthrough was made possible by combining advanced experimental techniques, such as neutron scattering at extremely low temperatures, with theoretical analysis. The researchers measured how neutrons interact magnetically with electron spins in the material, observing collective excitations of spins interacting strongly with lightlike waves.
"Fractional matter quasiparticles, which have long been theorized in quantum spin liquids, required significant advancements in experimental resolution to be convincingly tested in this material," said Romain Sibille, leader of the experimental team at the Paul Scherrer Institute in Switzerland. The experiment was conducted using a highly specialized spectrometer at the Institut Laue-Langevin in Grenoble, France, which provided the high-resolution data necessary for the study.
"Neutron scattering is a well-established tool for studying spins in magnets," explained Andriy Nevidomskyy, associate professor of physics and astronomy at Rice University, who conducted the theoretical analysis of the data. "However, it's very difficult to identify an unambiguous 'smoking gun' signature that would confirm the presence of a quantum spin liquid in the material."
In 2022, Nevidomskyy showed that narrowing the theoretical model to describe the experiment was a complex task, requiring detailed numerical analysis to fit the model to experimental data.
Spinons and Fractionalization
Electrons in quantum mechanics possess a property called spin, which behaves like a miniature magnet. When many electrons interact, their spins typically align or anti-align. However, certain crystal structures, like pyrochlores, can disrupt this alignment, preventing spins from settling into an ordered pattern. This phenomenon, known as "magnetic frustration," creates conditions for extraordinary quantum behavior, such as the emergence of quantum spin liquids.
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"Despite their name, quantum spin liquids are solid materials," Nevidomskyy said, explaining that the extreme geometric frustration in a quantum spin liquid forces electrons to form a quantum mechanical superposition. This creates fluidlike correlations between the electron spins, akin to spins being immersed in a liquid.
In a quantum spin liquid, the elementary excitations are not simple spin flips, but rather delocalized objects that carry half of a spin degree of freedom. These entities are known as spinons. The process where a single spin flip splits into two is called fractionalization.
Understanding how these fractional particles interact was crucial to this study. Spinons, which carry a magnetic charge, interact in a way similar to how electrically charged particles repel each other. At the quantum level, electrons interact by emitting and reabsorbing photons, and in a quantum spin liquid, the interaction between spinons involves the exchange of lightlike quanta.