Theoretical physicists have collaborated with experimental teams to discover evidence of a quantum spin liquid in a material called pyrochlore cerium stannate. They accomplished this feat by integrating cutting-edge experimental methods, such as neutron scattering at extremely low temperatures, with theoretical assessments. By examining how neutrons magnetically interact with electron spins in the pyrochlore material, the researchers detected collective excitations of spins that interact robustly with light-like waves.
For a long time, physicists have speculated about a unique state of matter called a quantum spin liquid. In this state, magnetic particles do not settle into a neatly organized pattern, even at absolute zero temperature. Instead, they persist in a state that is constantly fluctuating and entangled. This odd behavior follows intricate quantum rules and gives rise to emergent properties that echo fundamental aspects of our universe, such as the interactions between light and matter. Nonetheless, experimentally demonstrating the existence of quantum spin liquids and delving into their unique properties has proven to be quite difficult.
A recent study published in Nature Physics by an international team, which included experimental researchers from Switzerland and France and theoretical physicists from Canada and the U.S., including Rice University, presented evidence for this mysterious quantum spin liquid in the material known as pyrochlore cerium stannate. Their success stemmed from using advanced experimental techniques like neutron scattering at extremely low temperatures, combined with theoretical analysis. By measuring the interactions between neutrons and electron spins in pyrochlore, researchers detected the collective excitations of spins that strongly interact with light-like waves.
“Fractional matter quasiparticles, which have long been theorized in quantum spin liquids, required significant advancements in experimental precision to be convincingly validated in this material,” stated Romain Sibille, head of the experimental team at the Paul Scherrer Institute in Switzerland. “The actual neutron scattering experiments were carried out on a specialized spectrometer at the Institut Laue-Langevin in Grenoble, France, enabling us to gather exceptionally high-resolution data.”
“Neutron scattering is a reliable technique for examining the behavior of spins in magnets,” added Andriy Nevidomskyy, associate professor of physics and astronomy at Rice University, who performed the theoretical analysis of the data collected. “However, finding a definitive ‘smoking gun’ signature to confirm the presence of a quantum spin liquid in the material is quite challenging.”
Indeed, a 2022 study by Nevidomskyy demonstrated that refining the theoretical model to effectively match the experimental results is no simple task, requiring detailed numerical exploration of model parameters in conjunction with multiple experiments.
Understanding Spinons and Fractionalization
In the realm of quantum mechanics, electrons possess a feature known as spin, which functions like a tiny magnet. When multiple electrons interact, their spins typically align together or oppose one another. However, in certain crystal structures, like pyrochlores, this normal arrangement can be disrupted. This disruption, referred to as “magnetic frustration,” stops spins from stabilizing into regular patterns, creating conditions where quantum behaviors can occur in remarkable ways, consequently leading to the formation of quantum spin liquids.
“Despite the nomenclature, quantum spin liquids actually exist in solid materials,” noted Nevidomskyy, who has studied the quantum theory of frustrated magnets for countless years.
He explained that the severe geometric frustration within a quantum spin liquid compels electrons to arrange themselves into a quantum mechanical superposition, resulting in fluid-like correlations among electron spins, as if the spins were suspended in a liquid.
“Furthermore, the basic excitations do not consist of a single spin merely shifting from an upward to downward direction or the reverse,” Nevidomskyy elaborated. “Instead, they are these peculiar, delocalized entities that carry half of a single spin degree of freedom; we refer to them as spinons. This occurrence, where a single spin flip effectively divides into two halves, is termed fractionalization.”
A key aspect of this research collaboration between experimental and theoretical physicists involved understanding fractionalization and how these fractional particles interact with one another. Spinons can be compared to having a magnetic charge, and their interactions are similar to electrically charged electrons repelling each other.
“At the quantum level, electrons interact with one another by emitting and reabsorbing light quanta known as photons. In the case of a quantum spin liquid, however, the interaction between spinons is described through the exchange of light-like quanta,” stated Nevidomskyy.
This comparison bridges the study of quantum spin liquids with quantum electrodynamics (QED), which is the theory that details how electrons interact via the exchange of photons—the foundational principle of the Standard Model of particle physics. Likewise, the theory surrounding quantum pyrochlore magnets describes the interactions of spinons through emergent “photons.” However, contrary to the constancy of light speed in our universe, the “light” emergent in these magnets travels significantly slower—roughly 100 times slower than spinons. This major distinction leads to intriguing phenomena such as Cherenkov radiation and an increased likelihood of producing particle-antiparticle pairs. When combined with complementary findings from physicists at the University of Toronto, this research provided clear evidence for QED-like interactions in the experimental data.
“It is genuinely thrilling to see the outcome of this challenging experiment and the dedicated efforts of theorists culminating in such conclusions,” commented Sibille.
Potential Future Applications
This study offers some of the clearest experimental evidence yet regarding quantum spin liquid states and their fractionalized excitations. It validates that materials like cerium stannate can embody these exceptional phases of matter, which not only pique interest for fundamental physics but may also influence quantum technologies like quantum computing. The outcomes also imply that we could manipulate these materials to explore various quantum phenomena, such as the potential existence of dual particles, thereby paving the way for further research.
These dual particles, termed visons, differ from spinons as they carry electric rather than magnetic charge. They bear a resemblance to theoretical magnetic monopoles that were first suggested nearly a century ago by the pioneering quantum mechanic Paul Dirac, who predicted their quantization. Although magnetic monopoles have yet to be observed and high-energy theorists consider them unlikely, the idea continues to captivate modern physics.
“Following this discovery, the prospect of searching for evidence of monopole-like particles in a hypothetical universe composed of electron spins within a material is all the more exciting,” remarked Nevidomskyy.
This research received funding from the Swiss National Science Foundation (R.S. and V.P., Grant No. 200021_179150), the U.S. National Science Foundation Division of Materials Research under award DMR-1917511 (H.Y. and A.H.N.), as well as the Natural Sciences and Engineering Research Council of Canada (F.D. and YB.K.).