Computational police work by American and German physicists has confirmed that cerium pyrochloride is a three-dimensional quantum spin liquid.
Despite their name, quantum spin liquids are solid materials in which the quantum entanglement and geometric arrangement of atoms cancel out the natural tendency of electrons to magnetically classify each other. The geometric frustration in a quantum spin liquid is so severe that electrons fluctuate between quantum magnetic states no matter how cold they become.
Theoretical physicists regularly work with quantum mechanical models that produce quantum spin fluids, but finding convincing evidence that they exist in real natural materials has been a challenge for decades. While a number of two-dimensional or three-dimensional materials have been proposed as possible quantum rotation fluids, Rice University physicist Andriy Nevidomskyy said there is no established consensus among physicists that any of them meet the requirements.
Nevidomskyy hopes to change that based on a computer concept he and his colleagues from Rice, Florida State University and the Max Planck Institute for Physics of Complex Systems in Dresden, Germany, published this month in the open access journal npj Quantum Materials.
“Based on all the data we have today, this work confirms that the monocrystals of cerium pyrochloride that were identified as candidate 3D quantum spin liquids in 2019 are indeed quantum spin liquids with fractionated spin stimuli,” he said.
The inherent property of electrons that leads to magnetism is spin. Each electron behaves like a tiny north and south pole magnet, and when measured, the individual electron spins always point up or down. In most everyday materials, the rotations point up or down randomly. But electrons are antisocial in nature, and this can cause them to regulate their rotations relative to their neighbors in some cases. In magnets, for example, the rotations are collectively arranged in the same direction and in anti-iron magnets they are arranged in an up-down, up-down pattern.
At very low temperatures, quantum effects become more apparent, forcing electrons to arrange their spins collectively on most materials, even those where the spins would show random directions at room temperature. Quantum spin liquids are a counterexample, where rotations do not point in a specific direction – even up or down – no matter how cold the material becomes.
“A quantum spin liquid, by its very nature, is an example of a fractionated state of matter,” said Nevidomskyy, an associate professor of physics and astronomy and a member of both the Rice Quantum Initiative and the Rice Center for Quantum Materials (RCQM). . “Individual stimuli are not rotations from top to bottom or vice versa. It is these strange, displaced objects that carry half a degree of freedom of rotation. They are like half a rotation.”
Nevidomskyy was part of the 2019 study led by experimental physicist Rice, Pengcheng Dai, who found the first evidence that zirconium zinc pyrochloride was a quantum spin liquid. The samples of the group were the first of their kind: Pyrochlorides due to the ratio of 2-to-2-to-7 in cerium, zirconium and oxygen and single crystals because the atoms inside were arranged in a continuous, unbroken grid. Inelastic neutron scattering experiments by Dai and colleagues revealed a characteristic quantum spin fluid, a continuous spin excitation measured at temperatures as low as 35 millikelvin.
“You could argue that they found the suspect and charged him with the crime,” Nevidomskyy said. “Our job in this new study was to prove to the jury that the suspect is guilty.”
Nevidomskyy and his colleagues built their case using advanced Monte Carlo methods, precise bidding as well as analytical tools to perform dynamic rotation calculations for an existing quantum zirconium cerium pyrochloride model. The study was designed by Max Planck Nevidomskyy and Roderich Moessner, and the Monte Carlo simulations were performed by Anish Bhardwaj and Hitesh Changlani of Florida, with contributions from Rice’s Han Yan and Max Planck’s Shu Zhang.
“The framework for this theory was known, but the exact parameters, of which there are at least four, were not,” Nevidomskyy said. “In different compounds, these parameters could have different values. Our goal was to find these values for cerium pyrochloride and determine if they describe a quantum spin liquid.”
“He would be like a ballistics expert who uses Newton’s second law to calculate the orbit of a sphere,” he said. “Newton’s law is known, but it is only predictive if you provide the initial conditions, such as mass and the initial velocity of the sphere. These initial conditions are proportional to these parameters. We had to revise or find out” What are these? the initial conditions inside this cerium material? ». and, “Does this match the prediction of this liquid quantum spin?”
To create a convincing case, the researchers tested the model against the effects of thermodynamics, neutron scattering, and magnetization from previously published experimental studies of zirconium cerium pyrochloride.
“If you have only one piece of evidence, you may inadvertently find many models that still fit the description,” Nevidomskyy said. “We actually matched not one, but three different pieces of evidence. So only one candidate had to fit in all three experiments.”
Some studies have blamed the same type of quantum magnetic fluctuations that occur in quantum spin fluids as a possible cause of unconventional superconductivity. But Nevidomsky said computational findings are primarily of fundamental interest to physicists.
“This satisfies our innate desire, as naturalists, to learn how nature works,” he said. “There is no application that I know can benefit. It is not directly related to quantum computing, although there are ideas for using fractional excitations as a platform for logical qubits.”
He said a particularly interesting point for physicists is the deep connection between quantum spin liquids and the experimental implementation of magnetic monopoles, theoretical particles whose possible existence is still being debated by cosmologists and high-energy physicists.
“When people talk about fractionation, what they mean is that the system behaves like a physical particle, like an electron, splitting into two halves orbiting and then recombining somewhere later,” Nevidomskyy said. “And in pyrochloride magnets like the one we studied, these stray objects also behave like quantum magnetic monopoles.”
Magnetic monopoles can be represented as isolated magnetic poles, such as the pole of a single electron facing up or down.
“Of course, in classical physics one can never isolate only one end of a rod magnet,” he said. “North and south monopolies always come in pairs. But in quantum physics, magnetic monopolies can hypothetically exist, and quantum theorists constructed them almost 100 years ago to investigate fundamental questions about quantum mechanics.
“As far as we know, magnetic monopolies do not exist in raw form in our universe,” Nevidomskyy said. “But it turns out that there is a fancy version of monopolies in these quantum rotations in cerium pyrochloride. A single rotation creates two fractionated quasi-particles called spinons that behave like monopoles and orbit around the crystal lattice.”
The study also found evidence that monopole-like spinons were unusually generated in zirconium cerium pyrochloride. Because of the tetrahedral arrangement of magnetic atoms in pyrochloride, the study suggests that they develop octopolar magnetic moments — magnetic quasi-pole-like particles with eight poles — at low temperatures. Research has shown that spinnings in the material were produced both from these octopolar sources and from more conventional, bipolar rotations.
“Our modeling determined the exact proportions of the interactions of these two components,” said Nevidomskyy. “It opens a new chapter in the theoretical understanding not only of cerium pyrochloride materials but also of octapolar quantum spin liquids in general.”
Physicists find the first possible 3D quantum spin fluid
Anish Bhardwaj et al, Detection of exotic fluidity of quantum rotation in Ce2Zr2O7 pyrochloride magnet, npj Quantum Materials (2022). DOI: 10.1038 / s41535-022-00458-2
Provided by Rice University
Reference: Computer sleuthing confirms first 3D quantum liquid spin (2022, May 10) retrieved May 11, 2022 from https://phys.org/news/2022-05-sleuthing-3d-quantum-liquid.html
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