Computational detection work by American and German physicists has confirmed that cerium zirconium pyrochlore is a three-dimensional quantum spin liquid.
Despite the name, quantum spin liquids are solid materials in which quantum entanglement and the geometric arrangement of atoms thwart the natural tendency of electrons to magnetically arrange themselves with one another. The geometric frustration in a quantum spin liquid is so severe that electrons fluctuate between quantum magnetic states no matter how cold they get.
Theoretical physicists routinely work with quantum mechanical models that manifest quantum spin liquids, but finding convincing evidence that they exist in real physical materials has been a decades-long challenge. While a number of 2D or 3D materials have been proposed as possible quantum spin liquids, Rice University physicist Andriy Nevidomskyy has said that there is no established consensus among physicists that any of them qualify.
Nevidomskyy hopes that will change based on computational research that he and 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 evidence we have today, this work confirms that the cerium pyrochlore single crystals identified as 3D quantum spin liquid candidates in 2019 are indeed quantum spin liquids with fractional spin excitations,” he said.
The inherent property of electrons that leads to magnetism is spin. Each electron behaves like a tiny bar magnet with north and south poles, and when measured, the individual electron spins always point up or down. In most everyday materials, the turns point up or down randomly. But electrons are antisocial by nature, and this can cause them to organize their spins relative to their neighbors in some circumstances. In magnets, for example, the spins collectively arrange themselves in the same direction, and in antiferromagnets they arrange themselves in a top-down, top-down pattern.
At very low temperatures, quantum effects become more prominent and this causes the electrons to organize their spins collectively in most materials, even those where the spins would point in random directions at room temperature. Quantum spin liquids are a counterexample, where the spins don’t point in a definite direction, not even up or down, no matter how cold the material gets.
“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 the Rice Quantum Initiative and the Rice Center for Quantum Materials (RCQM). . . “Individual excitations are not spins from top to bottom or vice versa. It’s these strange, delocalized objects that have half a degree of freedom to spin. It’s like half a spin.”
Nevidomskyy was part of the 2019 study led by Rice experimental physicist Pengcheng Dai that found the first evidence that cerium zirconium pyrochlore was a quantum spin liquid. The team’s samples were the first of their kind: pyrochlores because of their 2:2:7 ratio of cerium, zirconium, and oxygen, and single crystals because the atoms inside them were arranged in a continuous, unbroken lattice. Inelastic neutron scattering experiments by Dai and his colleagues revealed a quantum spin liquid hallmark, a continuum of spin excitations measured at temperatures as low as 35 millikelvin.
“It could be argued 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 colleagues built their case using state-of-the-art Monte Carlo methods, exact diagonalization, and analytical tools to perform spin dynamics calculations for an existing quantum mechanical model of cerium and zirconium pyrochlore. The study was conceived by Nevidomskyy and Roderich Moessner of Max Planck, and the Monte Carlo simulations were performed by Anish Bhardwaj and Hitesh Changlani of Florida State, with contributions from Han Yan of Rice and Shu Zhang of Max Planck.
“The framework of this theory was known, but the exact parameters, of which there are at least four, were not known,” Nevidomskyy said. “In different compounds, these parameters could have different values. Our goal was to find those values for cerium pyrochlore and determine if they describe a quantum spin liquid.”
“It would be like a ballistics expert using Newton’s second law to calculate the trajectory of a bullet,” he said. “Newton’s law is known, but it only has predictive power if you provide the initial conditions, like the mass and the initial speed of the bullet. Those initial conditions are analogous to these parameters. We had to reverse engineer, or investigate, ‘What? What are those conditions inside this cerium material?’ and, ‘Does that match the prediction of this quantum spin liquid?'”
To build a convincing case, the researchers tested the model against thermodynamic, neutron scattering, and magnetization results from previously published experimental studies on cerium and zirconium pyrochlore.
“If you only have one piece of evidence, you may inadvertently find multiple models that still fit the description,” Nevidomskyy said. “In reality, we matched not one, but three different tests. So a single candidate had to match all three experiments.”
Some studies have implicated the same type of quantum magnetic fluctuations that arise in quantum spin liquids as a possible cause of unconventional superconductivity. But Nevidomskyy said the computational findings are primarily of fundamental interest to physicists.
“This satisfies our innate desire, as physicists, to find out how nature works,” he said. “I don’t know of any applications that could benefit. It’s not directly linked to quantum computing, although there are ideas to use fractional excitations as a platform for logical qubits.”
He said that a particularly interesting point for physicists is the deep connection between quantum spin liquids and the experimental realization of magnetic monopoles, theoretical particles whose potential existence is still debated by cosmologists and high-energy physicists.
“When people talk about fractionation, what they mean is that the system behaves as if a physical particle, like an electron, split into two halves that wander around and then recombine somewhere later,” Nevidomskyy said. “And in pyrochlore magnets like the one we studied, these wandering objects also behave like quantum magnetic monopoles.”
Magnetic monopoles can be visualized as isolated magnetic poles like the up or down pole of a single electron.
“Of course, in classical physics you can never isolate just one end of a bar magnet,” he said. “North and south monopoles always come in pairs. But in quantum physics, magnetic monopoles can hypothetically exist, and quantum theorists built them nearly 100 years ago to explore fundamental questions about quantum mechanics.
“As far as we know, magnetic monopoles do not exist in a pure form in our universe,” Nevidomskyy said. “But it turns out that there is an elegant version of monopoles in these cerium pyrochlore quantum spin liquids. A single spin change creates two fractionalized quasiparticles called spinons that behave like monopoles and wander the crystal lattice.”
The study also found evidence that monopole-like spinons were created in an unusual way in cerium and zirconium pyrochlore. Due to the tetrahedral arrangement of the magnetic atoms in pyrochlore, the study suggests that they develop octupole magnetic moments (spin-like magnetic quasiparticles with eight poles) at low temperatures. The investigation showed that the spinons in the material were produced both from these octupole sources and from more conventional dipole spin moments.
“Our model established the exact proportions of the interactions of these two components with each other,” said Nevidomskyy. “It opens a new chapter in the theoretical understanding of not only cerium pyrochlore materials, but octupolar quantum spin liquids in general.”
Physicists find the first possible three-dimensional quantum spin liquid
Anish Bhardwaj et al, investigating the exotic quantum spin liquidity in the pyrochlore magnet Ce2Zr2O7, npj quantum materials (2022). DOI: 10.1038/s41535-022-00458-2
Provided by Rice University
Citation: Computational Research Confirms First 3D Quantum Spin Liquid (May 10, 2022) Retrieved May 11, 2022 at https://phys.org/news/2022-05-sleuthing-3d-quantum-liquid. html
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