Digital detective work verifies liquid magnetic order in past experiments
Computer detective work by American and German physicists has confirmed that cerium-zirconium pyrochlore is a 3D quantum spin liquid.
Despite their name, quantum spin liquids are solid materials in which the quantum entanglement and geometric arrangement of atoms counteract the natural tendency of electrons to magnetically order themselves relative to each other. Geometric frustration in a quantum spin liquid is so severe that electrons fluctuate between quantum magnetic states, no matter how cold.
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, physicist Andriy Nevidomskyy of Rice University said there is no established consensus among physicists on the qualification of one of them.
Nevidomskyy hopes that will change based on computational research he and his colleagues at Rice, Florida State University, and the Max Planck Institute for Complex Systems Physics in Dresden, Germany, published this 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 split spin excitations,” did he declare.
The inherent property of electrons that leads to magnetism is spin. Each electron behaves like a small magnetic bar with a north pole and a south pole, and when measured, individual electron spins always point up or down. In most everyday materials, rotations point up or down at random. But electrons are antisocial in nature, which can cause them to organize their spins relative to their neighbors under certain circumstances. In magnets, for example, the spins are arranged collectively in the same direction, and in antiferromagnets they are arranged in an up-down, up-down pattern.
At very low temperatures, quantum effects become more prominent, causing 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 do not point in a definite direction – even up or down – no matter how cold the material is.
“A quantum spin liquid, by its very nature, is an example of a split 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 aren’t spin flips up and down or vice versa. They’re these weird, delocalized objects that carry half a spin degree of freedom. 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 were arranged in a continuous and uninterrupted network. Inelastic neutron scattering experiments by Dai and his colleagues revealed a liquid characteristic of quantum spin, a continuum of spin excitations measured at temperatures as low as 35 millikelvins.
“It could be argued that they found the suspect and charged him with the crime,” Nevidomskyy said. “Our job in this new investigation was to prove to the jury that the suspect is guilty.”
Nevidomskyy and his colleagues built their case using state-of-the-art Monte Carlo methods, exact diagonalization, and analytical tools to perform the spin dynamics calculations for an existing quantum mechanical model of cerium zirconium pyrochlore. The study was designed 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 Plank.
“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 might have different values. Our goal was to find these 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 such as the mass and initial velocity of the ball. These initial conditions are analogous to these conditions inside this cerium material?’ and, ‘Does this match the prediction of this quantum spin liquid?'”
To build a compelling case, the researchers tested the model against thermodynamic, neutron scattering, and magnetization results from previously published experimental studies of cerium-zirconium pyrochlore.
“If you only have one piece of evidence, you might inadvertently find multiple models that still match the description,” Nevidomskyy said. “We actually matched not one, but three different pieces of evidence. So, only one candidate had to match all three experiences.”
Some studies have implicated the same type of quantum magnetic fluctuations that occur in quantum spin liquids as a possible cause of unconventional superconductivity. But Nevidomskyy said computational discoveries are primarily of fundamental interest to physicists.
“It satisfies our innate desire as physicists to find out how nature works,” he said. “I don’t know of any application that could benefit from this. It’s not immediately related to quantum computing, although there are ideas for using split excitations as a platform for logic qubits.”
He said of particular interest to 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 splitting, what they mean is that the system behaves as if a physical particle, like an electron, splits into two halves that walk around and then recombine somewhere later,” Nevidomskyy said. “And in pyrochlore magnets like the one we studied, these wandering objects incidentally behave like quantum magnetic monopoles.”
Magnetic monopoles can be visualized as isolated magnetic poles as the upward or downward facing 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 raw form in our universe,” Nevidomskyy said. “But it turns out that a sophisticated version of monopoles exists in these cerium pyrochlore quantum spin liquids. A single spin flip creates two split quasiparticles called spinons that behave like monopoles and wander around the crystal lattice. .”
The study also found evidence that monopole-like spinons were unusually created in the cerium-zirconium pyrochlore. Due to the tetrahedral arrangement of the magnetic atoms in pyrochlore, the study suggests that they develop octupole magnetic moments – eight-pole spin-like magnetic quasiparticles – at low temperatures. The research showed that the spinons in the material were produced both from these octupole sources and from more conventional dipole spin moments.
“Our modeling established the exact proportions of the interactions of these two components with each other,” Nevidomskyy said. “This opens a new chapter in the theoretical understanding not only of cerium pyrochlore materials, but also of octupole quantum spin liquids in general.”
The research was funded by the Division of Materials Research of the National Science Foundation (1917511, 1644779, 2046570, 1742928, 1748958, 1607611), the Welch Foundation (C-1818) and the German Research Foundation (SFB-1143-247310070, EXC-2147-390858490). The scientists thank the Kavli Institute for Theoretical Physics and the Aspen Center for Physics where some of the research was done.
The RCQM draws on global partnerships and the strengths of more than 20 Rice research groups to answer questions related to quantum materials. The RCQM is supported by the Offices of the Provost and Vice Provost for Research at Rice, the Wiess School of Natural Sciences, the Brown School of Engineering, the Smalley-Curl Institute, and the departments of Physics and Astronomy, Engineering electrical and computer science and materials science. and Nanoengineering.