Computational detective work by American and German physicists has confirmed that zirconium pyrochlore is a three-dimensional quantum spin fluid.
Despite the name, quantum spin fluids are solid materials in which quantum entanglement and the geometric arrangement of atoms prevent the natural tendency of electrons to magnetically order themselves relative to each other. The geometric frustration in quantum spin fluid is so severe that electrons oscillate between quantum magnetic states, no matter how cold they become.
Theoretical physicists typically work with quantum mechanical models that detect quantum spin fluids, but finding convincing evidence that they exist in real physical materials has been a challenge for decades. While a number of 2D or 3D materials have been suggested as possible quantum spin fluids, Rice University physicist Andrei Nevidomsky said there is no consensus among physicists that any of them qualifies.
Unknown hopes this will change based on calculations he and colleagues from Rice, the University of Florida and the Max Planck Institute for Physics of Complex Systems in Dresden, Germany, published this month in an 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 candidates for 3D quantum spin fluids in 2019 are indeed quantum spin fluids with fractionalized spin excitations,” he said.
An integral property of electrons that leads to magnetism is spin. Each electron behaves like a tiny magnet with a north and south pole, and when measuring individual spins the electron is always directed up or down. In most everyday materials rotation is shown up or down randomly. But electrons are antisocial in nature, and this can cause them to organize their backs in relation to their neighbors in certain circumstances. In magnets, for example, the backs are arranged together in one direction, and in antiferromagnets they are arranged up-down, up-down.
At very low temperatures quantum effects become more pronounced, and this causes electrons to organize their spins together in most materials, even in those where the spins will point in random directions at room temperature. Quantum spin fluids are a counterexample when the spins do not point in a certain direction – even up or down – no matter how cold the material becomes.
“Quantum spin fluid is by nature an example of the fractionalized state of matter,” said Nevidomsky, an associate professor of physics and astronomy, a member of the Rice Quantum initiative and the Rice Quantum Materials Center (RCQM). . “Individual arousals are not top-down coups or vice versa. It’s these weird, provocative objects that carry half of one degree of back freedom. It’s like half of the back.”
Unknown participated in a 2019 study led by physicist-experimenter Rice Penchen Day, which revealed the first evidence that zirconium zirconium cerium was a quantum spin fluid. The team samples were the first of their kind: pyrochlores because of their ratio of cerium, zirconium and oxygen 2 to 2 to 7 and single crystals because the atoms inside were placed in a continuous intact lattice. Experiments on inelastic neutron scattering conducted by Dai and his colleagues found a hallmark of quantum spin fluid – a continuum of spin excitations measured at temperatures up to 35 milliquels.
“It can be said that the suspect was found and charged with a crime,” Nevidomsky said. “Our job in this new study was to prove to the jury that the suspect is guilty.”
Nevidomsky and his colleagues built their case using modern Monte Carlo methods, accurate diagonalization, and analytical tools to perform spin dynamics calculations for the existing quantum-mechanical model of cerium pyrchlorine and zirconium. The study was conceived by Nevidsky and Roderick Messner of Max Planck, and the Monte Carlo simulation was performed by Anisha Bhardwaj of Florida and Hitsch Changlani with the participation of Khan Yan Rice and Shu Zhang of Max Planck.
“The foundations of this theory were known, but there were no exact parameters, at least four,” – said Nevidomsky. “In different connections, these parameters may have different values. Our goal was to find these values for cerium pyrochlore and to determine whether they describe quantum spin fluid.
“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 has predictive power only if you provide initial conditions such as the mass and initial velocity of the bullet. These initial conditions are similar to these parameters. conditions inside this cerium material? ‘ and “Does this coincide with the prediction of this quantum spin fluid?”
To construct a compelling case, the researchers tested the model by comparing thermodynamic results, neutron scattering, and magnetization with previously published experimental studies of cerium pyrchlorine and zirconium.
“If you have only one piece of evidence, you may inadvertently find several models that still fit the description,” Nevidomsky said. “We actually compared not one but three different pieces of evidence. Therefore, a single candidate must meet all three experiments. “
In some studies, the same type of quantum magnetic fluctuations that occur in quantum spin fluids are a possible cause of unconventional superconductivity. But Nevidomsky said the results of the calculations 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 be useful. It’s not immediately related to quantum computing, although there are ideas of using fractional excitations as a platform for logical qubits.”
He said that a particularly interesting point for physicists is the deep connection between quantum spin fluids and the experimental implementation of magnetic monopoles, theoretical particles, the possible existence of which is still being discussed by cosmologists and high-energy physicists.
“When people talk about fractionalization, they mean that the system behaves as if a physical particle, like an electron, splits into two halves that wander and then recombine somewhere,” Nevidomsky said. “And in pyrochlorine magnets like the one we studied, these wandering objects also behave like quantum magnetic monopolies.”
Magnetic monopoles can be visualized as isolated magnetic poles, as the up or down pole of a single electron.
“Of course, in classical physics, one end of a rod magnet cannot be isolated,” he said. “Northern and southern monopolies are always in pairs. But in quantum physics, magnetic monopolies can hypothetically exist, and quantum theorists built them nearly 100 years ago to investigate fundamental questions of quantum mechanics.
“As far as we know, magnetic monopolies in our universe do not exist in the raw form,” – said Nevidomsky. “But it turned out that in these quantum spin liquids of cerium pyrochloride there is an intricate version of monopolies. One spin revolution creates two fractionated quasiparticles, called spinons, that behave like monopolies and roam the crystal lattice. ”
The study also found evidence that monopoly spinons were created in an unusual way in cerium and zirconium pyrochlore. Due to the tetrahedral arrangement of magnetic atoms in pyrochlores, the study suggests that they develop eight-polar magnetic moments – spin-like magnetic quasiparticles with eight poles – at low temperatures. The study found that the spinons in the material were derived from both these eight-polar sources and from more conventional, dipolar spin moments.
“Our simulations have established the exact proportions of the interaction of these two components with each other,” said Nevidomsky. “This opens a new chapter in the theoretical sense of not only cerium pyrochlore materials but also octapolar quantum spin fluids in general.”
The study was funded by the Materials Research Division of the National Science Foundation (1917511, 1644779, 2046570, 1742928, 1748958, 1607611), the Welch Foundation (C-1818) and the German Research Foundation 3SFB-1703 SFB-2017. -390858490). The scientists thank the Cowley Institute for Theoretical Physics and the Aspen Physical Center, where part of the research was conducted.
RCQM uses the global partnership and strengths of more than 20 Rice research groups to address issues related to quantum materials. The RCQM is supported by the offices of Vice-Chancellor and Vice-Vice-Chancellor for Rice Research, the Wiss School of Natural Sciences, the Brown School of Engineering, the Smolly-Curl Institute and the Departments of Physics and Astronomy, Electrical Engineering and Computer Science and Materials Science. and NanoEngineering.