May 18, 2022
First 3D quantum spin fluid confirmation computation test
The computational detective work by American and German physicists has confirmed cerium zirconium pyrochlore to be a 3D quantum spin liquid.

The computational detective work by American and German physicists has confirmed cerium zirconium pyrochlore to be a 3D quantum spin liquid.

Despite the name, quantum spin liquid is the solid material in which quantum entanglement and the geometrical arrangement of the atoms discourages the electrons’ natural tendency to align themselves magnetically in relation to each other. The geometric frustration in quantum spin fluids is so severe that electrons oscillate between quantum magnetic states no matter how cold they become.

A 3D representation of the spin excited continuum – a possible signature of a quantum spin liquid – was observed in 2019 in a single crystal sample of cerium zirconium pyrochlore. Image credit: Tong Chen / Rice University

Theoretical physicists often work with quantum mechanical models that exhibit quantum spin fluids, but finding convincing evidence that they exist in actual physical materials has been a challenge. spanning decades. While some 2D or 3D materials have been suggested as possible quantum spin liquids, the Rice University physicist Andriy Nevidomskyy says there is no consensus among physicists that any of them qualify.

Nevidomskyy hopes that will change based on the results of calculations by him and colleagues from Rice, Florida State University and the Max Planck Institute for Complex Systems Physics in Dresden, Germany, published this month in the open access journal npj Quantum Materials.

“Based on all the evidence that we have today, this work confirms that the single crystals of cerium pyrochlore identified as a candidate 3D quantum spin liquid in 2019 are indeed,” he said. quantum spin fluids with fractional spin excitations.

The inherent property of electrons that lead to magnetism is spin. Each electron behaves like a tiny bar magnet with north and south poles, and when measured, the individual electron spins are always pointing up or down. In most everyday materials, the rotations point up or down randomly. But electrons are antisocial by nature, and this can cause them to arrange their spins in relation to neighboring electrons in some cases. For example, in magnets, the spins are arranged in the same direction, and in antiferromagnetism, they are arranged in an upward, downward pattern.

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At very low temperatures, quantum effects become more prominent, and this causes electrons to align their spins collectively in most materials, even those where the spins would point in directions. randomly at room temperature. Quantum spin fluids are an opposite example where the spins do not point in a definite direction – even up or down – no matter how cold the material becomes.

Nevidomskyy, associate professor of physics and astronomy and a member of the Rice Quantum Initiative and the Rice Center for Quantum Materials ( RCQM). “Individual stimuli are not top-down rotations or vice versa. They are strange, positioned objects bearing half of a degree of free rotation. It’s like a half-turn.”

Nevidomskyy is part of Research in 2019 led by experimental physicist Rice Pengcheng Dai found the first evidence that cerium zirconium pyrochlore is a quantum spin liquid. The team’s samples are the first of their kind: Pyrochlores because of their 2-7 ratio of cerium, zirconium and oxygen, and single crystals because the atoms inside are arranged in a continuous, unbroken lattice. Dai et al.’s inelastic neutron scattering experiment reveals a quantum spin liquid signaturea continuum of spin excitations measured at temperatures as low as 35 millikelvin.

“You could argue that they found the suspect and charged him,” Nevidomskyy said. “Our job in this new study is to prove to the jury that the suspect is guilty.”

Nevidomskyy and colleagues built their case using The Monte Carlo method, exact diagonal as well as analytical tools to perform rotational dynamics calculations for an existing quantum mechanical model of cerium zirconium pyrochlore. Research conceived by Nevidomskyy and Max Planck’s Roderich Moessnerand Monte Carlo simulations performed by Florida State Anish Bhardwaj and Hitesh Changlani with contributions from Rice’s Han Yan and Max Planck’s Shu Zhang.

“The framework for this theory is known, but the exact parameters, of which there are at least four, are not,” says Nevidomskyy. “In different compounds, these parameters can have different values. Our goal is to find those values ​​for cerium pyrochlore and determine if they describe a quantum spin liquid.

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“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 is only predictive if you provide initial conditions such as the initial mass and velocity of the bullet. The initial conditions are similar for these parameters. We had to reverse engineer, or retest, ‘What are the initial conditions inside this cerium material?’ and “Does that match this prediction of quantum spin fluids?”

To build a convincing case, the researchers tested the model based on thermodynamics, neutron scattering, and magnetization results from previously published experimental studies of cerium zirconium pyrochlore. .

“If you only have one proof, you might accidentally find several models that still match the description,” says Nevidomskyy. “We actually matched not one, but three different pieces of evidence. So a candidate must match all three experiments. ”

Several studies have hinted at the same kind of quantum magnetic field oscillations arising in quantum spin liquids as Possible causes for unusual superconductivity. But Nevidomskyy says the computational findings are mainly of interest to physicists.

“This satisfies our innate desire, as physicists, to figure out how nature works,” he said. “There is no app that I know of that would be beneficial. It is not immediately tied to quantum computing, although there has been an idea of ​​using fractional excitations as the basis for logical qubits.”


p id=”caption-attachment-494549″>American and German physicists have found evidence that cerium zirconium pyrochlore crystals are “octpole quantum spin liquids” in which octapole magnetic moments (red and blue) contribute to fractional magnetism. Image credit: A. Nevidomskyy / Rice University

One particularly interesting point for physicists, he says, is the profound connection between quantum spin fluids and the experimental experiments of quantum physics. magnetic monopoletheoretical particles whose potential existence is still being debated by cosmologists and high-energy physicists.

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“When people talk about fractionalization, they mean the system behaves as if a particle of matter, such as an electron, splits into two halves as if to wander off and then recombine,” says Nevidomskyy. somewhere again,” said Nevidomskyy. “And in pyrochlore magnets like the one we’ve studied, these vagabonds behave like quantum magnetic monopoles.”

Magnetic monopoles can be visualized as isolated magnetic poles like the up or down poles of a single electron.

“Of course, in classical physics, one can never isolate just one end of a bar magnet,” he said. “North and south monopoles always go in pairs. But in quantum physics, magnetic monopoles can hypothetically exist, and quantum theorists formulated these nearly 100 years ago to explore fundamental questions about quantum mechanics.

“As far as we know, magnetic monopoles do not exist in their raw form in our universe,” said Nevidomskyy. “But it turns out that a fancy version of monopole exists in these cerium pyrochlore quantum spin fluids. A single spin flip will produce two fractions quasiparticles summon spinons behaves like monopolies and roams around the lattice. ”

The study also found evidence that unipolar spinons are generated in an unusual way in cerium zirconium pyrochlore. Because of the tetrahedral arrangement of the magnetic atoms in pyrochlores, the study shows that they develop octahedral magnetic moments – spin-like quasipars with eight poles – at low temperatures. The study shows that spinons in the material are generated from both these octapole sources and the more conventional dipole torques.

“Our model established the exact ratio of these two components to each other,” says Nevidomskyy. “It opens a new chapter in the theoretical understanding not only of cerium pyrochlore materials but also of eight-maximum quantum spin fluids in general.”

The source: Rice University

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