Ultracold atoms reveal a new type of quantum magnetic behavior


MIT and Harvard researchers have studied how the primary units of magnetism, called spins (black arrows), move around and interact with other atoms, in a series of single atoms (colored spheres). The background shows a true image of the spin, revealing a high contrast periodic modulation of the blue (spin up) atoms. Sincerely: Courtesy of Researchers

The findings may help researchers design “spintronic” devices and novel magnetic materials.

A new study illuminates the amazing choreography between spinning atoms. In a paper published in the journal Nature, From researchers MIT And Harvard University explains how magnetic forces in quantum affect the atomic scale of how atoms orient their spin.

In experiments with ultraviolet lithium atoms, researchers observed the different ways in which atoms’ spins develop. Positioning back upward like the tippy ballerinas, the spinning atoms return to an equilibrium orientation that depends on the magnetic forces between the individual atoms. For example, atoms can balance in an extremely fast, “ballistic” fashion or in a slower, more diffuse pattern.

Researchers found that these behaviors, which were not observed until now, could be described mathematically by the Heisenberg model, a set of equations commonly used to predict magnetic behavior. Their results address the fundamental nature of magnetism, revealing the diversity of behavior in one of the simplest magnetic materials.

This better understanding of magnetism can help engineers design “spintronic” devices to transmit, process and store information using the spin of quantum particles rather than the flow of electrons.

“By studying one of the simplest magnetic materials, we have advanced the understanding of magnetism,” John D. of Physics at MIT. Says Arthur Professor and MIT team leader Wolfgang Catterall. “When you find a new phenomenon in one of the simplest models of physics for magnetism, you have a chance to fully describe and understand it. It’s what gets me out of bed in the morning, and excites me. Does. “

Ketelaar co-authors are MIT graduate students and MIT postdocs at JC-Ametto Grill, Ivana Dimitrova, both MIT postdocs, Wayne Wei Ho, Harvard University and Stanford University, and MIT graduate student with physics professor and lead author . At Harvard. There are researchers at the MIT-Harvard Center for all ultracold atoms. The MIT team is affiliated with the Department of Physics and Research Laboratory of the Institute of Electronics.

Sprain wire

Quantum spin is considered a microscopic unit of magnetism. On a quantum scale, atoms can spin clockwise or counterclockwise, which gives them an orientation like a compass needle. In magnetic materials, the spin of several atoms can show a wide variety of phenomena, including equilibrium states, where Atom Spins are aligned, and have dynamic behavior, where several atoms resemble a wave-like pattern.

It is this latter pattern that was studied by the researchers. The dynamics of the wavelength spin pattern is very sensitive to magnetic forces between atoms. The wavy pattern for the isotropic magnetic forces dissipated much faster than for isotropic forces. (Isotropic forces do not depend on how all spins are oriented in space).

Ketterley’s group conducted an experiment aimed at studying this phenomenon, in which they used laser-cooling technology for the first time, in which lithium atoms were cooled to about 50 nanocelvins – 10 million times more than in interstellar space.

At such ultracold temperatures, the atoms are frozen near a constant standstill, so that researchers can see in detail any magnetic effects that would otherwise be masked by the thermal motion of the atoms. The researchers then used a system of lasers to trap and arrange multiple strings with 40 atoms, like beads on a string. Altogether, they produced a mesh of about 1,000 wires, containing about 40,000 atoms.

“You can think of lasers as tweezers that hold atoms, and they will survive if they are heated,” Jepsen explains.

He then applied a pattern of radio waves and a pulsed magnetic force to the entire wave, which induced each atom with a string to tilt its spin in a helical (or wavelike) pattern. The wave-like patterns of these stars correspond to the periodic density modulation of the “spin up” atoms simultaneously forming a pattern of stripes, which the researchers can image on the detector. They then saw how the striped patterns disappeared as the individual spins of atoms got closer to their equilibrium positions.

Catterall compared the experiment to tying the strings of a guitar. If researchers were to see the sprain of atoms in equilibrium, it would not tell them much about the magnetic forces between the atoms, just as a guitar string would not tell much about its physical properties at rest. By plucking the string, bringing it out of balance, and seeing how it vibrates and eventually returns to its original position, one can learn something fundamental about the physical properties of the string.

“What we’re doing here, we’re like dropping a string of spins. We’re putting in this helix pattern, and then seeing how this pattern behaves as a function of time,” says Ketrell . “This allows us to see the effect of different magnetic forces between spins.”

Ballistics and ink

In their experiment, the researchers changed the force of the pulsed magnetic force exerted by them, varying the width of the stripes in the nuclear spin pattern. They measured how quickly, and in what ways, the pattern faded. Depending on the nature of the magnetic forces between the atoms, they behaved differently on how quantum spins returned to equilibrium.

They discovered a transition between ballistic behavior, where spines quickly shot back into an equilibrium state, and controversial behavior, where spines propagate more incorrectly, and the overall stripe pattern gradually propagates into equilibrium, As the ink slowly dissolves in water.

Some of this behavior has been predicted theoretically, but has never been seen in detail until now. Some other results were completely unexpected. What’s more, the researchers found their observations mathematically fit to what they calculated with the Heisenberg model for their experimental parameters. He worked closely with theorists at Harvard, who performed cutting-edge calculations of spin dynamics.

“It was interesting to see that there were properties that were easy to measure, but difficult to calculate, and other properties could be calculated, but not measured,” Ho says.

In addition to advancing the understanding of magnetism at the fundamental level, the team’s results can be used to explore the properties of new materials, as a kind of quantum simulator. Such a platform can function like a special-purpose quantum computer that calculates the behavior of materials, in a way that exceeds the capabilities of today’s most powerful computers.

“With the current excitement about quantum information science’s promise to solve practical problems in the future, it’s really great to see such work coming in today,” John Gillesafi, Department of Physics The program officer at the National Science Foundation, a treasurer of research.

Reference: “Spin transport in tunable Heisenberg model with ultrahold atoms” on 16 December 2020 by Paul Nickless Jepsen, Jesse Ameto-Grill, Ivana Dimitrova, Wen Wei Ho, Eugene Daimler and Wolfgang Centrel. Nature.
DOI: 10.1038 / s41586-020-3033-y

The research was also supported by the Department of Defense and the Gordon and Betty Moore Foundation.

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