Quantum particle, quantum mechanics

3D illustration: Quantum particle, quantum mechanics. (© Peter Jurik - stock.adobe.com)

PRINCETON, N.J. — When you think of magnets, you likely picture the simple bar magnets stuck to your fridge door or the rock-like lodestones that first sparked the discovery of magnetism. But at the deepest quantum levels, magnetism can take on vastly stranger forms than those familiar objects.

In a pioneering study published in the journal Nature, researchers from Princeton University reveal an entirely new mechanism for magnetism that arises from the quantum motion of atoms – a phenomenon they’ve dubbed “kinetic magnetism.”

“This is very exciting,” says Waseem Bakr, the study’s senior author, in a media release. “The origins of the magnetism have to do with the motion of impurities in the atomic array, hence the name kinetic magnetism. This motion is highly unusual and leads to magnetism that is robust even at very high temperatures.”

Conventional magnets get their magnetic properties from the spin of electrons. But kinetic magnetism emerges from the collective quantum behavior of many interacting particles. Specifically, it comes from the coordinated drift of dopant atoms moving through a rigid lattice of other atoms. To study this bizarre form of magnetism at the most fundamental scales, the Princeton team used an advanced setup in their lab to create model quantum systems of ultracold lithium atoms trapped in laser-generated crystals called optical lattices.

“We have the capability in our lab to look at this system at the single atom and single site level in the lattice and take ‘snapshots’ of the subtle quantum correlations between the particles,” Bakr explains.

Through precise quantum control, the researchers could introduce impurities or “dopants” into the lattice by removing some atoms to create vacant holes, or adding extra atoms. This allowed them to observe the exotic physics at play.

They discovered that kinetic magnetism arises from strange quantum objects called “magnetic polarons” that form around these dopant particles. A polaron is an amalgam of a particle and the disturbance it creates in its surrounding quantum environment.

“We find that a hole dopant surrounds itself with anti-aligned spins as it moves around, while a particle dopant does the opposite, surrounding itself with aligned spins,” says Benjamin Spar, a graduate researcher on the study.

Researchers at Princeton have directly imaged the microscopic origins of a novel type of magnetism.
Researchers at Princeton have directly imaged the microscopic origins of a novel type of magnetism. (CREDIT: Max Prichard, Waseem Bakr group at Princeton University)

Imagine a lattice of upright arrows representing the spin of atoms. Introducing a “hole dopant” causes that vacancy to roam around the lattice, with the arrows nearby flipping to point down as it passes by. This creates a quasiparticle made of the hole plus its associated pattern of flipped arrows – the magnetic polaron.

The same happens in reverse for an extra particle dopant, which wanders through, creating a polaron surrounded by arrows aligning in the same direction. Despite not being actual particles themselves, these polarons behave like particles with properties such as mass and charge. Their unique structure and dynamics give rise to the robust magnetism in kinetic systems.

“The most exciting part of this research is that it truly is concurrent with studies in the condensed matter community,” explains Max Prichard, another co-lead author of the study. “We are in the unique position to provide insight to a timely problem from a totally different angle, and all parties benefit.”

While hints of kinetic magnetism were recently spotted in exotic 2D materials with moiré patterns, those experiments could only measure the bulk magnetic behavior. The Princeton team’s ultracold atom microscope allowed them to directly visualize the magnetic polarons driving the effect at the most fundamental quantum level. Understanding and controlling quantum magnetism like this could open new technological frontiers.

“Combined with the tunability of the magnetism with doping—the addition or removal of particles—kinetic magnetism is very promising for device applications in real materials,” Bakr says.

Beyond magnetism, polarons may also play a key role in other quantum phenomena like high-temperature superconductivity, where current flows with zero resistance. Some theories propose hole dopants could pair up through polaron distortions.

“We’ve simply taken snapshots of the polaron, which is only the first step,” Prichard concludes. “We’re now interested in doing a spectroscopic measurement of the polarons. We want to see how long the polarons live in the interacting system, to measure the energy binding together a polaron’s constituents and its effective mass as it propagates in the lattice. There is a lot more to do.”

By simulating and visualizing the quantum underpinnings of kinetic magnetism, the researchers have pioneered a new frontier for exploring magnetism in its strangest quantum realms. Who knows what other exotic quantum forms of magnetism may lurk in the vast unexplored corners of the natural world?

Article reviewed by StudyFinds Editor Chris Melore.

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