Unveiling the Mysteries of Kinetic Magnetism: Princeton Physicists Make Startling Discoveries

15 May 2024 2382
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Princeton University researchers have made new advances in understanding kinetic magnetism. They used ultracold atoms within a laser-constructed lattice to create an image of a unique type of polaron. This image demonstrates how impurity movement within an atomic array can generate robust magnetism at high temperatures.

The microscopic object behind this magnetism, a unique form of polaron, was directly captured on camera by the research team.

Our understanding of magnetism is not one-dimensional. For instance, the magnets we typically find stuck to the doors of our fridges are formed through electronic interactions that have been understood for roughly a century, dating back to the early stages of quantum mechanics. However, nature is teeming with diverse forms of magnetism, and the mechanisms that power them continue to be discovered by scientists.

In a significant development, Princeton University physicists have now made unprecedented strides in understanding kinetic magnetism. To do this, they used ultracold atoms held in place by an artificial laser-created lattice. The findings of the experiments, documented in a recent Nature publication, allowed the researchers to directly image the unique type of polaron, or quasiparticle, that brings this magnetism to life within an interactive quantum system.

Waseem Bakr, a physics professor at Princeton and the paper's senior author, finds these findings thrilling. He explains that the origins of such magnetism are tied to the movements of impurities within the atomic array, which explains the term "kinetic magnetism." This movement pattern is highly unusual and results in high-temperature resistant magnetism. As the magnetism is adjustable through doping (either adding or removing particles), kinetic magnetism shows immense potential for real-world device applications.

In their research, Bakr and his team examined this newfound form of magnetism at a level of detail that had not been previously achieved. Due to the preciseness of ultracold atomic systems, they were able to visualize, for the first time, the intricate physics that lead to kinetic magnetism.

Bakr said that their lab has the ability to analyze this system at a single atom and single-site level in the lattice, allowing them to capture 'snapshots' of the delicate quantum correlations among the system particles.

For years, Bakr and his research team have examined quantum states through experimentation with ultracold fermions (subatomic particles) within a vacuum chamber. They created a complex apparatus that can cool atoms down to ultracold levels and place them into artificial crystals, also known as optical lattices, through the use of laser beams. This system allowed them to delve into many fascinating aspects of the quantum world, such as the emergence of behavior amongst interacting particle groups.

A mechanism involving magnetism formally proposed and known as Nagaoka ferromagnetism was a significant proponent for the team’s current line of experiments. Yosuke Nagaoka, the discoverer, named it after himself. However, Nagaoka theorized a different mechanism leading to ferromagnetism, driven by the movement of intentionally-inserted impurities, or "dopants," could also exist. This mechanism can be best explained by visualizing a two-dimensional square lattice where each lattice site, apart from one, is occupied by an electron. The unoccupied site (or hole dopant) moves around within the lattice.

According to Nagaoka, if the hole moves in a milieu of aligned spins, or a ferromagnet, the various possible paths of the hole's movement can overlap and interfere with each other quantum mechanically. This increased diffusion of the hole's quantum position lowers the kinetic energy, which is a desirable outcome.

The Nagaoka theorem gained swift recognition due its rare, rigorous demonstrations that explain ground states of strongly linked electron systems. However, its experimental application has been challenging due to strict model requirements. The theorem specifies that the interactions must be infinitely strong and only one single dopant is permissible. A realization dawned half a century after the proposition of the theorem that these unrealistic conditions could be effectively subdued in lattices with triangular geometry.

Researchers utilized lithium-6 atom vapors for the experiment. This lithium isotope comprises three euthrons, three protons and three electrons. A Princeton University physics graduate student and a paper co-lead author, Benjamin Spar, stated that the isotope's odd total number categorizes it as a fermionic isotope that behaves similarly to electrons in a solid-state system.

When the gases are super cooled by lasers to just a few billionths of a degree above absolute zero, quantum mechanics principles start controlling their behavior as opposed to familiar classical mechanics. “Upon achievement of this quantum system, the next step is the loading of the atoms into a triangular optical lattice. This cold atom setup allows us to regulate the speed of atom movement as well as their interaction intensity,” said Spar.

A prevalent configuration in strongly interacting systems is the Mott insulator, a state of matter wherein a single particle occupies each lattice site. In this state of matter, the spin of electrons on neighboring sites prompts weak antiferromagnetic interactions due to superexchange. However, instead of utilizing a Mott insulator, researchers leveraged a technique known as "doping" which either eradicates some particles creating "holes" in the lattice or adds extra particles.

Bakr, a researcher, explained that their experiment doesn’t start with one atom per site. Instead, they dope the lattice with holes or particles. Doing this interestingly results in a more robust form of magnetism, observed in systems with higher energy scale than typical superexchange magnetism. This type of magnetism involves atom hopping in the lattice.

With the help of an optical microscope, the researchers could examine single-site level activities due to the larger lattice site spacing in optical lattices compared to real materials. Their observations led to the identification of a new type of magnetic polaron.

"A polaron represents a quasiparticle emerging in a quantum system with numerous interacting constituents. Similar to a regular particle, a polaron possesses properties like charge, a spin and effective mass. However, it is not a particle like an atom. It can be described as a dopant that travels around causing a disturbance in its magnetic environment or the alignment of spins around it," further explained Bakr.

This unique form of magnetism has previously been observed in moire materials, comprised of stacked two-dimensional crystals, only in the recent past. However, the available magnetism probes for these materials are limited. Max Prichard, a graduate student and another co-lead author of the paper, said that what makes this research exciting is that it proceeds in lockstep with condensed matter community studies.

Researchers can provide insight into a timely problem from a unique perspective, beneficial to all parties. Moving forward, researchers plan to probe this new, exotic form of magnetism and investigate the spin polaron in more detail.

“In this first experiment, we’ve simply taken snapshots of the polaron, which is only the first step,” said Prichard. “But 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.” 

Other members of the team are Zoe Yan, now at the University of Chicago, and theorists Ivan Morera, University of Barcelona, Spain, and Eugene Demler, Institute of Theoretical Physics in Zurich, Switzerland. The experimental work was supported by the National Science Foundation, the Army Research Office and the David and Lucile Packard Foundation.