Princeton physicists unlock secrets of kinetic magnetism

Physicists from Princeton University They directly imaged the microscopic object responsible for this magnetism, an unusual type of polaron.

Not all magnets are the same. When we think of magnetism, we usually think of magnets that stick to the refrigerator door. For these types of magnets, the electronic interactions that give rise to magnetism have been understood for about a century, since the early days of quantum mechanics. But there are many different forms of magnetism in nature, and scientists are still discovering the mechanisms that drive them.

Now, physicists from Princeton University have made significant progress in understanding a form of magnetism known as kinetic magnetism, using ultracold atoms linked to an artificial lattice made with a laser. Their experiences are chronicled in a research paper published this week in the journal natureIt allowed the researchers to directly image the microscopic object responsible for this magnetism, an unusual type of polaron, or quasiparticle, that appears in an interacting quantum system.

Understanding kinetic magnetism

“This is very exciting,” said Waseem Bakr, a professor of physics at Princeton University and lead author of the study. “The origins of magnetism have to do with the movement of impurities in the atomic matrix, hence the name Kinetics Magnetism. This motion is highly unusual and results in strong magnetism even at very high temperatures. Combined with the possibility of tuning magnetism with doping – adding or removing particles – kinetic magnetism is very promising for device applications in real materials.

Bakr and his team studied this new form of magnetism in a level of detail not achieved in previous research. Thanks to the control provided by ultracold atomic systems, researchers were able, for the first time, to visualize the precise physics that gives rise to kinetic magnetism.

Researchers at Princeton University have directly imaged the microscopic origins of a new type of magnetism. Image credit: Max Pritchard, Waseem Bakr Collection at Princeton University

Advanced tools for quantum discoveries

“We have the ability in our lab to look at this system individually corn “The researchers are monitoring the level of a single location in the network and taking snapshots of the precise quantum correlations between particles in the system,” Baker said.

For several years, Bakr and his research team have studied quantum states by experimenting with ultracold subatomic particles known as fermions in a vacuum chamber. They have created a sophisticated device that cools atoms to cryogenic temperatures and holds them in artificial crystals known as optical lattices created using lasers. This system has allowed researchers to explore many interesting aspects of the quantum world, including the emergent behavior of groups of interacting particles.

Theoretical foundations and experimental insights

One of the early theoretically proposed mechanisms for magnetism that laid the foundation for the team’s current experiments is known as Nagaoka ferromagnetism, named after its discoverer Yosuke Nagaoka. Ferromagnets are those in which all electron spin states point in the same direction.

While a ferromagnet with aligned spins is the most common type of magnet, in the simplest theoretical setting, strongly interacting electrons on the lattice actually tend toward antiferromagnetism, with the spins lining up in alternating directions. This preference to resist the alignment of neighboring spins occurs as a result of indirect coupling of neighboring electron spins known as superexchange.

However, Nagaoka theorized that ferromagnetism might also result from a completely different mechanism, one determined by the movement of intentionally added impurities, or doping. This can be best understood by imagining a two-dimensional square lattice, where every lattice site, except one, is occupied by an electron. An unoccupied site (or similar hole) roams the network.

Nagaoka found that if the hole moves in an environment with parallel spins or ferromagnets, the different paths of the quantum hole’s motion mechanically interfere with each other. This enhances the out-of-site propagation of the quantum hole and reduces the kinetic energy, which is a positive result.

Nagaoka’s legacy and modern quantum mechanics

Nagaoka’s theory quickly gained recognition because there were few rigorous proofs that claimed to explain the fundamental states of systems of strongly interacting electrons. But monitoring the consequences through experiments was a difficult challenge due to the strict requirements of the model. In theory, the reactions should be infinitely strong and only one dopant is allowed. Over the five decades after Nagaoka proposed his theory, other researchers realized that these unrealistic conditions could be significantly alleviated in networks with triangular geometry.

Quantum experiment and its effects

To conduct the experiment, the researchers used vapors of lithium-6 atoms. This lithium isotope has three electrons, three protons and three neutrons. “The odd total number makes this a fermionic isotope, which means the atoms behave similarly to electrons in a solid-state system,” said Benjamin Spar, a graduate student in physics at Princeton University and a co-author of the study.

When these gases are cooled using lasers to extreme temperatures of only a few billionths of a degree Absolute zeroTheir behavior begins to obey the principles of quantum mechanics rather than the more familiar classical mechanics.

Exploring quantum states through cold atom settings

“Once we achieve this quantum system, the next thing we do is load the atoms into the triangular optical lattice,” Spar says. “In the cold-atom setup, we can control how fast the atoms move or how strongly they interact with each other.”

In many highly interacting systems, the particles in the lattice are organized in a “death insulator,” a state of matter in which a single particle occupies each site of the lattice. In this case, there are weak ferromagnetic interactions due to superfluous exchange between the spins of electrons in adjacent sites. But instead of using a dying buffer, the researchers used a technique called “grafting,” which either removes some molecules, thus leaving “holes” in the mesh, or adds additional molecules.

Uncovering new forms of quantum magnetism

“We don’t start with one seed per site in our experiment,” Baker said. “Instead, we cover the lattice with holes or molecules. And when you do that, you find that there is a much stronger form of magnetism that is observed in these systems at a higher energy scale than the usual superexchange magnetism. This energy scale has to do with atoms jumping in the lattice.”

By taking advantage of the larger distances between lattice sites in optical networks compared to real materials, the researchers were able to see what was happening at the single-site level using optical microscopy. They found that the objects responsible for this new form of magnetism are a new type of magnetic pole.

The role of polarons in quantum systems

“A polaron is a quasiparticle that appears in a quantum system with many interacting components,” Baker said. “It behaves very much like a regular particle, meaning it has properties like charge, spin, and effective mass, but it’s not an actual particle like an atom. In this case, it’s a doping material that moves with a disturbance in its magnetic environment, or how the spins are aligned.” around them relative to each other.

In real materials, this new form of magnetism has previously been observed in so-called moiré materials made up of stacked 2D crystals, and this happened only in the last year.

Investigate deeper into quantum magnetism

“The magnetism probes available for these materials are limited. Experiments with moiré materials have measured the macroscopic effects associated with how a large piece of material responds when a magnetic field is applied,” Spar said. “With the cold-atom setup, we can delve deeper into the physics Microstructures responsible for magnetism. We have captured detailed images that reveal the spin correlations around mobile doping. For example, a hole-filled surround surrounds itself with anti-alignment spin as it moves, while an enhanced particle does the opposite, surrounding itself with coherent spin.

This research has far-reaching implications for condensed matter physics, even beyond understanding the physics of magnetism. For example, it has been hypothesized that more complex versions of these polarons give rise to hole-doping coupling mechanisms, which could lead to superconductivity at high temperatures.

Future directions in quantum magnetism research

“The most exciting part about this research is that it really coincides with studies in the condensed matter community,” said Max Pritchard, a graduate student and co-author of the paper. “We are uniquely positioned to provide timely insight into a problem from a completely different angle, and all parties will benefit.”

Looking to the future, researchers are already coming up with new and innovative ways to further explore this strange new form of magnetism – and investigate spin polarity in more detail.

Next steps in Polaron research

“In this first experiment, we simply took snapshots of the polaron, which is just the first step,” Pritchard said. “But now we are interested in doing a spectroscopic measurement of polarons. We want to see how long the polarons live in the interacting system, to measure the energy binding the electrode components and their effective mass as they propagate in the lattice. There is a lot more to do.”

Other members of the team are Zoe Yan, now in University of Chicagoand theorists Ivan Moreira, University of Barcelona, ​​Spain, and Eugene Demmler, Institute for 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.

Reference: “Direct imaging of spin poles in a kinetically frustrated Hubbard system” by Max L. Pritchard, Benjamin M. Spar, Ivan Moreira, Eugene Demmler, Zoe Z. Yan, and Wasim S. Bakr, May 8, 2024, nature.
doi: 10.1038/s41586-024-07356-6