Princeton Physicists Unveil Microscopic Origins of Novel Quantum Magnetism

Physicists at Princeton University, led by Professor Waseem Bakr, have significantly advanced in understanding kinetic magnetism, a novel form of magnetism. Using ultracold atoms in a laser-built lattice, they could directly image the microscopic object responsible for this magnetism, a unique type of polaron. This research could have significant implications for device applications in real materials due to the robustness of kinetic magnetism at high temperatures and its tunability with doping. The team includes Max Prichard, Benjamin Spar, Zoe Yan, Ivan Morera, and Eugene Demler. Their findings were published in the journal Nature.

Unveiling the Microscopic Basis of a New Quantum Magnetism

Physicists from Princeton University have made significant strides in understanding a form of magnetism known as kinetic magnetism. This form of magnetism is driven by the motion of impurities in an atomic array, hence the name kinetic magnetism. The researchers have been able to directly image the microscopic object responsible for this magnetism, an unusual type of polaron, or quasiparticle that emerges in an interacting quantum system.

The team, led by Waseem Bakr, a professor of physics at Princeton, used ultracold atoms bound in an artificial laser-built lattice to study this novel form of magnetism. The control provided by ultracold atomic systems allowed the researchers to visualize the finely-grained physics that gives rise to kinetic magnetism. This level of detail was previously unattainable in past research.

The Role of Ultracold Atoms and Optical Lattices

Bakr and his team have been experimenting with ultracold atomic gases in a vacuum chamber for several years. They have developed a sophisticated apparatus that cools atoms to ultracold temperatures and loads them into artificial crystals known as optical lattices created using laser beams. This system has allowed the researchers to explore many interesting aspects of the quantum world involving the emergent behavior of ensembles of interacting particles.

The researchers used vapors of lithium-6 atoms for the experiment. When these gases are cooled down using laser beams to extreme temperatures only a few billionths of a degree above absolute zero, their behavior begins to be governed by the principles of quantum mechanics rather than the more familiar classical mechanics.

The Concept of Doping and the Emergence of a New Magnetic Polaron

The researchers used a technique called “doping,” which either removes some particles, thereby leaving “holes” in the lattice, or adds extra particles. This technique allowed the researchers to observe a much more robust form of magnetism that is observed in these systems with higher energy scale than the usual superexchange magnetism.

The researchers were able to see what was occurring on the single-site level with an optical microscope. They found that the objects responsible for this new form of magnetism are a new type of magnetic polaron. A polaron is a quasiparticle that emerges in a quantum system with many interacting constituents. It acts very much like a regular particle, in the sense it has properties like a charge, a spin, and effective mass, but it is not an actual particle like an atom.

Implications and Future Directions

This research has far-reaching implications in condensed matter physics, even beyond understanding the physics of magnetism. More complex versions of these polarons have been hypothesized to lead to mechanisms for hole dopants to pair up, which may result in superconductivity at high temperatures.

Looking forward, the researchers are already devising new and innovative ways to further probe this new, exotic form of magnetism—and investigate the spin polaron in greater detail. They are interested in doing a spectroscopic measurement of the polarons to measure the energy binding together a polaron’s constituents and its effective mass as it propagates in the lattice.

The team included 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. The study was published online in the journal Nature on May 8, 2024.