Scientists unveil direct imaging of spin polarons

A research team from Princeton University directly imaged the microscopic object - a type of polaron or quasiparticle -responsible for kinetic magnetism and reported their finding in the journal Nature. 

Overview of kinetic magnetism

We are all familiar with electromagnetism which is responsible for almost everything in our lives, sustaining the existence of atoms and molecules. If you think of the word “magnetism” separately, it is most likely that the image of magnets on the fridge door comes to your mind. The magnetism arises from the electronic interactions inside the atoms. But this is not the only type of magnetism: There are also more in nature! One of them is “Kinetic Magnetism”.

According to the lead author of the study, Professor Waseem Bakr, the motion of impurities in the atomic array is responsible for kinetic magnetism; also, this motion is highly unusual and leads to magnetism which is robust even at high temperatures. Controlling the magnetism with doping – the addition and removal of particles – kinetic magnetism results in promising advantages for device applications. 

Reaching ultracold temperatures

Researchers used vapors of lithium-6 atoms. This kind of isotope contains three electrons, three protons, and three neutrons. They cooled the gases by employing laser beams to extreme temperatures only a few billionths of a degree above absolute zero temperature. At this extreme temperature, the behavior of the atoms starts to be governed by quantum mechanics rather than classical mechanics. Bakr and his research team have several years of experience in cooling down atoms to ultracold temperatures with sophisticated tools and loading them into artificial crystals known as optical lattices using laser beams.

In this recent study, they load the atoms into the triangular optical lattice. “In this cold atom setup, we can control how fast atoms move around and how strongly they interact with each other,” said Benjamin Spar, a graduate student in physics at Princeton University and a co-author of the paper. 

Observing a new type of polaron

Doping this lattice with holes or particles results in a robust form of magnetism with a higher energy scale than usual magnetism that arises from the superexchange between the spin of electrons on neighboring sites. 

Leveraging the much larger lattice site spacings in the optical lattices compared to real materials, the physicist could see the process occurring at the single-site level with an optical microscope. As a result, they found that the objects responsible for the observed new type of magnetism are also a new type of polaron. 

A little bit about the Polaron

A polaron is a quasiparticle that emerges in a quantum system with many interacting constituents. Like regular particles, it has a charge, spin, and effective mass, however, it is not an actual particle like an atom. “In this case, it is a dopant that moves around with a disturbance to its magnetic environment, or how the spins around it are aligned relative to each other,” said Bakr. Also, Spar noted “We have taken detailed images revealing the spin correlations around mobile dopants. For example, 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”. 

Future directions

The impact of this research work is even beyond understanding the physics of magnetism. For instance, the more complex versions of these polarons lead to pairing up hole dopants and resulting in superconductivity at high temperatures. 

One of the authors of this study, a graduate student Max Prichard said that they are now interested in doing a spectroscopic measurement of the polarons and seeing 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. This is just the first step. 

Full article:

Prichard, M.L., Spar, B.M., Morera, I. et al. Directly imaging spin polarons in a kinetically frustrated Hubbard system. Nature 629, 323–328 (2024). https://doi.org/10.1038/s41586-024-07356-6

Source:

https://scitechdaily.com/chilling-discoveries-princeton-physicists-unlock-secrets-of-kinetic-magnetism/