#polarons

This Algorithm Just Solved One of Physics’ Most Infamous Problems

Caltech scientists have found a fast and efficient way to add up large numbers of Feynman diagrams, the simple drawings physicists use to represent particle interactions. The new method has already enabled the researchers to solve a longstanding problem in the materials science and physics worlds known as the polaron problem, giving scientists and engineers a way to predict how electrons will flow in certain materials, both conventional and quantum. In the 1940s, physicist Richard Feynman first proposed a way to represent the various interactions that take place between electrons, photons, and other fundamental particles using 2D drawings that involve straight and wavy lines intersecting at vertices. Though they look simple, these Feynman diagrams allow scientists to calculate the probability that a particular collision, or scattering, will take place between particles.

To describe how matter works at infinitesimal scales, researchers designate collective behaviors with single concepts -- like calling a group of birds flying in sync a "flock" or "murmuration." Known as quasiparticles, the phenomena these concepts refer to could be the key to next-generation technologies. In a recent study published in Science Advances, a team of researchers led by Shengxi Huang, associate professor of electrical and computer engineering and materials science and nanoengineering at Rice, describe how one such type of quasiparticle -- polarons -- behaves in tellurene, a nanomaterial first synthesized in 2017 that is made up of tiny chains of tellurium atoms and has properties useful in sensing, electronic, optical and energy devices.

Study uncovers the fundamental mechanisms underlying the formation of polarons in 2D atomic crystals

Polarons are localized quasiparticles that result from the interaction between fermionic particles and bosonic fields. Specifically, polarons are formed when individual electrons in crystals distort their surrounding atomic lattice, producing composite objects that behave more like a massive particles than electron waves.

Feliciano Giustino and Weng Hong Sio, two researchers at the University of Texas at Austin, recently carried out a study investigating the processes underpinning the formation of polarons in 2D materials. Their paper, published in Nature Physics, outlines some fundamental mechanisms associated with these particles' formation that had not been identified in previous works.

"Back in 2019, we developed a new theoretical and computational framework to study polarons," Feliciano Giustino, one of the researchers who carried out the study, told Phys.org. "One thing that caught our attention is that many experimental papers discuss polarons in 3D bulk materials, but we could find only a couple of papers reporting observations of these particles in 2D. So, we were wondering whether this is just a coincidence, or else polarons in 2D are more rare or more elusive than in 3D, and our recent paper addresses this question."

Material Makes Photons Bulky

Photons in certain materials can form large, strongly interacting quasiparticles, boosting nonlinear effects that could be useful in quantum optics.

Nonlinear effects will be a crucial ingredient of future optical quantum information platforms. By allowing different information-carrying photons to interact, these effects can be useful for performing basic tasks like shifting the photons’ color, modifying their statistics, and inducing entanglement between them. Nonlinear processes are ubiquitous in laser optics, but in typical materials they are too inefficient to have a measurable effect at the single-photon level, as would be required for advanced quantum optics applications. Now, Li Bing Tan of the Swiss Federal Institute of Technology (ETH) in Zurich and co-workers report on a novel way to boost the optical nonlinearity of a medium [1]. Their idea is to couple, or “dress,” photons with suitable material excitations in an atomically thin semiconductor layer. The coupling results in quasiparticles called polaron-polaritons, which extend over large spatial scales and can interact surprisingly strongly with each other. The researchers’ measurements show that this coupling boosts the optical nonlinearity of the material by a factor of 50.

A new method to study polarons in insulators and semiconductors

A team of researchers at the University of Oxford have recently introduced a new way to model polarons, a quasiparticle typically used by physicists to understand interactions between electrons and atoms in solid materials. Their method, presented in a paper published in Physical Review Letters, combines theoretical modeling with computational simulations, enabling in-depth observations of these quasiparticles in a wide range of materials.

Essentially, a polaron is a composite particle comprised of an electron surrounded by a cloud of phonons (i.e. lattice vibrations). This quasiparticle is heavier than the electron itself and due to its substantial weight it can sometimes become trapped in a crystal lattice.

Polarons contribute to the electric currentthat powers several technological tools, including organic light-emitting diodes and touchscreens. Understanding their properties is thus of key importance, as it could help to develop the next generation of various devices for lighting and optoelectronics.

Source: Abstract

Predicting the emergent properties of impurities immersed in a quantum bath is a fundamental challenge that can defy quasiparticle treatments. Here, we measure the spectral properties and real-time dynamics of mobile impurities injected into a weakly interacting homogeneous Bose-Einstein condensate, using two broad Feshbach resonances to tune both the impurity-bath and intrabath interactions. For attractive impurity-bath interactions, the impurity spectrum features a single branch, which away from the resonance corresponds to a well-defined attractive polaron; near the resonance, we observe dramatic broadening of this branch, suggesting a breakdown of the quasiparticle picture. For repulsive impurity-bath interactions, the spectrum features two branches: the attractive branch that is dominated by excitations with energy close to that of the Feshbach dimer, but has a many-body character, and the repulsive polaron branch. Our measurements show that the behavior of impurities in weakly interacting baths is remarkably universal, controlled only by the bath density and a single dimensionless interaction parameter.