Polaroni: from their stability, the beginning of a new physics

THE polarons they are a turning point. To solve one of the problems oldest in the world of physicsa “flawed” theory that physicists today use to study the interactions of electrons in materials, a group of researchers from the Federal Polytechnic of Lausanne (EPFL) managed to develop a new approach, focusing on the stabilization of polaronsnamely the quasi-particles produced by electron-phonon interactions in materials. The method, described in two articles published in journals Physical Review Letters And Physical Review Bwill open the doors to a new one physicsand more precisely a more detailed calculations of polarons in large systemssystematic studies of large sets of materials and molecular dynamics that evolve over a long time.

But let’s clarify: in the world of quantum mechanics, particles can also be described as waves. A common example is the photon, the particle associated with light. In crystals, that is, in ordered structures, also the electrons they can be described as waves that spread through the entire system. As they move through the crystal, ions, which are atoms carrying a negative or positive charge, are periodically arranged in space. If a electron in addition to the crystal, its negative charge could cause the ions surrounding it to move away from their equilibrium positions.

At this point, the electron charge would localize in space and couple to the “lattice” of the crystalgiving rise to a new particle known as polaron. “Technically, a polaron is a quasi-particle, made up of an electron ‘dressed’ by its self-induced phonons, which represent the quantized vibrations of the crystal”explains Stefano Fallettaamong the authors of the study. “The stability of polarons arises from a competition between two energy contributions: the gain due to the localization of the charge and the cost due to the distortions of the lattice. When the polaron is destabilized, theelectron in addition, it delocalizes on the entire system, while the ions return to their positions of equilibrium ”.

The method in question, the one that physicists use today to examine the interactions of electrons but with important limitations, is called density functional theory or Dft (Density functional theory) and is used not only in physics, but also in chemistry and materials sciences to study the electronic structure of systems with many atoms and molecules. Although it is a powerful tool, the Dft it has some limitations, including the “Self-interaction error” (Sie), translated as “self-interaction error”, ie the residual interaction of a electron with itself, which often leads to an incorrect description of the polaronswhich are often destabilized.

“In our work, we introduce a theoretical formulation for electronic self-interaction that solves the problem of location of the polaron in the theory of the density functional “, Clarifies Falletta. “This allows for stability accurate polarons within a computationally efficient scheme. Our study paves the way for unprecedented calculations of polarons in large systems, for systematic studies involving large sets of materials and molecular dynamics that evolve over long periods of time “.