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EPFL Physicists Aim to Stabilize Polarons and Advance Quantum Mechanics

Physicists at EPFL have devised a method to solve the persistent issue of electron self-interaction when analyzing polarons - quasiparticles fashioned by interactions of electron-phonon in materials.

EPFL Physicists Aim to Stabilize Polarons and Advance Quantum Mechanics

A polaron forming in magnesium oxide atoms. Image Credit: S. Falletta (EPFL).

The study could pave the way to unparalleled calculations of polarons in big systems, methodical studies of huge sets of materials, and molecular dynamics developing over long periods.

One of the numerous distinctiveness of quantum mechanics is that particles can also be illustrated as waves. A typical example is a photon, a particle related to light.

In ordered structures, called crystals, electrons can be observed and illustrated as waves that disperse across the whole system, which s relatively harmonious imagery. As electrons travel through the crystal, ions, atoms holding a positive or negative charge, are occasionally arranged in space.

Currently,  if an additional electron were incorporated into the crystal, its negative charge could render the ions around it travel away from their equilibrium positions. The electron charge would localize in space and link with the nearby structural “lattice” distortions of the crystal, inducing a new particle called a 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.

Stefano Falletta, School of Basic Sciences, École Polytechnique Fédérale de Lausanne

Falletta continues: “The stability of polarons arises from a competition between two energy contributions: the gain due to charge localization, and the cost due to lattice distortions. When the polaron destabilizes, the extra electron delocalizes over the entire system, while the ions restore their equilibrium positions.”

Collaborating with Professor Alfredo Pasquarello at EPFL, they have published two papers in Physical Review Letters and Physical Review B, explaining a new method for cracking a major inadequacy of a well-proven theory that physicists use to explore the interactions of electrons in materials.

The technique is known as density functional theory (DFT). It is used in chemistry, physics, and materials science to explore the electronic structure of many-body systems, such as molecules and atoms.

DFT is a robust tool for carrying out ab-initio calculations of materials by streamlined treatment of the electron interactions. However, DFT is vulnerable to false interactions of the electron with itself, which is what physicists call the “self-interaction problem.” This self-interaction is one of the biggest limitations of DFT, frequently resulting in an incorrect illustration of polarons, which are repeatedly destabilized.

“In our work, we introduce a theoretical formulation for the electron self-interaction that solves the problem of polaron localization in density functional theory,” says Falletta.

This gives access to accurate polaron stabilities within a computationally-efficient scheme. Our study paves the way to unprecedented calculations of polarons in large systems, in systematic studies involving large sets of materials, or in molecular dynamics evolving over long time periods.

Stefano Falletta, School of Basic Sciences, École Polytechnique Fédérale de Lausanne

Journal References:

Falletta, S & Pasquarello, A. (2022) Many-Body Self-Interaction and Polarons. Physical Review Letters.

Falletta, S & Pasquarello, A. (2022) Polarons free from many-body self-interaction in density functional theory. Physical Review B.


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