COUPE - COUPling Excitations | COUPE | Proyecto | Ficha informativa | H2020 | CORDIS

Descripción del proyecto

Nuevo enfoque teórico para describir el acoplamiento de electrones y bosones

La interacción de electrones y bosones influye en las propiedades espectrales de los sólidos de manera profunda. Los bosones pueden ser vibraciones de redes cristalinas (fonones), fluctuaciones colectivas de la densidad de carga (plasmones) o estados ligados de electrones y huecos de electrones (excitones). La aproximación GW produce cálculos precisos de las energías de las cuasipartículas en diferentes materiales. Las excitaciones bosónicas se describen según la teoría del funcional de la densidad dependiente del tiempo (TDDFT, por sus siglas en inglés). En la actualidad, existen algunos enfoques que describen el acoplamiento de electrones y bosones. El proyecto COUPE, financiado con fondos europeos, propone una nueva combinación de GW y TDDFT que acople cuasipartículas a todo tipo de excitaciones bosónicas. Esta combinación se basa en una «teoría de conectores» desarrollada recientemente que permite usar los resultados del gas electrónico homogéneo para describir materiales inhomogéneos.

Objetivo

Spectroscopic characterization of matter is ubiquitous in modern material science. Increased resolution and precise control over radiation sources have led to numerous discoveries of previously inaccessible spectroscopic signatures, often linked to complex excitation processes. The main challenge in their interpretation is the fact that excitations are coupled through interactions. A striking signature for this is that spectra often consist of one or more dominant peaks (called quasi-particle), followed by a series of secondary structures (called satellites). The latter, which can be very strong, are pure interaction effects and may be interpreted in terms of coupling of a quasiparticle to a bosonic excitation of the material: a plasmon, exciton, phonon, or any other boson. To solve the electron-boson problem, and, first of all, to set up the description in terms of the most pertinent quasiparticles and bosons, is a major challenge of condensed matter theory. COUPE will face this challenge. Today, quasiparticles are most often described in the GW approximation to many-body perturbation theory, whereas for bosonic excitations one can resort to time-dependent density-functional theory (TDDFT). Only a few approaches exist to describe the coupled problem, with severe limitations. For example, a cumulant expansion on top of GW describes plasmon satellites, but cannot couple to spin excitations, and there is yet no systematic way for improvement. COUPE proposes a new combination of GW and TDDFT which couples quasiparticles to all kinds of bosonic excitations. This combination relies on a recently developed “connector theory” (CT), which allows using results from the homogeneous electron gas to describe inhomogeneous materials. COUPE will further apply it to the simulation of phase transitions in magnetic and superconducting materials under study at the synchrotron SOLEIL in Paris. Preliminary model results confirm the great potential of the approach envisaged in COUPE.