Nonperturbative Green’s function method to study electron-phonon interactions in the strong coupling regime

A fundamental area of solid state physics is the study of electrons, nuclei vibrations (phonons), and their interactions in crystals, known as electron-phonon interactions (EPIs).

EPIs determine the temperature dependence of the bandgap, and lead to conventional superconductivity, among many other important phenomena. EPIs are usually studied using lowest order perturbation theory, which is only valid when the coupling is weak. However, strong EPIs have been reported in many key materials.

For example, there is a huge gap renormalization of about 3 eV in high-pressure hydrogen that makes it metallic, and understanding metallic hydrogen is considered a substantial step towards understanding high-temperature superconductivity in hydrides.

Research on the gap renormalization and temperature dependence of the gap of diamond is also very active, with exciting possibilities in the development of LEDs for use in extreme environments like high temperatures. However, existing calculations are not well justified, or perturbative approaches do not match experimental data.

In GreenNP, we will study these and other effects due to EPIs using a rigorous nonperturbative adiabatic Green's function approach that the experienced researcher (ER) Dr. Jean Paul Nery has recently developed.

Strong assets of the project will be the use of first-principles as opposed to models, and the inclusion of anharmonicities.

First-principle calculations will be implemented into one of the prime open-source packages of the community, in collaboration with its renowned leading group at a nearby university; and a secondment will be carried out with a specialist in anharmonicities.

By the end of the project, through the study of elemental semiconductors and insulators, there will be a clear understanding of the effects of strong EPIs in electronic properties.

The ER will be taken to a leadership position in the field of EPIs and higher-order effects, with excellent future career prospects.