Three-particle mechanism for unconventional superconductivity with potential applications in moiré materials

Abstract

A novel electronic mechanism for superconductivity is introduced, wherein pairing between two conduction electrons is mediated by the virtual motion of a third one originating from the valence band. Relying on interband polarization rather than singularities near the Fermi level, this mechanism bypasses the limitations that other electronic models for superconductivity encounter at low charge carrier concentrations. As a result, it is able to capture the full crossover from Bardeen-Cooper- Schrieffer behaviors to the Bose-Einstein condensation of pairs, which has recently attracted a lot of experimental attention. Furthermore, the pairing potential obtained is non-retarded and non-perturbative in the Coulomb interaction strength, leading to strong coupling behaviors and, in particular, to large critical temperature to Fermi energy ratios. Finally, we argue that this mechanism should apply to some moiré materials, but also to ZrNCl and WTe₂, for which it predicts an experimentally observable spin-triplet order parameter.

Faithfully describing two of the most important class of unconventional superconductors – dilute superconductors with large [formula] ratios and spin-triplet superconductors – the so-coined “three-particle” mechanism for superconductivity studied throughout this thesis appears pivotal to future research on high-𝑇꜀ and topological superconductors.