Electron pairing: from metastable electron pair to bipolaron

TL;DR

This paper investigates how electron-electron and electron-phonon interactions can stabilize metastable electron pairs, leading to bipolaron formation in crystals, with implications for understanding correlated electron systems.

Contribution

It introduces a detailed analysis of exchange-correlation effects and polaron corrections that enable the stabilization of electron pairs as bipolarons in a crystal lattice.

Findings

Electron-phonon interaction counteracts Coulomb repulsion.

Metastable electron pairs form a bipolaron band above the single-electron band.

Interactions lead to electron pairs on the Fermi surface.

Abstract

Starting from the shell structure in atoms and the significant correlation within electron pairs, we distinguish the exchange-correlation effects between two electrons of opposite spins occupying the same orbital from the average correlation among many electrons in a crystal. In the periodic potential of the crystal with lattice constant larger than the effective Bohr radius of the valence electrons, these correlated electron pairs can form a metastable energy band above the corresponding single-electron band separated by an energy gap. In order to determine if these metastable electron pairs can be stabilized, we calculate the many-electron exchange-correlation renormalization and the polaron correction to the two-band system with single electrons and electron pairs. We find that the electron-phonon interaction is essential to counterbalance the Coulomb repulsion and to stabilize the electron pairs. The interplay of the electron-electron and electron-phonon interactions, manifested in the exchange-correlation energies, polaron effects, and screening, is responsible for the formation of electron pairs (bipolarons) that are located on the Fermi surface of the single-electron band.