Electron trimer states in conventional superconductors

TL;DR

This paper explores the formation of electron trimer states in conventional superconductors by extending the Cooper problem to include a third electron, revealing conditions for their emergence and competition with Cooper pairs.

Contribution

It introduces a theoretical model demonstrating the existence of electron trimer states and identifies key parameters influencing their formation in superconductors.

Findings

Trimer states form with strong inter-band attraction.

Critical interaction strength depends on Fermi velocity, electron mass, and Debye energy.

Trimer states can be transiently created via optical pumping.

Abstract

We expand the Cooper problem by including a third electron in an otherwise empty band. We demonstrate the formation of a trimer state of two electrons above the Fermi sea and the third electron, for sufficiently strong inter-band attractive interaction. We show that the critical interaction strength is the lowest for small Fermi velocities, large masses of the additional electron and large Debye energy. This trimer state competes with the formation of the two-electron Cooper pair, and can be created transiently via optical pumping.