From local to macroscopic coherence in systems with composite quasi-particles

TL;DR

This paper explores how local coherent quantum states in strongly interacting systems evolve into macroscopic states like superfluidity, focusing on electron-phonon interactions and the transition from localized to delocalized charge carriers.

Contribution

It provides a detailed analysis of the transformation of local quasi-particle states into macroscopic coherence, with implications for understanding superconductivity and lattice dynamics.

Findings

Significant changes in phonon clouds during phase transition

Localization of charge carriers becomes delocalization in superconducting phase

Proposes experimental methods to verify lattice dynamic features

Abstract

Strongly interacting systems are characterized by heavily dressed entities with internal degrees of freedom, which, on a local level, can be described in terms of coherent quantum states. We examine the modification of these local coherent quantum states when such entities condense into a macroscopic coherent quantum state, such as superfluidity. As an example, we consider a system of electrons coupled to local lattice deformations. Significant changes in the phonon clouds surrounding the charge carriers occur when the system develops into a spatially phase-locked state. The question of localized self-trapped charge carriers (bipolarons) in the normal state becoming delocalized upon entering the superconducting phase is discussed in terms of squeezing of the local coherent phonon states. Suggestions for experimental verifications of these features associated with the lattice dynamics are made.