Superconductivity in 2D systems enhanced by nonadiabatic phonon-production effects

TL;DR

This paper explores how nonadiabatic phonon-production effects, driven by electron-phonon interactions and charge fluctuations, can enhance superconductivity in two-dimensional systems by shifting phonon spectral weight and increasing transition temperatures.

Contribution

It introduces a comprehensive dynamical framework for analyzing electron-phonon coupling effects on 2D superconductors, emphasizing the role of phonon production and charge fluctuations.

Findings

Phonon spectral weight shifts to low frequencies due to coupling with 2D plasmons.

Electron-phonon coupling enhances transition temperature through phonon production.

The impact of phonon production increases with higher electron density and effective mass.

Abstract

We investigate the dynamical effects of electron-phonon coupling (EPC) on the superconducting properties of two-dimensional (2D) systems, calculating the Eliashberg function in terms of dynamically renormalized phonons. By studying different approximations for the phonon self-energy, we identify the important role of charge fluctuations in shaping the superconductivity properties, not only through the renormalization of phonon frequencies and damping rates but also through structural changes in the phonon spectral function. With the dynamical effects treated consistently, we argue that a part of the phonon spectral weight necessarily shifts to low frequencies due to the coupling to the 2D gapless plasmon. Furthermore, we find that the EPC leads to excess phonon spectral weight as well - i.e., phonon production - which generally tends to enhance the transition temperature. Our calculations point out that the influence of phonon production becomes greater as the density and the effective mass of electrons increase.