Why does maximum Tc occur at the cross-over from weak to strong electron-phonon coupling in high temperature superconductors?

TL;DR

This paper investigates why the maximum critical temperature (Tc) in high-temperature cuprate superconductors occurs at a specific electron-phonon coupling strength, highlighting a crossover from weak to strong coupling as key to high Tc.

Contribution

It proposes that the maximum Tc arises at the crossover from weak to strong electron-phonon coupling, challenging conventional theories and suggesting a two-component carrier scenario.

Findings

Maximum Tc occurs at the crossover from weak to strong coupling.

Tc is a non-linear function of electron-phonon coupling lambda.

Carrier localization onset correlates with Tc maximum.

Abstract

In cuprate superconductors, a pronounced maximum of superconducting Tc is observed in compounds that have an in-plane Cu-O distance close to ~1.92 Angstroms. On the other hand, direct measurements of the electron-phonon coupling lambda as a function of Cu-O distance show a clear linear correlation, implying that Tc is a strongly non-linear function of lambda. Conventional superconductivity theories based on the electron-phonon interaction predict a monotonic dependence of Tc on electron phonon coupling constant, which makes them incompatible with the observed behaviour. The observed cross-over behaviour as a function of lambda suggests that Tc occurs at the cross-over from weak to strong coupling, which is also associated with the onset of carrier localization. A coexistence, with a dynamical exchange of localized and itinerant carriers in a two-component superconductivity scenario are in agreement with the observed anomalous behavior and are suggested to be the key to understanding the mechanism for achieving high Tc.