Quantifying nonadiabaticity in major families of superconductors

TL;DR

This paper classifies various superconductors based on their degree of nonadiabaticity, characterized by the ratio of phononic to electronic energy scales, and explores how this affects their superconducting transition temperatures.

Contribution

It introduces a classification scheme for superconductors according to nonadiabaticity strength, linking it to their transition temperatures and expanding understanding beyond traditional adiabatic theories.

Findings

Major classes of superconductors are classified by nonadiabaticity strength.

A correlation between nonadiabaticity and superconducting transition temperature is discussed.

The classification scheme relates to existing frameworks and broadens understanding of superconductivity mechanisms.

Abstract

The classical Bardeen-Cooper-Schrieffer and Eliashberg theories of the electron-phonon-mediated superconductivity are based on the Migdal theorem, which is an assumption that the energy of charge carriers, $k{_B}T{_F}$, significantly exceeds the phononic energy, $\hbar{\omega{_D}} $, of the crystalline lattice. This assumption, which is also known as adiabatic approximation, implies that the superconductor exhibits fast charge carriers and slow phonons. This picture is valid for pure metals and metallic alloys because these superconductors exhibit $\hbar{\omega{_D}}$/$k{_B}T{_F}<0.01$. However, n-type doped semiconducting $SrTiO_3$ was the first superconductor which beyond this adiabatic approximation, because this material exhibits $\hbar{\omega{_D}}$/$k{_B}T{_F} $~$ 50$. There is growing number of newly discovered superconductors which also beyond the adiabatic approximation. Here, leaving apart pure theoretical aspects of nonadiabatic superconductors, we classified major classes of superconductors (including, elements, A-15 and Heusler alloys, Laves phases, intermetallics, noncentrosymmetric compounds, cuprates, pnictides, highly-compressed hydrides and oxygen, and magic-angle twisted bilayer graphene) by the strength of nonadiabaticity (for which the ratio of the Debye temperature to the Fermi temperature, $T{_\theta}/T{_F}$, is used as a criterion for the nonadiabaticity) versus the superconducting transition temperature, $T{_c}$. The discussion of this classification scheme and its relation to other known classification counterparts is given.