Multi-Band Superconductivity and the Steep Band -- Flat Band Scenario

TL;DR

This paper explores multi-band superconductivity, highlighting how interband interactions can enhance transition temperatures and alter isotope effects, especially in systems with flat and steep bands like cuprates and MgB2.

Contribution

It provides a theoretical framework for understanding multi-band superconductivity, including effects of polaronic coupling and the advantages over single-band models.

Findings

Interband interactions can significantly increase superconducting transition temperatures.

Isotope effects are substantially different from BCS predictions in polaronic systems.

Coexistence of flat and steep bands influences superconducting properties.

Abstract

The basic features of multi-band superconductivity and its implications are derived. In particular, it is shown that enhancements of the superconducting transition temperature take place due to interband interactions. In addition, isotope effects differ substantially from the typical BCS scheme as soon as polaronic coupling effects are present. Special cases of the model are polaronic coupling in one band as realized e.g., in cuprates, coexistence of a flat band and a steep band like in MgB2, crossovers between extreme cases. The advantages of the multiband approach as compared to the single band BCS model are elucidated and its rather frequent realization in actual systems discussed