Anomalous density of states in multiband superconductors near Lifshitz transition

TL;DR

This paper investigates how the density of states in multiband superconductors behaves near a Lifshitz transition, revealing divergent or vanishing features depending on the transition side, with implications for iron-based superconductors.

Contribution

It provides a theoretical analysis of the density of states near Lifshitz transition in multiband superconductors, explaining experimental observations in iron pnictides and selenides.

Findings

Density of states diverges at the induced gap on one side of the transition.

Density of states vanishes at the gap on the other side.

The model explains recent experimental gap structures in iron-based superconductors.

Abstract

We consider a multiband metal with deep primary bands and a shallow secondary one. In the normal state the system undergoes Lifshitz transition when the bottom of the shallow band crosses the Fermi level. In the superconducting state Cooper pairing in the shallow band is induced by the deep ones. As a result, the density of electrons in the shallow band remains finite even when the bottom of the band is above the Fermi level. We study the density of states in the system and find qualitatively different behaviors on the two sides of the Lifshitz transition. On one side of the transition the density of states diverges at the energy equal to the induced gap, whereas on the other side it vanishes. We argue that this physical picture describes the recently measured gap structure in shallow bands of iron pnictides and selenides.