Low-energy interband transitions in the infrared response of Ba(Fe1-xCox)2As2

TL;DR

This study investigates the infrared response of Ba(Fe1-xCox)2As2, revealing that low-energy interband transitions, rather than magnetic interactions or pseudogaps, dominate the IR features and are linked to the material's multiband electronic structure and Lifshitz transition.

Contribution

It clarifies the origin of IR bands in Ba(Fe1-xCox)2As2, associating them with interband transitions and providing bulk evidence for a Lifshitz transition near the suppression of superconductivity.

Findings

The 1000 cm-1 band is due to interband transitions, not magnetic interactions.

This band vanishes near the doping level where superconductivity disappears.

A second band around 2300 cm-1 highlights the multiband nature of the material.

Abstract

We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around 1000 cm-1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and T-dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm-1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.