High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy

TL;DR

This study uses optical spectroscopy to reveal a high-energy pseudogap in Fe-based superconductors, showing its evolution with doping and linking it to Hund's rule coupling effects, indicating coexistence of itinerant and localized electrons.

Contribution

It provides new insights into the high-energy pseudogap and its doping dependence in Fe-based superconductors, highlighting the role of Hund's rule coupling and electron localization.

Findings

Presence of a high-energy pseudogap near 0.6 eV in normal state.

Pseudogap evolution correlates with doping and bond angle changes.

Coexistence of itinerant and localized electrons in the compounds.

Abstract

We report optical spectroscopic measurements on electron- and hole-doped BaFe2As2. We show that the compounds in the normal state are not simple metals. The optical conductivity spectra contain, in addition to the free carrier response at low frequency, a temperature-dependent gap-like suppression at rather high energy scale near 0.6 eV. This suppression evolves with the As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that the feature is caused by the strong Hund's rule coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. Our experiments demonstrate the coexistence of itinerant and localized electrons in iron-based compounds, which would then lead to a more comprehensive picture about the metallic magnetism in the materials.