From Hunds insulator to Fermi liquid: Optical spectroscopy study of K doping in BaMn$_2$As$_2$

TL;DR

This study uses optical spectroscopy and theoretical calculations to explore the electronic properties of BaMn$_2$As$_2$ and its potassium-doped variants, revealing a large charge gap and Fermi liquid behavior, and discussing the implications for Hund's metal physics.

Contribution

It demonstrates that BaMn$_2$As$_2$ is a Mott-Hund's insulator with a large charge gap and shows that doping does not yet produce a strongly correlated Hund's metal state.

Findings

Charge gap of 0.86 eV in BaMn$_2$As$_2$

Doped BaMn$_2$As$_2$ exhibits Fermi liquid behavior

Doping does not realize a Hund's metal state

Abstract

We present optical transmission measurements that reveal a charge gap of 0.86 eV in the local moment antiferromagnetic insulator BaMn$_2$As$_2$, an order of magnitude larger than previously reported. Density functional theory plus dynamical mean field theory (DFT+DMFT) calculations correctly reproduce this charge gap only when a strong Hund's coupling is considered. Thus, BaMn$_2$As$_2$ is a member of a wider class of Mn pnictide compounds that are Mott-Hund's insulators. We also present optical reflectance for metallic 2% K doped BaMn$_2$As$_2$ that we use to extract the optical conductivity at different temperatures. The optical conductivity $\sigma_1$($\omega$) exhibits a metallic response that is well described by a simple Drude term. Both $\sigma$($\omega$$\rightarrow$0, T) and $\rho$(T) exhibit Fermi liquid temperature dependencies. From these measurements, we argue that a more strongly correlated Hund's metal version of the parent compounds of the iron pnictide superconductors has not yet been realized by doping this class of Hund's insulators.