Optical properties of the infinite-layer La$_{1-x}$Sr$_{x}$NiO$_{2}$ and hidden Hund's physics

TL;DR

This study uses DFT+DMFT to analyze the optical properties of La$_{1-x}$Sr$_x$NiO$_2$, revealing a Hund's metallic state with unusual spectral weight behavior influenced by orbital fluctuations.

Contribution

It provides new insights into the optical response and Hund's physics of La$_{1-x}$Sr$_x$NiO$_2$, highlighting the role of orbital fluctuations and contrasting with Mott insulators.

Findings

Spectral weight decreases with doping and increases with temperature.

Optical conductivity decreases with temperature due to coherence-incoherence crossover.

Supports a Hund's metallic state with dynamical orbital fluctuations.

Abstract

We investigate the optical properties of the normal state of the infinite-layer La$_{1-x}$Sr$_x$NiO$_2$ using DFT+DMFT. We find a correlated metal which exhibits substantial transfer of spectral weight to high energies relative to the density functional theory. The correlations are not due to Mott physics, which would suppress the charge fluctuations and integrated optical spectral weight as we approach a putative insulating state. Instead we find the unusual situation, that the integrated optical spectral weight {\it decreases} with doping and {\it increases } with increasing temperature. We contrast this with the coherent component of the optical conductivity, which {\it decreases} with increasing temperature as a result of a coherence$-$incoherence crossover. Our optical studies support a picture of a Hund's metallic state, where dynamical orbital fluctuations are visible at intermediate energies, even if at low energies the Fermi surface has primarily $d_{x^2 - y^2}$ character and we propose a low-energy two-band model with atom centered $e_g$ states.