Optical properties and electronic correlations in La$_3$Ni$_2$O$_7$ bilayer nickelates under high pressure

TL;DR

This study uses density functional theory to analyze the optical properties and electronic correlations in La3Ni2O7 bilayer nickelates, revealing moderate correlations, effects of oxygen vacancies, and structural transitions under high pressure relevant to high-Tc superconductivity.

Contribution

It provides a detailed theoretical investigation of optical spectra, electronic correlations, and structural effects in La3Ni2O7, connecting experimental observations with microscopic insights.

Findings

Agreement with experimental spectra at U=3eV

Oxygen vacancies mainly at inner apical sites

Structural transition enhances Drude weight and hole pocket formation

Abstract

We explore the optical properties of La3Ni2O7 bilayer nickelates by using density functional theory including a Coulomb repulsion term. Convincing agreement with recent experimental ambient-pressure spectra is achieved for U=3eV, which permits tracing the microscopic origin of the characteristic features. Simultaneous consistency with angle-resolved photoemission spectroscopy and x-ray diffraction suggests the notion of rather moderate electronic correlations in this novel high-Tc superconductor. Oxygen vacancies form predominantly at the inner apical sites and renormalize the optical spectrum quantitatively, while the released electrons are largely accommodated by a defect state. We show that the structural transition occurring under high pressure coincides with a significant enhancement of the Drude weight and a reduction of the out-of-plane interband contribution that act as a fingerprint of the emerging hole pocket. We further calculate the optical spectra for various possible magnetic phases including spin-density waves and discuss the results in the context of experiment. Finally, we investigate the role of the 2-2 versus 1-3 layer stacking and compare the bilayer nickelate to La4Ni3O10, La3Ni2O6, and NdNiO2, unveiling general trends in the optical spectrum as a function of the formal Ni valence in Ruddlesden-Popper versus reduced Ruddlesden-Popper nickelates.