Spectroscopic signatures of molecular orbitals on a honeycomb lattice

TL;DR

This paper demonstrates that combined photoemission and optical spectroscopy can identify molecular orbitals in honeycomb lattice transition metal oxides, providing experimental evidence for their formation in SrRu$_{2}$O$_{6}$.

Contribution

It proposes a method to detect molecular orbitals using spectroscopic techniques, clarifying their presence in SrRu$_{2}$O$_{6}$ and addressing previous experimental gaps.

Findings

Optical transitions are suppressed between certain molecular orbitals due to symmetry.

Photoemission and inverse photoemission are insensitive to specific molecular orbitals.

Spectroscopic comparison reveals signatures indicative of molecular orbital formation.

Abstract

A tendency to form benzene-like molecular orbitals has been recently shown to be a common feature of the $4d$ and $5d$ transition metal oxides with a honeycomb lattice. This tendency competes with other interactions such as the spin-orbit coupling and Hubbard correlations, and can be partially or completely suppressed. In the calculations, SrRu$_{2}$O$_{6}$ presents the cleanest, so far, case of well-formed molecular orbitals, however, direct experimental evidence for or against this proposition has been missing. In this paper, we show that combined photoemission and optical studies can be used to identify molecular orbitals in SrRu$_{2}$O$_{6}$. Symmetry-driven election selection rules suppress optical transitions between certain molecular orbitals, while photoemission and inverse photoemission measurements are insensitive to them. Comparing the photoemission and optical conductivity spectra one should be able to observe clear signatures of molecular orbitals.