Quasiparticles and charge transfer at the two surfaces of the honeycomb iridate Na$_2$IrO$_3$

TL;DR

This study investigates the complex electronic structure of Na$_2$IrO$_3$, revealing surface-dependent charge transfer effects and the existence of bulk quasiparticles through advanced spectroscopic techniques.

Contribution

It provides a detailed experimental analysis showing how surface termination affects charge transfer and quasiparticle visibility in Na$_2$IrO$_3$, clarifying previous conflicting interpretations.

Findings

Anomalous band from charge transfer identified

Bulk quasiparticles confirmed but surface-dependent

Surface termination influences electronic states

Abstract

Direct experimental investigations of the low-energy electronic structure of the Na$_2$IrO$_3$ iridate insulator are sparse and draw two conflicting pictures. One relies on flat bands and a clear gap, the other involves dispersive states approaching the Fermi level, pointing to surface metallicity. Here, by a combination of angle-resolved photoemission, photoemission electron microscopy, and x-ray absorption, we show that the correct picture is more complex and involves an anomalous band, arising from charge transfer from Na atoms to Ir-derived states. Bulk quasiparticles do exist, but in one of the two possible surface terminations the charge transfer is smaller and they remain elusive.