Molecular collapse in graphene: sublattice symmetry effect

TL;DR

This study explores how sublattice symmetry influences molecular collapse phenomena in graphene with charge-tunable vacancies, revealing specific spectral shifts and features that depend on vacancy type and symmetry breaking.

Contribution

It demonstrates that sublattice symmetry affects satellite and vacancy-specific collapse peaks, providing a way to distinguish vacancy types experimentally.

Findings

Satellite peaks shift due to sublattice symmetry breaking.

Vacancy characteristic collapse peaks only occur in A-B type vacancies.

Collapse features merge with main peaks at higher charge, energy, and separation.

Abstract

Atomic collapse can be observed in graphene because of its large "effective" fine structure constant, which enables this phenomenon to occur for an impurity charge as low as $Z_c\sim 1-2$. Here, we investigate the effect of the sublattice symmetry on molecular collapse in two spatially separated charge tunable vacancies, that are located on the same (A-A type) or different (A-B type) sublattices. We find that the broken sublattice symmetry: (1) does not affect the location of the main bonding and anti-bonding molecular collapse peaks, (2) but shifts the position of the satellite peaks, because they are a consequence of the breaking of the local sublattice symmetry, and (3) there are vacancy characteristic collapse peaks that only occur for A-B type vacancies, which can be employed to distinguish them experimentally from the A-A type. As the charge, energy, and separation distance increase, the additional collapse features merge with the main molecular collapse peaks. We show that the spatial distribution around the vacancy site of the collapse states allows us to differentiate the molecular from the frustrated collapse.