Intertwined Lattice Deformation and Magnetism in Monovacancy Graphene

TL;DR

This study uses density functional theory to explore how vacancies in graphene influence local magnetic moments and lattice structure, revealing two competing configurations with distinct magnetic and geometric properties.

Contribution

It predicts the existence of both planar and non-planar vacancy structures in graphene, detailing their magnetic states and the underlying electronic interactions, which was not previously understood.

Findings

Planar structure has a saturated magnetic moment of 1.5 μB.

Non-planar structure is energetically close and non-magnetic.

Vacancy-induced magnetism can be tuned by lattice deformation.

Abstract

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground state planar configuration with a saturated local moment of 1.5 $\mu_B$, and a metastable non-planar configuration with a vanishing magnetic moment, at a modest energy expense of ~50 meV. Though non-planarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states (v$\sigma$, v$\pi$). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin-polarization of v$\pi$ yielding a fractional moment, which is aligned parallel to the unpaired v$\sigma$ electron through Hund's coupling. In the non-planar configuration, the absence of orthogonal symmetry allows interaction between v$\sigma$ and local d$\pi$ states, to form a hybridized v$\sigma^\prime$ state. The non-orthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between v$\sigma$ and v$\pi$ lowers the energy. The gradual spin reversal of v$\pi$ with increasing non-planarity opens up the possibility of an intermediate structure with balanced v$\pi$ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to v$\sigma$ based spin-1/2 paramagnetism.