Dirac Half-Semimetallicity and Antiferromagnetism in Graphene Nanoribbon/Hexagonal Boron Nitride Heterojunctions

Abstract

Half-metals have been envisioned as active components in spintronic devices by virtue of their completely spin-polarized electrical currents. Actual materials hosting half-metallic phases, however, remain scarce. Here, we predict that recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity originates from the transfer of charges from hexagonal boron nitride to the embedded graphene nanoribbon. These charges give rise to opposite energy shifts of the states residing at the two edges, while preserving their intrinsic antiferromagnetic exchange coupling. Upon doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge controlling the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize tunable one-dimensional conducting channels of spin-polarized Dirac fermions seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.

One-Dimensional Magnetic Conduction Channels across Zigzag Graphene Nanoribbon/Hexagonal Boron Nitride Heterojunctions

“…On the one hand, in type-I and type-II heterojunctions, the ferromagnetic phase is more stable than the antiferromagnetic phase by 3.2 and 4.0 meV per unit cell, respectively. On the other hand, in the type-III heterojunction, the antiferromagnetic phase is more stable than the ferromagnetic phase by 2.4 meV per unit cell, qualitatively similar to hydrogen-terminated zigzag graphene nanoribbons where this value is 3.6 meV …”

“…The acquired (semi)­metallicity is present at all widths of the embedded ZGNR, as shown in Notes S7 and S8 in the Supporting Information, in agreement with recent charge transport measurements performed on these heterojunctions, in which signatures of nonzero magnetic conductance at the Fermi level have been detected in wide ZGNR embedded in h BN. The (semi)­metallic character of these heterojunctions is different from type-III heterojunctions, which possess a vanishing density of states at the Fermi level and a Dirac half-semimetallic character where the insulating behavior in one spin orientation is accompanied by a Dirac semimetallic behavior in the other, as discussed in both Note S3 in the Supporting Information and prior studies. − In Note S9 in the Supporting Information, we present the local density of states, proportional to the STM signal, of these heterojunctions, which is found to be localized along the edges of the embedded nanoribbon and to reach its maximum at the ZGNR/ h BN interfaces. We thus suggest that STM imaging can effectively differentiate between pristine regions of h BN and those hosting ZGNRs and can be used to precisely quantify the width of the embedded ZGNR.…”

“…This approach yields atomically precise, in-plane heterojunctions of ZGNRs of defined width seamlessly embedded in a continuous h BN matrix. Previous computational work has established the occurrence of a Dirac half-semimetallic phase in these heterojunctions, where one spin orientation is semimetallic while the other is semiconducting. − This intriguing phase has proven to be robust against structural disorder and can be modulated by charge doping, thus holding promise for antiferromagnetic spintronics and related applications. , However, these investigations have been limited to asymmetric configurations of the interfaces between the constituent materials, in which one edge of the nanoribbon is terminated by boron atoms and the other by nitrogen atoms of the surrounding hexagonal boron nitride. Conversely, symmetric configurations in which both edges of the embedded nanoribbon are terminated by the same chemical species, either boron or nitrogen atoms, have received little attention.…”

“…On the basis of this ab initio analysis of the interfacial charge transfer, we propose a simple model to capture the essential features of the electronic structure of these heterojunctions. Similar to previous works, ,, our model relies on the mean-field Hubbard Hamiltonian and is restricted to the p z -electrons of the ZGNR, without explicitly accounting for h BN scriptĤ = prefix− t ∑ ⟨ i , j ⟩ , σ [ ĉ i σ † ĉ j σ + hc ] + prefix∑ i , σ ϵ i ĉ i σ † ĉ i σ + U ∑ i [ n̂ i ↑ false⟨ n̂ i false↓ false⟩ + false⟨ n̂ i false↑ false⟩ n̂ i ↓ ] where ĉ i σ ( ĉ i σ † ) is the annihilation (creation) operator for a p z -electron with spin σ at lattice site i (hc denotes the hermitian conjugate), t is the hopping amplitude between nearest-neighboring lattice sites i and j , ϵ i is the on-site potential acting on lattice site i , n̂ i σ = ĉ i σ † ĉ iσ is the spin density at lattice site i , and U is the strength of the on-site Coulomb repulsion between a pair of p z -electrons located at the same lattice site. Following earlier ab initio calculations and experimentally inferred values obtained in sp 2 -hybridized carbon chains, we set U = t = 2.75 eV and use ϵ i to control the interfacial charge transfer, as we describe below.…”

“…In both cases, the shift promotes the formation of a metallic phase. A closely related mechanism is operative in type-III heterojunctions as well as in freestanding ZGNRs under transverse external and built-in electric fields . The main difference in those cases is that the energy shift of the two pairs of edge states occurs in opposite directions due to the ambipolar charge transfer at the interfaces, which results from the different chemical terminations of the edge carbon atoms.…”