Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions

We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our {\em ab initio} calculations for the neutral system reveal previously unknown stable 2D $\epsilon$-B and $\omega$-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of ...

Systematic ab initio structure prediction was applied for the first time to predict low energy surface reconstructions by employing the minima hopping method on the \alpha-boron (111) surface. Novel reconstruction geometries were identified and carefully characterized in terms of structural and electronic properties. Our calculations predict the formation of a planar, mono-layer sheet at the surface, which is responsible for conductive surface states. Furthermore, the isolate...

Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of boron nanostructures were found to be metallic, despite of their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by the symmetry breaking that divides boron atoms into cations and ani...

A computation methodology based on ab initio evolutionary algorithms and the spin-polarized density functional theory was developed to predict two-dimensional (2D) magnetic materials. Its application to a model system borophene reveals an unexpected rich magnetism and polymorphism. A stable borophene with nonzero thickness was an antiferromagnetic (AFM) semiconductor from first-principles calculations, which can be further turned into a half metal by finite electron doping. I...

Recently, the crystal symmetry-protected topological semimetals have aroused extensive interests, especially for the nonsymmorphic symmetry-protected one. We list the possible nonmagnetic topological semimetals and develop their k.p Hamiltonian in all layer groups with multiple screw axes in the absence of spin-orbital coupling. We find a novel cat's cradle-like topological semimetal phase, which looks like multiple hourglass-like band structures staggered together. Furthermo...

Thin flakes of black phosphorus (BP) are a two-dimensional (2D) semiconductor whose energy gap is predicted being sensitive to the number of layers and external perturbations. Very recently, it was found that a simple method of potassium (K) doping on the surface of BP closes its band gap completely, producing a Dirac semimetal state with a linear band dispersion in the armchair direction and a quadratic one in the zigzag direction. Here, based on first-principles density fun...

Silicene, an analogue of graphene, was so far predicted to be the only two-dimensional silicon (2D-Si) with massless Dirac fermions. Here we predict a brand new 2D-Si Dirac semimetal, which we name siliconeet [silik'ni:t]. Unexpectedly, it has a much lower energy than silicene and robust direction-dependent Dirac cones with Fermi velocities comparable to those in graphene. Remarkably, its peculiar structure based on pentagonal rings and fivefold coordination plays a critical ...

We present a new class of boron sheets, composed of triangular and hexagonal motifs, that are more stable than structures considered to date and thus are likely to be the precursors of boron nanotubes. We describe a simple and clear picture of electronic bonding in boron sheets and highlight the importance of three-center bonding and its competition with two-center bonding, which can also explain the stability of recently discovered boron fullerenes. Our findings call for rec...

The 8-Pmmn borophene, a boron analogue of graphene, hosts tilted and anisotropic massless Dirac fermion quasiparticles owing to the presence of the distorted graphene-like sublattice. First-principles calculations show that the stacked 8-Pmmn borophene is transformed into the fused three-dimensional borophene under pressure, being accompanied by the partially bond-breaking and bond-reforming. Strikingly, the fused 8-Pmmn borophene inherits the Dirac band dispersion resulting ...

First-principles calculations on monolayer 8-{\it Pmmn} borophene are reported to reveal unprecedented electronic properties in a two-dimensional material. Based on a Born effective charge analysis, 8-{\it Pmmn} borophene is the first single-element based monolayered material exhibiting two sublattices with substantial ionic features. The observed Dirac cones are actually formed by the p$_z$ orbitals of one of the inequivalent sublattices composed of uniquely four atoms, yiel...