2D Ladder polyborane: an ideal Dirac semimetal with a multi-feld-tunable band gap

Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and non-trivial Berry phase play the pivotal role in the remarkable electronic, optical, mechanical and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of SOC. Here we report a route to establishing robust Di...

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...

Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and opto-electrical anisotropy. Very recently, a novel semi-hydrogenated borophene, called $\alpha'$-4H, was synthesized in large-scale freestanding samples, which exhibits excellent air-stability and semiconducting nature. Herein, using the density functional theory (DFT) and many-body perturbation theory (MBPT), we investigate the electronic and exci...

Lateral superlattices have attracted major interest as this may allow one to modify spectra of two dimensional electron systems and, ultimately, create materials with tailored electronic properties. Previously, it proved difficult to realize superlattices with sufficiently short periodicity and weak disorder, and most of the observed features could be explained in terms of commensurate cyclotron orbits. Evidence for the formation of superlattice minibands (so called Hofstadte...

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 ...

Hyperbolic materials are receiving significant attention due to their ability to support electromagnetic fields with arbitrarily high momenta and, hence, to achieve very strong light confinement. Here, based on first-principles calculations and many-body perturbation theory, we explore the characteristic of two-dimensional plasmon modes and its hyperbolic properties for two phases of single layer boron hosting tilted Dirac cone, namely, the $hr$-$sB$ and $8Pmmn$ borophene. In...

Inspired by the great development of graphene, more and more works have been conducted to seek new two-dimensional (2D) materials with Dirac cones. Although 2D Dirac materials possess many novel properties and physics, they are rare compared with the numerous 2D materials. To provide explanation for the rarity of 2D Dirac materials as well as clues in searching for new Dirac systems, here we review the recent theoretical aspects of various 2D Dirac materials, including graphe...

Two-dimensional semimetals with tilted Dirac cones in the electronic band structure are shown to exhibit spatial separation of carriers belonging to different valleys under illumination. In stark contrast to gapped Dirac materials this optovalleytronic phenomenon occurs in systems with intact inversion and time-reversal symmetry that host massless Dirac cones in the band structure, thereby retaining the exceptional graphene-like transport properties. As a result we demonstrat...

The band gap of periodically-doped graphene with hydrogen is investigated. It is found through a tight-binding model (TB) that for certain periodicities, called here NGPs (non-gap periodicities), no gap is opened at the Dirac point. This result is confirmed by Density Functional Theory (DFT) calculations. DFT results show that a tiny gap is opened for NGPs due to exchange effects, not taken into account in the TB model. However, this tiny gap is one or two orders of magnitude...

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...