Seeing Dirac electrons and heavy fermions in new boron nitride monolayers

Most three-dimensional(3D)and two-dimensional(2D)boron nitride(BN)structures are wide-band-gap insulators.Here,we propose two BN monolayers having Dirac points and flat bands,respectively.One monolayer is named as 5-7 BN that consists of five-and seven-membered rings.The other is a Kagome BN made of triangular boron rings and nitrogen dimers.The two structures show not only good dynamic and thermodynamic stabilities but also novel electronic properties.The 5-7 BN has Dirac points on the Fermi level,indicating that the structure is a typical Dirac material.The Kagome BN has double flat bands just below the Fermi level,and thus there are heavy fermions in the structure.The flat-band-induced ferromagnetism is also revealed.We analyze the origination of the band structures by partial density of states and projection of orbitals.In addition,a possible route to experimentally grow the two structures on some suitable substrates such as the PbO2(111)surface and the CdO(111)surface is also discussed,respectively.Our research not only extends understanding on the electronic properties of BN structures,but also may expand the applications of BN materials in 2D electronic devices.

Bilayer twisting as a mean to isolate connected flat bands in a kagome lattice through Wigner crystallization

The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy.Alternatively,one may start with a 2 D kagome lattice,which already possesses flat bands at the Fermi level,but unfortunately these bands connect quadratically to other(dispersive)bands,leading to undesirable effects.Here,we propose,by first-principles calculation and tight-binding modeling,that the same bilayer twisting approach can be used to isolate the kagome flat bands.As the starting kinetic energy is already vanishingly small,the interlayer vd W potential is always sufficiently large irrespective of the twisting angle.As such the electronic states in the(connected)flat bands become unstable against a spontaneous Wigner crystallization,which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.

Tunable bandgaps and flat bands in twisted bilayer biphenylene carbon

Owing to the interaction between the layers,the twisted bilayer two-dimensional(2 D)materials exhibit numerous unique optical and electronic properties different from the monolayer counterpart,and have attracted tremendous interests in current physical research community.By means of first-principles and tight-binding model calculations,the electronic properties of twisted bilayer biphenylene carbon(BPC)are systematically investigated in this paper.The results indicate that the effect of twist will not only leads to a phase transition from semiconductor to metal,but also an adjustable band gap in BPC(0 me V to 120 me V depending on the twist angle).Moreover,unlike the twisted bilayer graphene(TBG),the flat bands in twisted BPC are no longer restricted by"magic angles",i.e.,abnormal flat bands could be appeared as well at several specific large angles in addition to the small angles.The charge density of these flat bands possesses different local modes,indicating that they might be derived from different stacked modes and host different properties.The exotic physical properties presented in this work foreshow twisted BPC a promising material for the application of terahertz and infrared photodetectors and the exploration of strong correlation.

Robust and intrinsic type-Ⅲnodal points in a diamond-like lattice

An ideal type-Ⅲnodal point is generated by crossing a completely flat band and a dispersive band along a certain momentum direction.To date,the type-Ⅲnodal points found in two-dimensional(2D)materials have been mostly accidental and random rather than ideal cases,and no one mentions what kind of lattice can produce ideal nodal points.Here,we propose that ideal type-Ⅲnodal points can be obtained in a diamond-like lattice.The flat bands in the lattice originate from destructive interference of wavefunctions,and thus are intrinsic and robust.Moreover,the specific lattice can be realized in some 2D carbon networks,such as T-graphene and its derivatives.All the carbon structures possess type-ⅢDirac points.In two of the structures,consisting of triangular carbon rings,the type-ⅢDirac points are located just on the Fermi level and the Fermi surface is very clean.Our research not only opens a door to finding the ideal type-ⅢDirac points,but also provides 2D materials for exploring their physical properties experimentally.