Coexistence of Dirac and Weyl points in non-centrosymmetric semimetal NbIrTe_(4)

Using angle-resolved photoemission spectroscopy and density functional theory calculations methods,we investigate the electronic structures and topological properties of ternary tellurides NbIrTe_(4),a candidate for type-II Weyl semimetal.We demonstrate the presence of several Fermi arcs connecting their corresponding Weyl points on both termination surfaces of the topological material.Our analysis reveals the existence of Dirac points,in addition to Weyl points,giving both theoretical and experimental evidences of the coexistence of Dirac and Weyl points in a single material.These findings not only confirm NbIrTe_(4) as a unique topological semimetal but also open avenues for exploring novel electronic devices based on its coexisting Dirac and Weyl fermions.

Multiple Dirac points including potential spin-orbit Dirac points in nonsymmorphic HfGe0.92Te

The search for new materials with Dirac points has been a fascinating subject of condensed matter physics.Here we first report the growth and band structure of HfGe_(0.92)Te single crystals featuring three different types of Dirac points.HfGe_(0.92)Te crystallizes in a nonsymmorphic tetragonal space group P4/nmm(No.129),having a square Ge-atom plane with vacancies of about 8%.Using angle-resolved photoemission spectroscopy(ARPES),the Dirac nodal line composed of conventional Dirac points vulnerable to spin-orbit coupling(SOC)is observed,accompanied by robust Dirac points protected by the nonsymmorphic symmetry against SOC and vacancies.In particular,spin-orbit Dirac points(SDPs)originating from the surface formed under significant SOC could exist based on ARPES and calculations.Quasi-two-dimensional(quasi-2D)characteristics are confirmed by angular-resolved magnetoresistance.HfGe_(0.92)Te bulk crystals can be easily exfoliated to flakes with a thickness of approximately 5 nm for the quasi-2D nature.Thus,HfGe_(0.92)Te provides a good platform to explore exotic topological phases or topological properties with three different types of Dirac points,which is a potential candidate to achieve novel 2D SDPs.

Movement of Dirac points and band gaps in graphyne under rotating strain

The introduction of lattice anisotropy causes Dirac cones to shift in response to the applied strain, leaving a pseudogap at the original Dirac points. Here, a group-theory analysis is combined with first-principles calculations to reveal the movement characteristics of Dirac points and band gaps in various graphynes under rotating uniaxial and shear strains. Graphene, where linear effects dominate, is different from α-,β-, and γ-graphynes, which generate strong nonlinear responses due to their bendable acetylenic linkages. However, the linear components of the electronic response, which are essential in determining material performance such as intrinsic carrier mobility due to electron-phonon coupling, can be readily separated, and are well described by a unified theory. The movement of the Dirac points in α-graphyne is circular under a rotating strain, and the pseudogap opening is isotropic with a magnitude of only 2% that in graphene. In comparison, the movement in β-graphyne is elliptical and the center is displaced from the origin. For γ-graphyne, three branches of gaps change with the applied strains with a sine/cosine dependence on the strain angle. The developed methodology is useful in determining the electronic response to various strains of Dirac materials and two-dimensional semiconductors,

Photonic Dirac cone and topological transition in a moving dielectric slab

The moving media theory is applied to a photonic confined structure which is a continuous dielectric slab waveguide with the uniaxial anisotropy and without the discrete translational symmetry.The moving effect not only brings about non-reciprocity to the whole photonic band structure in the co-moving and counter-moving directions,but also leads to the topological transition of local degenerate points within the band diagram.We demonstrate through calculation that the type-ⅡDirac point can be turned into type-ⅠDirac point when the uniaxial slab is moving over certain speed.Our results provide a new approach to regulate the topology of degeneracy for two-dimensional photonic bands in the continuous translational symmetry condition.

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.

Low-Frequency Noise in Gate Tunable Topological Insulator Nanowire Field Emission Transistor near the Dirac Point

Low-frequency flicker noise is usually associated with material defects or imperfection of fabrication procedure. Up to now, there is only very limited knowledge about flicker noise of the topological insulator, whose topologically protected conducting surface is theoretically immune to back scattering. To suppress the bulk conductivity we synthesize antimony doped Bi2Se3 nanowires and conduct transport measurements at cryogenic temperatures. The low-frequency current noise measurement shows that the noise amplitude at the high-drain current regime can be described by Hooge＇s empirical relationship, while the noise level is significantly lower than that predicted by Hooge＇s model near the Dirac point. Furthermore, different frequency responses of noise power spectrum density for specific drain currents at the low drain current regime indicate the complex origin of noise sources of topological insulator.

Topological states switching and group velocity control in two-dimensional non-reciprocal Hermitian photonic lattice

We proposed a model with non reciprocal coupling coefficients, in which the imaginary parts γ indicate the phase delay or exceed term. The distributions of band structure and the group velocity are both characterized as a function of the coupling. we studied the system’s topological states and group velocity control. The results show that the movement and breaking of Dirac points exist in the energy band of the system. By changing the coupling coefficients, the conversion between any topological states corresponds to different Chern number. Topological edge states exist in topological nontrivial systems that correspond to the two different Chern numbers. Besides, it is also found that both the coupling coefficient and the wave vector can cause the oscillation of the pulse group velocity. At the same time, the topological state can suppress the amplitude of the group velocity profiles. Our findings enrich the theory of light wave manipulation in high-dimensional photonic lattices and provide a novel view for realizing linear localization and group velocity regulation of light waves,which has potential application in high-speed optical communication and quantum information fields.

Gap opening and tuning in single-layer graphene with combined electric and magnetic field modulation

The energy band structure of single-layer graphene under one-dimensional electric and magnetic field modulation is theoretically investigated. The criterion for bandgap opening at the Dirac point is analytically derived with a two-fold degeneracy second-order perturbation method. It is shown that a direct or an indirect bandgap semiconductor could be realized in a single-layer graphene under some specific configurations of the electric and magnetic field arrangement. Due to the bandgap generated in the single-layer graphene, the Klein tunneling observed in pristine graphene is completely suppressed.

Novel conductance step in carbon nanotube with wing-like zigzag graphene nanoribbons

Connecting one armchair carbon nanotube（CNT） to several zigzag graphene nanoribbons（ZGNRs） we find that the topologically-protected edge states of ZGNRs and the massless Dirac particle inherited from CNT still hold from the analysis of the band structure and the edge state. Furthermore, the lowest conductance step at the valley bottom increases proportionally with increasing the number of ZGNR wings. A novel conductance step of a peak occurs in the valley, which is two steps higher than the lowest step at the valley bottom. In addition, with increasing the number of ZGNR wings the width of the novel conductance step becomes narrow.

Electronic and topological properties of kagome lattice LaV3Si2

Topological kagome lattice at the frontier of fundamental physics plays a key role in non-trivial topological quantum state.Here,we predict and investigate kagome lattice rare-earth vanadium-based quantum material LaV3Si2using density functional theory calculations.Both phonon spectrum and crystal transformation show stability of this material,which may be grown by experimental method.Dirac fermions,flat bands,and van Hove points as some basic features are presented in band structure and surface states.Further,symmetry-based compatibility relations support enforced semi-metal for occupied electron numbers with strong Berry curvature.Our results suggest that rare-earth vanadium-based RV3Si2can be treated as a new family kagome lattice.

Dual topology in van der Waals-type superconductor Nb_(2)S_(2)C

Nb_(2)S_(2)C is a van der Waals type layered superconductor with a transition temperature Tc=7.6 K.In this paper,detailed calculations of the electronic structure and topological properties of Nb_(2)S_(2)C were performed from first principles.We find that Nb2S2C is a highly anisotropic metal with multi-band characteristics.In the absence of spin-orbit coupling(SOC),there appears one pair of triply degenerate points created by band inversion along the Γ-A line.When SOC is considered,the triple points are gapped.Intriguingly,two distinct types of topological states,including topological Dirac semimetal and topological insulator states,co-emergence in the vicinity of Fermi level.Moreover,the topology of Nb_(2)S_(2)C is robust to external pressure and the Fermi level can be shifted downward to the topological Dirac semimetal state and topological insulator state at 10 GPa and 14 GPa,respectively.The results herein provide a new platform not only for the studies of physics of low-dimensional superconductor but also for further investigations of topological superconductivity.

Light response and adsorption interaction of black phosphorus quantum dots and single‑layer graphene phototransistor

Black phosphorus quantum dots(BPQDs)are synthesized and combined with graphene sheet.The fabricated BPQDs/graphene devices are capable of detecting visible and near infrared radiation.The adsorption efect of BPQDs in graphene is clarifed by the relationship of the photocurrent and the shift of the Dirac point with diferent substrate.The Dirac point moves toward a neutral point under illumination with both SiO_(2)/Si and Si(3)N_(4)/Si substrates,indicating an anti-doped feature of photo-excitation.To our knowledge,this provides the frst observation of photoresist induced photocurrent in such systems.Without the infuence of the photoresist the device can respond to infrared light up to 980 nm wavelength in vacuum in a cryostat,in which the photocurrent is positive and photoconduction efect is believed to dominate the photocurrent.Finally,the adsorption efect is modeled using a frst-principle method to give a picture of charge transfer and orbital contribution in the interaction of phosphorus atoms and single-layer graphene.