Magnetic Manipulation of Massless Dirac Fermions in Graphene Quantum Dot

We have theoretically analyzed the quasibound states in a Mraphene quantum dot （GO, D） with a magnetic flux -φ in the centre. It is shown that the two-fold time reversal degeneracy is broken and the quasibound states of GQD with positive^negative angular momentum shifted upwards/downwards with increasing the magnetic flux. The variation of the quasibound energy depends linearly on the magnetic flux, which is quite different from the parabolic relationship for SchrSdinger electrons. The GQD＇s quasibound states spectrum shows an obvious Aharonov-Bohm （AB） oscillations with the magnetic flux. It is also shown that the quasibound state with energy equal to the barrier height becomes a bound state completely confined in GQD.

Highly anisotropic Dirac fermion and spin transport properties in Cu-graphane

Inspired by the successful synthesis of h Hv-graphane[Nano Lett.15903(2015)],a new two-dimensional(2D)Janus material Cu-graphane is proposed based on the first-principles calculations.Without the spin-orbit coupling(SOC)effect,Cu-graphane is a Dirac semimetal with a highly anisotropic Dirac cone,whose Fermi velocity ranges from 0.12×10^(5)m/s to2.9×10^(5)m/s.The Dirac cone near the Fermi level can be well described with an extended 2D Dirac model Hamiltonian.In the presence of the SOC effect,band splitting is observed around the Fermi level,and a large intrinsic spin Hall conductivity(ISHC)with a maximum value of 346(h/e)S/cm is predicted.Moreover,the spin Hall transport can be regulated by slightly adjusting the Fermi energy,e.g.,grid voltage or chemical doping.Our work not only proposes a new 2D Janus material with a highly anisotropic Dirac cone and a large ISHC,but also reveals that a large ISHC may exist in some Dirac systems.

Long-range adsorbate interactions mediated by two-dimensional Dirac fermions

We provide here an analytical formalism to describe the indirect interaction between adsorbed atom or molecule pairs mediated by two-dimensional(2D)Dirac fermions.We show that in contrast to the case of traditional 2D electron gas,in the 2D Dirac system,the long-range interaction behaves as 1/r^(3) decaying Friedel oscillation.This analytical formalism is fully consistent with a tight-binding numerical calculation of honeycomb lattices.Our formalism is suitable for the realistic 2D Dirac materials,such as graphene and surface states of three-dimensional topological insulators.

Maximal symmetry and mass generation of Dirac fermions and gravitational gauge field theory in six-dimensional spacetime

The relativistic Dirac equation in four-dimensional spacetime reveals a coherent relation between the dimensions of spacetime and the degrees of freedom of fermionic spinors. A massless Dirac fermion generates new symmetries corresponding to chirality spin and charge spin as well as conformal scaling transformations. With the introduction of intrinsic W-parity, a massless Dirac fermion can be treated as a Majorana-type or Weyl-type spinor in a six-dimensional spacetime that reflects the intrinsic quantum numbers of chirality spin. A generalized Dirac equation is obtained in the six-dimensional spacetime with a maximal symmetry. Based on the framework of gravitational quantum field theory proposed in Ref. [1] with the postulate of gauge invariance and coordinate independence, we arrive at a maximally symmetric gravitational gauge field theory for the massless Dirac fermion in six-dimensional spacetime. Such a theory is governed by the local spin gauge symmetry SP（1,5） and the global Poincar′e symmetry P（1,5）= SO（1,5） P^1,5 as well as the charge spin gauge symmetry SU（2）. The theory leads to the prediction of doubly electrically charged bosons. A scalar field and conformal scaling gauge field are introduced to maintain both global and local conformal scaling symmetries. A generalized gravitational Dirac equation for the massless Dirac fermion is derived in the six-dimensional spacetime. The equations of motion for gauge fields are obtained with conserved currents in the presence of gravitational effects. The dynamics of the gauge-type gravifield as a Goldstone-like boson is shown to be governed by a conserved energy-momentum tensor, and its symmetric part provides a generalized Einstein equation of gravity. An alternative geometrical symmetry breaking mechanism for the mass generation of Dirac fermions is demonstrated.

Recent progress on non-Abelian anyons:from Majorana zero modes to topological Dirac fermionic modes

Majorana zero modes(MZMs)have been intensively studied in recent decades theoretically and experimentally as the most promising candidate for non-Abelian anyons supporting topological quantum computation(TQC).In addition to the Majorana scheme,some non-Majorana quasiparticles obeying non-Abelian statistics,including topological Dirac fermionic modes,have also been proposed and therefore become new candidates for TQC.In this review,we overview the non-Abelian braiding properties as well as the corresponding braiding schemes for both the MZMs and the topological Dirac fermionic modes,emphasizing the recent progress on topological Dirac fermionic modes.A topological Dirac fermionic mode can be regarded as a pair of MZMs related by unitary symmetry,which can be realized in a number of platforms,including the one-dimensional topological insulator,higher-order topological insulator,and spin superconductor.This topological Dirac fermionic mode possesses several advantages compared with its Majorana cousin,such as superconductivity-free and larger gaps.Therefore,it provides a new avenue for investigating non-Abelian physics and possible TQC.

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.

Thermodynamic properties of massless Dirac–Weyl fermions under the generalized uncertainty principle

The Dirac–Weyl equation characterized quasi-particles in the T3 lattice are studied under external magnetic field using the generalized uncertainty principle(GUP). The energy spectrum of the quasi-particles is found by the Nikiforov–Uvarov method. Based on the energy spectrum obtained, the thermodynamic properties are given, and the influence of the GUP on the statistical properties of systems is discussed. The results show that the energy and thermodynamic functions of massless Dirac–Weyl fermions in the T3 lattice depend on the variation of the GUP parameter.

Fabrication of honeycomb AuTe monolayer with Dirac nodal line fermions

Two-dimensional honeycomb lattices show great potential in the realization of Dirac nodal line fermions(DNLFs).Here,we successfully synthesized a gold telluride(AuTe)monolayer by direct tellurizing an Au(111)substrate.Low energy electron diffraction measurements reveal that it is(2×2)AuTe layer stacked onto(3×3)Au(111)substrate.Moreover,scanning tunneling microscopy images show that the AuTe layer has a honeycomb structure.Scanning transmission electron microscopy reveals that it is a single-atom layer.In addition,first-principles calculations demonstrate that the honeycomb AuTe monolayer exhibits Dirac nodal line features protected by mirror symmetry,which is validated by angle-resolved photoemission spectra.Our results establish that monolayer AuTe can be a good candidate to investigate 2D DNLFs and provides opportunities to realize high-speed low-dissipation devices.

石墨烯中Dirac费米子量子输运特性研究

利用紧束缚模型的方法研究了石墨烯中Di-rac费米子的势垒相关输运特性,分析了石墨烯中Di-rac费米子的势垒透射率与其入射角度、入射能量、势垒高度和势垒厚度等对应关系。通过理论分析和数值计算表明,石墨烯中Dirac费米子的隧穿系数随矩形势垒的高度和厚度的变化都呈现出明显的振荡效应,隧穿系数与入射角度依赖关系证实了Klein效应。在一定参数下,Dirac费米子的势垒透射率随Dirac费米子入射能量的变化经历从"0"～"1"的突变,显示良好的"电导开关效应",该效应在微电子器件中将有很好的应用前景。

Nonlinear Schroedinger Solitons in Massive Yang-Mills Theory and Partial Localization of Dirac Matter

We investigate the classical dynamics of the massive SU(2) Yang-Mills field in the framework of multiple scale perturbation theory. We show analytically that there exists a subset of solutions having the form of a kink soliton, modulated by a plane wave, in a linear subspace transverse to the direction of free propagation. Subsequently, we explore how these solutions affect the dynamics of a Dirac field possessing an SU(2) charge. We find that this class of Yang- Mills configurations, when regarded as an external field, leads to the localization of the fermion along a line in the transverse space. Our analysis reveals a mechanism for trapping SU(2) charged fermions in the presence of an external Yang-Mills field indicating the non-abelian analogue of Landau localization in electrodynamics.

D Dimensions and N ×N Matrix Representations of Fermions

The connection between the number of dimensions and the size of the representation matrices in the Dirac equation has been discussed thoroughly and the restriction N2 = 2D was derived. In this summary, the result is brought again, this time with emphasis on the importance of irreducibility of the representations. As a counter example, the case of the neutrino is discussed where the above restriction does not hold, indicating that the Dirac equation, in this case, is reducible.

The Approximation of Bosonic System by Fermion in Quantum Cellular Automaton

In one-dimensional multiparticle Quantum Cellular Automaton (QCA), the approximation of the bosonic system by fermion (boson-fermion correspondence) can be derived in a rather simple and intriguing way, where the principle to impose zero-derivative boundary conditions of one-particle QCA is also analogously used in particle-exchange boundary conditions. As a clear cut demonstration of this approximation, we calculate the ground state of few-particle systems in a box using imaginary time evolution simulation in 2nd quantization form as well as in 1st quantization form. Moreover in this 2nd quantized form of QCA calculation, we use Time Evolving Block Decimation (TEBD) algorithm. We present this demonstration to emphasize that the TEBD is most natu-rally regarded as an approximation method to the 2nd quantized form of QCA.

拓扑绝缘体二维纳米结构与器件

拓扑绝缘体是一种全新的量子功能材料,具有绝缘性体能带结构和受时间反演对称性保护的自旋分辨的金属表面态,属于Dirac粒子系统,将在新原理纳电子器件、自旋器件、量子计算、表面催化和清洁能源等方面有广泛的应用前景.理论和实验相继证实Sb2Te3,Bi2Se3和Bi2Te3单晶具有较大的体能隙和单一Dirac锥表面态,已经迅速成为了拓扑绝缘体研究中的热点材料.然而,利用传统的高温烧结法所制成的拓扑绝缘体单晶块体样品常存在大量本征缺陷并被严重掺杂,拓扑表面态的新奇性质很容易被体载流子掩盖.拓扑绝缘体二维纳米结构具有超高比表面积和能带结构的可调控性,能显著降低体态载流子的比例和凸显拓扑表面态,并易于制备高结晶质量的单晶样品,各种低维异质结构以及平面器件.近年来,我们一直致力于发展拓扑绝缘体二维纳米结构的控制生长方法和物性研究.我们发展了拓扑绝缘体二维纳米结构的范德华外延方法,实现了高质量大比表面积的拓扑绝缘体二维纳米结构的可控制备,并实现了定点与定向的表面生长.开展拓扑绝缘体二维纳米结构的谱学研究,利用角分辨光电子能谱直接观察到拓扑绝缘体狄拉克锥形的表面电子能带结构,发现了拉曼强度与位移随层数的依赖关系.设计并构建拓扑绝缘体纳米结构器件,系统研究其新奇物性,观测到拓扑绝缘体Bi2Se3表面态的Aharonov-Bohm(AB)量子干涉效应等新奇量子现象,通过栅电压实现了拓扑绝缘体纳米薄片化学势的调控,并将拓扑绝缘体纳米结构应用于柔性透明导电薄膜.本文首先简单介绍拓扑绝缘体的发展现状,然后系统介绍我们开展的拓扑绝缘体二维纳米结构的范德华外延生长、谱学、电学输运特性以及透明柔性导电薄膜应用的研究,最后对该领域所面临的机遇和挑战进行简要的展望.

Nanoscale strain engineering of graphene and graphene-based devices

Structural distortions in nano-materials can induce dramatic changes in their electronic properties. This situation is well manifested in graphene, a two-dimensional honeycomb structure of carbon atoms with only one atomic layer thickness. In particular, strained graphene can result in both charging effects and pseudo-magnetic fields, so that controlled strain on a perfect graphene lattice can be tailored to yield desirable electronic properties. Here, we describe the theoretical foundation for strain-engineering of the electronic properties of graphene, and then provide experimental evidence for strain-induced pseudo-magnetic fields and charging effects in monolayer graphene. We further demonstrate the feasibility of nano-scale strain engineering for graphene-based devices by means of theoretical simulations and nano-fabrication technology.

A review of the growth and structures of silicene on Ag(111)

Ag(111) is currently the most often used substrate for growing silicene films. Silicene forms a variety of different phases on the Ag(111) substrate. However, the structures of these phases are still not fully understood so far. In this brief review we summarize the growth condition and resulting silicene phases on Ag(111), and discuss the most plausible structural model and electronic property of individual phases. The existing debates on silicene on Ag(111) system are clarified as mush as possible.

Graphene-like physics in optical lattices

Graphene has attracted enormous attention over the past years in condensed matter physics. The most interesting feature of graphene is that its low-energy excitations are relativistic Dirac fermions. Such feature is the origin of many topological properties in graphene-like physics. On the other hand, ultracold quantum gas trapped in an optical lattice has become a unique setting for quantum simulation of condensed matter physics. Here, we mainly review our recent work on quantum simulation of graphene-like physics with ultracold atoms trapped in a honeycomb or square optical lattice, including the simulation of Dirac fermions and quantum Hall effect with and without Landau levels. We also present the related experimental advances.

费米子与Schwarzschild黑洞

本文证实,Schwarzschild黑洞周围不存在费米子束缚态;并利用渐近解讨论了费未子的Hawking蒸发过程.

费米子隧穿辐射研究

本文在对标量粒子和矢量粒子隧穿辐射研究的基础上探讨了费米子的隧穿辐射。首先对霍金辐射研究作了综述,然后讨论了狄拉克方程,利用WKB近似,研究了二维时空中费米子的隧穿辐射,通过对隧穿率的讨论,最后求得黑洞的霍金温度。研究结果表明:黑洞的霍金温度同样可以通过费米子的隧穿行为得到。

狄拉克对物理学的主要贡献

论述了狄拉克(P.A.M.Dirac,1902-1984)在量子力学、量子电动力学、相对论性电子理论和反物质理论等4方面作出的贡献,以缅怀他光荣伟大的一生以及为科学而献身奋斗的高尚品德。

双层转角石墨烯的结构和电学性质

石墨烯是第一个被发现的层状二维材料,它的电学性质对层数和层间的堆垛方式有很强的依赖性。而层间的转角可以被看作是调控石墨烯电学性质的又一个可控的自由度。本文主要综述双层转角石墨烯的基本结构与性质,重点讲述转角对双层石墨烯电学性质的影响。我们将从理论与实验结合的角度出发,介绍双层转角石墨烯中Dirac费米子的新奇物理特性,重点阐述如何通过转角来调控石墨烯的电学性质。最后我们对双层转角石墨烯的研究现状进行总结和展望。