磁性硅烯超晶格中电场调制的谷极化和自旋极化

研究了基于硅烯的静电势超晶格、铁磁超晶格、反铁磁超晶格中谷极化、自旋极化以及赝自旋极化的输运性质,分析了铁磁交换场、反铁磁交换场以及化学势对输运性质的影响,讨论了电场对谷极化、自旋极化以及赝自旋极化的调控作用.结果表明:当3种超晶格的晶格数达到10以上时,在硅烯超晶格中很容易实现100%的谷极化、自旋极化和赝自旋极化,而且通过调节超晶格上的外加电场可以使极化方向发生翻转,从而在硅烯超晶格中实现外电场对谷自由度、自旋自由度以及赝自旋自由度的操控.     

Light-induced interaction between graphene and magnetic film in a cavity

An interaction mechanism between graphene and magnetic film in cavity is presented in this work. The pseudospin in graphene can indirectly interact with the spins in magnetic film by the media of a circularly polarized photons under the conditions of high temperature and intense laser field. The interaction energy as well as the average values of pseudospin and spin components are calculated according to a generating functional approach. This interaction mechanism provides a scheme of detecting the pseudospin polarization effect.     

Transport in magnetic graphene superlattice with Rashba spin-orbit interaction

Based on the transfer-matrix method, we have investigated the spin-dependent transport properties of magnetic graphene superlattice in the presence of Rashba spin-orbit interaction (RSOI). It is shown that the angular range of the spin transmission probability through magnetic graphene superlattice can be efficiently controlled by the number of barriers. As the number of magnetic barriers increases, the angular range of the transmission through the magnetic superlattice decreases, the gaps in the transmission and conductivity versus energy become wider. It is also found that the spin conductivities oscillate with the Fermi energy and RSOI strength. Specifically, when a magnetic field is present, the spin polarisation can be observed, whereas for the RSOI alone it is zero. Application of such a phenomenon to design a spin polarised electron device based on the graphene material is anticipated.     

包含非中心电耦极矩的环状非谐振子势场赝自旋对称性的三对角化表示

提出了一个包含非中心电耦极矩分量的环状非谐振子势模型,在能够负载Dirac波动算子三对角化表示的完全平方可积L~2空间讨论了这一势场的赝自旋对称性.利用三对角化矩阵方案,使得求解Dirac方程转换为寻求波函数展开系数满足的三项递推关系式.角向波函数和径向波函数分别以Jacobi多项式和Laguerre多项式表示.由径向分量展开系数递推关系式的对角化条件得到束缚态的能量谱,显示出这一势模型具有严格的赝自旋对称性     

Zero-energy states and fragmentation of spin in the easy-plane antiferromagnet on a honeycomb lattice

The core of the vortex in the Néel order parameter for an easy-plane antiferromagnet on a honeycomb lattice is demonstrated to bind two zero-energy states. Remarkably, a single electron occupying this midgap band has its spin fragmented between the two sublattices: Whereas it yields a vanishing total magnetization, it shows a finite Néel order, orthogonal to the one of the assumed background. The requisite easy-plane anisotropy may be introduced by a magnetic field parallel to the graphene layer, for example. The results are relevant for spin-1/2 fermions on the graphene's or optical honeycomb lattice, in the strongly interacting regime.     

Spin-to-orbital angular momentum conversion and spin-polarization filtering in electron beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.     

Spin remagnetization excitations in semiconductors with Rashba and Dresselhaus spin–orbit coupling

Spin remagnetization modes in paramagnetic materials with Rashba and Dresselhaus spin–orbit interaction are studied by analytically solving the kinetic equations for the spin-density matrix. These eigenmodes, which are induced by an in-plane electric field, lead to a rotation of the spin magnetic moment. The specific character of the spin remagnetization modes depends on the details of the excitation mechanism. By applying the approach to another system, namely to a model for graphene, pseudospin excitations are identified.     

Anisotropic quantum transport in monolayer graphene in the presence of Rashba spin–orbit coupling

We have studied spin-dependent electron tunneling through the Rashba barrier in a monolayer graphene lattices. The transfer matrix method, have been employed to obtain the spin dependent transport properties of the chiral particles. It is shown that graphene sheets in the presence of Rashba spin–orbit barrier will act as an electron spin-inverter.     

Valley-contrasting physics in graphene: magnetic moment and topological transport

We investigate physical properties that can be used to distinguish the valley degree of freedom in systems where inversion symmetry is broken, using graphene systems as examples. We show that the pseudospin associated with the valley index of carriers has an intrinsic magnetic moment, in close analogy with the Bohr magneton for the electron spin. There is also a valley dependent Berry phase effect that can result in a valley contrasting Hall transport, with carriers in different valleys turning into opposite directions transverse to an in-plane electric field. These effects can be used to generate and detect valley polarization by magnetic and electric means, forming the basis for the valley-based electronics applications.     

Weak localization of inelastically reflected electrons under auger emission conditions

A new type of weak localization of electrons emerging during electron emission is considered. It is manifested in singularities of the angular spectra of particles reflected inelastically from a solid and causing Auger ionization of the atoms. The orientational dependences in this case appear as a result of interference of two types of processes. In one case, an electron from the primary beam penetrates the solid, undergoes inelastic scattering, ionizes an atom, and is then scattered elastically through a large angle, after which it leaves the solid. In the other case, elastic scattering of an electron precedes its inelastic scattering due to the Auger ionization of an atom. The azimuthal angular dependences of currents created by inelastically reflected electrons contain information on new processes of weak localization of particles.     

Electron-spin and electron-orbital dependence of the tunnel coupling in laterally coupled double vertical dots

We employ a new laterally coupled, vertical double dot with a tunable tunnel-coupling gate in a parallel configuration to study the electron spin and orbital dependence of quantum mechanical tunnel coupling on the size of the honeycomb vertices in the small electron numbers regime. We find a transition from the weak coupling regime, where fluctuations in tunnel coupling due to varying electron configuration dominate the anticrossings, to a regime where the two dots coalesce. We apply a magnetic field to ascertain the orbital angular momenta of the Fermi surface eigenstates, which correlate with anticrossing size, and we identify spin pairs with congruent behavior.     

Silicene on substrates:A theoretical perspective

Silicene, as the silicon analog of graphene, is successfully fabricated by epitaxially growing it on various substrates.Like free-standing graphene, free-standing silicene possesses a honeycomb structure and Dirac-cone-shaped energy band,resulting in many fascinating properties such as high carrier mobility, quantum spin Hall effect, quantum anomalous Hall effect, and quantum valley Hall effect. The existence of the honeycomb crystal structure and the Dirac cone of silicene is crucial for observation of its intrinsic properties. In this review, we systematically discuss the substrate effects on the atomic structure and electronic properties of silicene from a theoretical point of view, especially with emphasis on the changes of the Dirac cone.     

Distinguishing spontaneous quantum Hall states in bilayer graphene

Chirally stacked N-layer graphene with N≥2 is susceptible to a variety of distinct broken symmetry states in which each spin-valley flavor spontaneously transfers charge between layers. In mean-field theory, one of the likely candidate ground states for a neutral bilayer is the layer antiferromagnet that has opposite spin polarizations in opposite layers. In this Letter, we analyze how the layer antiferromagnet and other competing states are influenced by Zeeman fields that couple to spin and by interlayer electric fields that couple to layer pseudospin, and comment on the possibility of using Zeeman responses and edge state signatures to identify the character of the bilayer ground state experimentally.     

Superconducting states of pure and doped graphene

We study the superconducting phases of the two-dimensional honeycomb lattice of graphene. We find two spin singlet pairing states; s wave and an exotic p+ip that is possible because of the special structure of the honeycomb lattice. At half filling, the p+ip phase is gapless and superconductivity is a hidden order. We discuss the possibility of a superconducting state in metal coated graphene.     

Approximate <Emphasis Type="Italic">κ</Emphasis>-state solutions to the Dirac-Yukawa problem based on the spin and pseudospin symmetry

Using an approximation scheme to deal with the centrifugal (pseudo-centrifugal) term, we solve the Dirac equation with the screened Coulomb (Yukawa) potential for any arbitrary spin-orbit quantum number κ. Based on the spin and pseudospin symmetry, analytic bound state energy spectrum formulas and their corresponding upper- and lower-spinor components of two Dirac particles are obtained using a shortcut of the Nikiforov-Uvarov method. We find a wide range of permissible values for the spin symmetry constant C _s from the valence energy spectrum of particle and also for pseudospin symmetry constant C _ps from the hole energy spectrum of antiparticle. Further, we show that the present potential interaction becomes less (more) attractive for a long (short) range screening parameter α. To remove the degeneracies in energy levels we consider the spin and pseudospin solution of Dirac equation for Yukawa potential plus a centrifugal-like term. A few special cases such as the exact spin (pseudospin) symmetry Dirac-Yukawa, the Yukawa plus centrifugal-like potentials, the limit when α becomes zero (Coulomb potential field) and the non-relativistic limit of our solution are studied. The nonrelativistic solutions are compared with those obtained by other methods.     

Exact solution of the Dirac equation with the Mie-type potential under the pseudospin and spin symmetry limit

We investigate the exact solution of the Dirac equation for the Mie-type potentials under the conditions of pseudospin and spin symmetry limits. The bound state energy equations and the corresponding two-component spinor wave functions of the Dirac particles for the Mie-type potentials with pseudospin and spin symmetry are obtained. We use the asymptotic iteration method in the calculations. Closed forms of the energy eigenvalues are obtained for any spin-orbit coupling term κ. We also investigate the energy eigenvalues of the Dirac particles for the well-known Kratzer-Fues and modified Kratzer potentials which are Mie-type potentials.     

Spin and spin-valley Hall effects in a honeycomb lattice with antiferromagnetism and spin-orbit couplings

We study linear response to a longitudinal electric field on an antiferromagnetic honeycomb lattice with intrinsic and Rashba spin-orbit couplings (SOCs). It is found that the spin-valley Hall effect could emerge alone or coexist with the spin Hall effect. The spin and spin-valley Hall conductivities exhibit some peculiarities that depend on the distinct topological states of the graphene lattice. Furthermore, the spin and spin-valley Hall conductivities could be remarkably modulated by changing the Fermi level. Our findings suggest that the antiferromagnetic honeycomb lattice with SOCs is an excellent platform for potential applications of spintronics and valleytronics.     

Self-organization of ultra-thin uranium film

We have studied the adsorption behaviors of uranium (U) atoms coated graphene at different coverage ratios using first-principles calculations. Isolated U atom is demonstrated to be more likely to stay on the hollow site of graphene with a large adsorption energy of 2.80 eV and high magnetic moment of 5.07 ${\mu }_{\text{B}}$. It seems that two U atoms tend to adsorb on the nearest neighbor hollow sites of graphene. As the concentration of U atom increases, no U-dimer or U-cluster appears with increasing coverage ratio. In the case of coverage ratio of 2/3, U atoms on graphene is the most stable configuration, where each U atoms are uniformly-distributed in U network with honeycomb lattice. The adsorption energy is as large as 2.57 eV per U atom. Moreover, the spin–orbit coupling effect on electronic band structures is outstanding, which induces the degeneracy bands splitting. Our calculations will provide profound background to understand the adsorption behaviors of U atoms on graphene.     

Substrate orientation: A way towards higher quality monolayer graphene growth on 6H-SiC(0 0 0 1)

The influence of substrate orientation on the morphology of graphene growth on 6H-SiC(0 0 0 1) was investigated using low-energy electron and scanning tunneling microscopy (LEEM and STM). Large area monolayer graphene was successfully furnace-grown on these substrates. Larger terrace widths and smaller step heights were obtained on substrates with a smaller mis-orientation from on-axis (0.03°) than on those with a larger (0.25°). Two different types of a carbon atom networks, honeycomb and three-for-six arrangement, were atomically resolved in the graphene monolayer. These findings are of relevance for various potential applications based on graphene-SiC structures.     

Dirac Equation with a New Tensor Interaction under Spin and Pseudospin Symmetries

The approximate analytical solutions of the Dirac equation under spin and pseudospin symmetries are examined using a suitable approximation scheme in the framework of parametric Nikiforov-Uvarov method. Because a tensor interaction in the Dirac equation removes the energy degeneracy in the spin and pseudospin doublets that leads to atomic stability, we study the Dirac equation with a Hellmann-like tensor potential newly proposed in this study.The newly proposed tensor potential removes the degeneracy from both the spin symmetry and pseudospin symmetry completely. The proposed tensor potential seems better than the Coulomb and Yukawa-like tensor potentials.