Yuan LI

Hangzhou Dianzi University · Department of physics

The double Weyl semimetal (DWSM) is a newly proposed topological material that hosts Weyl points with chiral charge n=2. The disorder effect in DWSM is investigated by adopting the tight-binding Hamiltonian. Using the transfer matrix method and the noncommutative Kubo formula, we numerically calculate the localization length and the Hall conductivi...

We study the band structure and transport properties of ferromagnetic tetragonal silicene nanoribbons by using the non-equilibrium Green's function method. The band structure and spin-dependent conductance are discussed under the combined effect of the external electric field, potential energy, exchange field and the spin-orbit coupling. One can ea...

The double Weyl semimetal (DWSM) is a newly proposed topological material that hosts Weyl points with chiral charge n=2. The disorder effect in DWSM is investigated by adopting the tight-binding Hamiltonian. Using the transfer matrix method and the noncommutative Kubo formula, we numerically calculate the localization length and the Hall conductivi...

We study the effect of the strain on the energy bands of TaIrTe4 sheet and the photocurrent in the Cu-TaIrTe4-Cu heterojunction by using the quantum transport simulations. It is found that the Weyl points can be completely broken with increasing of the strain along z dirction. One can obtain a large photocurrent in the Cu-TaIrTe4-Cu heterojunction...

We study the effect of the strain on the band structure and the valley-dependent transport property of graphene heterojunctions. It is found that valley-dependent separation of electrons can be achieved by utilizing the strain and on-site energies. In the presence of the strain, the values of the transmission can be effectively adjusted by changing...

We study electrical modulation of transport properties of silicene nanoconstrictions with different geometrical structures. We investigate the effects of the position and width of the central scattering region on the conductance with increasing Fermi energy. It is found that the conductance significantly depends on the position and the width of the...

We build a Cu-MoTe2-Cu device model and use first-principles density functional theory to study the transport properties of single-layer Td-MoTe2. We obtained the effect of strain on the energy band structure, transport properties, and photocurrent. The strain-induced photocurrent shows an anisotropy that reflects the modulation of the energy bands...

By using the tight-binding model and nonequilibrium Green's function method (NEGF), we study the band structures and transport properties of a silicene nanoribbon with a line defect where a bulk energy gap is opened due to the sublattice symmetry breaking. The flat subband bends downwards or upwards due to the effect of the line defect. The spin-or...

We study the effect of the strain on the band structure and the valley-dependent transport property of graphene heterojunctions. It is found that valley-dependent separation of electrons can be achieved by utilizing the strain and on-site energies. In the presence of the strain, the values of the transmission can be effectively adjusted by changing...

By using the tight-binding model and non-equilibrium Green's function method (NEGF), we study the band structures and transport properties of a silicene nanoribbon with a line defect where a bulk energy gap is opened due to the sublattice symmetry breaking. The flat subband bends downwards or upwards due to the effect of the line defect. The spin-o...

We used first-principles density functional theory to study the physical properties of TaAs sheet. The effect of the strain on the energy band structure and transport properties was investigated. The results show that the Weyl point of TaAs can be destroyed by the strain, and new Weyl points are generated as the strain increases. The electron trans...

We study the electrical modulation of the transport properties of the silicene constrictions with different geometrical structures by adopting the tight-binding model and non-equilibrium Green's function method. The band structure and the transmission properties are discussed under the influence of the external electric field and potential energy....

We adopt the tight-binding model and the mode-matching method to study the electrical modulation of the valley polarization in strained silicene constrictions. The effects of the potential energy, the external electric field, and the strain on the band structures and the transport property are investigated. The conductance shows a clear valley pola...

We adopt the tight-binding mode-matching method to study the strain effect on silicene heterojunctions. It is found that valley- and spin-dependent separation of electrons cannot be achieved by the electric field only. When a strain and an electric field are simultaneously applied to the central scattering region, not only are the electrons of vall...

We obtain the effective Hamiltonian of the phosphorene including the effect of Rashba spin-orbit coupling in the frame work of the low-energy theory. The spin-splitting energy bands show an anisotropy feature for the wave vectors along kx and ky directions, where kx orients to ΓX direction in the k space. We numerically study the optical absorption...

The properties of NiFe/Cu/NiFe trilayers with different buffer and cap layers are investigated. It is found that the properties of the NiFe/Cu/NiFe trilayers with a 50 Å NiFeCr buffer layer and a 50 Å NiFeCr cap layer exhibits a triangle transfer curve with a negligible hysteresis, showing a GMR ratio of 5.4%, and a saturation field of 272 Oe. In c...

Atomic-layer MoS2 ultrathin films are synthesized using a hot filament chemical vapor deposition method. A combination of atomic force microscopy (AFM), x-ray diffraction (XRD), high-resolution transition electron microscopy (HRTEM), photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS) characterization methods is applied to investigat...

We report the observation of a small but finite valley Hall effect (VHE) signal in two-dimensional MoS2 channels which is grown on SiO2/Si substrates under the circularly polarized light. And the angular dependence of VHE in two-dimensional MoS2 is studied. The VHE signal is a periodic function (period π) but with a phase shift, which confirms the...

We investigate Klein tunneling in graphene heterojunctions under the influence of a perpendicular magnetic field via the non-equilibrium Green's function method. We find that the angular dependence of electron transmission is deflected sideways, resulting in the suppression of normally incident electrons and overall decrease in conductance. The off...

For graphene heterojunctions, perfect transmission of the electrons injected at normal direction can occur across an arbitrarily high and wide barrier, a phenomenon known as Klein tunneling [1-2]. Due to the effect of Klein tunneling, it is difficult to confine the massless Dirac fermions using the electrostatic methods [3-4]. Recently, it is repor...

The surface states of topological insulators (TI) possess a linear dispersion in the low-energy approximation, and are protected against time-reversal-invariant perturbations such as non-magnetic impurities, defects [1-3]. The chiral nature of TI surface states can induce perfect quantum tunneling for Dirac fermions incident in the normal direction...

We investigate the tunneling transport across a graphene p-n junction under the influence of a perpendicular magnetic field (B field). We observe a sideway deflection of the transmission profile, which can be quantitatively explained by invoking the classical Lorentz force. By considering the trajectory of the Dirac fermions along their cyclotron o...

Zn and Co multi-doped CeO2 thin films have been prepared using an anodic electrochemical method. The structures and magnetic behaviors are characterized by several techniques, in which the oxygen states in the lattice and the absorptive oxygen bonds at the surface are carefully examined. The absorptive oxygen bond is about 50% of the total oxygen b...

We study the electronic structure and transport for a quasi-one-dimensional channel constructed via two ferromagnetic (FM) stripes on the surface of a three-dimensional (3D) topological insulator (TI) in parallel (P) or antiparallel (AP) magnetization configuration along the vertical zz-direction. We demonstrate that the confined states which are l...

GMR/TMR materials are firstly introduced, and then the related devices including spintronic sensors, isolators and magnetic random access memory (MRAM) are reviewed from a perspective of materials, design, functionalities as well as device applications. Furthermore, the suggestions are given on how to commercialize spintronic devices.

Time-periodic perturbation can be used to modify the transport properties of the surface states of topological insulators, specifically their chiral tunneling property. Using the scattering matrix method, we study the tunneling transmission of the surface states of a topological insulator under the influence of a time-dependent potential and finite...

The exchange coupling effect between the ferromagnetic pinned layer and antiferromagnetic pinning layer is considered to be one of the key factors for fabricating high performance spin-valves. In this study, the dependence of the exchange coupling field (Hex) in CrPtMn-based spin valves on the deposition pressure and subsequent anneal treatment has...

The effects of different relevant layers, especially the insertion layers (which are between Ta buffer layer and IrMn pinning layer) and free layers, on the magnetic properties of IrMn bottom-pinning spin valves are investigated. Spin valve with a NiFe insertion layer exhibits a higher GMR ratio of ~ 6.0% than that of 2.0% for the spin valve with a...

We investigate the transport properties of a topological insulator (TI) waveguide in the presence of an external magnetic field. We demonstrate that the magnetic field can modulate the energy gap induced by the transverse confinement and may even result in the closure of the band gap. The dependence of the magnetic field and the temperature on the...

We propose a spin current generator based on a topological insulator current-in-plane spin valve, consisting of a 3D topological insulator sandwiched between two ferromagnetic insulator layers. The "on" and "off" states of the spin current can be toggled by switching the magnetization configuration of the two ferromagnetic insulator layers which ar...

The counterpropagating edge states of a two-dimensional topological insulator (TI) carry electrons of opposite spins. We investigate the transport properties of edge states in a two-dimensional TI which is contacted to ferromagnetic leads. The application of a side-gate voltage induces a constriction or quantum point contact (QPC) which couples the...

We investigate the spin transport in a spin-diode structure consisting of a quantum dot (QD) coupled to a ferromagnetic lead and a nonmagnetic (NM) lead. Electron transport through the QD system is investigated theoretically by means of the nonequilibrium Keldysh Green’s function technique. The presence of a spin bias in the NM lead induces a charg...

The conductance through a mesoscopic system of interacting electrons coupled to two adjacent leads is conventionally derived via the Keldysh nonequilibrium Green’s function technique, in the limit of noninteracting leads [Y. Meir, N.S. Wingreen, Phys. Rev. Lett. 68 (1992) 2512]. We extend the standard formalism to cater for a quantum dot system wit...

ZnO based diluted magnetic semiconductor particles (ZnO:Co) have been grown using a hydrothermal method with good crystallinity. The atomic percentage of Co presented in the specimen is about 0.01. Based on the x-ray diffraction and high-resolution transition electron, Co is found to be incorporated into ZnO lattice without evidence of obvious Co p...

We investigate the transport properties of an interacting quantum dot modulated by spin bias by means of the Keldysh nonequilibrium Green’s function method. The charge and spin differential conductance (Gc and Gs, respectively) are calculated for three kinds of spin-bias configurations. Our calculations show the presence of conductance plateaus and...

It has been shown that three-terminal quantum dot (TTQD) devices and their variants can generate a rather high spin current in the presence of Rashba spin-orbit coupling (RSOC) at low temperature. The effect of contacting ferromagnetic (FM) electrodes to the TTQD devices is studied theoretically via the Keldysh non-equilibrium Green’s function meth...

We study the spatial distribution of spin polarization and charge transmission for a T-shaped waveguide formed by gate electrodes on the surface of three-dimensional (3D) topological insulator (TI). We demonstrate that an energy gap depending on waveguide geometry parameters is opened and the spin surface-locking is broken by the electrical confine...

In this work, the effective anisotropy Heff in the free layer of patterned spin-valve resistor has been investigated. A magnetic analysis is first conducted to explain the effective anisotropy, a mixed effect of the uniaxial anisotropy and the shape anisotropy. The experiment is then performed to verify the model analysis. The effective anisotropy...

We investigate the spin-bias-induced transport through a quantum dot coupled to a ferromagnetic lead and a nonmagnetic semiconductor lead. Electron transport through this quantum dot is analyzed theoretically by means of the master equation method. It is shown that the polarization direction and the magnitude of the spin bias can be detected throug...

Spin dynamics of Rashba-Dresselhaus two-dimensional electron systems is studied by taking account of electron-electron interactions under the D’yakonov-Perel’ mechanism. The diffusion equations for charge and spin densities are obtained through decoupling of the interactions using the auxiliary Bose field. We show that the electron-electron interac...

We show that the efficiency of manipulating electron spins in semiconductor quantum wells can be enhanced by tuning strain strengths. The combined effects of intrinsic and strain-induced spin-orbit couplings vary for different quantum wells, which provide an alternative route to understand the experimental phenomena brought in by the strain. The co...

We study the spin dynamics of two dimensional electron gases (2DEGs) with Rashba spin-orbit coupling by taking account of electron-electron interactions. The diffusion equations for charge and spin densities are derived by making use of the path-integral approach and the quasiclassical Green's function. Analyzing the effect of the interactions, we...

We show that the efficiency of manipulating electron spin in semiconductor quantum wells can be enhanced by tuning the strain strength. The effect combining intrinsic and strain-induced spin splitting varies for different systems, which provides an alternative route to understand the experimental phenomena brought in by the strain. The types of spi...

The density matrix formalism is applied to calculate the spin-relaxation time for two-dimensional systems with a hierarchy of spin-orbit couplings, such as Rashba-type, Dresselhaus-type and so on. It is found that the spin-relaxation time can be infinite if those coupling strengths $\alpha$, $\beta$, $\gamma_1$ and $\gamma_2$ satisfy either conditi...

We theoretically study the electron transport property for a semiconductor quantum wire irradiated under a longitudinally polarized electromagnetic field within a finite range. We obtain the non-perturbative solutions of the single-electron time-dependent Schrödinger equation both inside and outside the field-irradiated region of the wire according...

We study theoretically the electron transport properties for an open rectangular quantum dot under an external electromagnetic field illumination in the ballistic regime. Using the effective mass free-electron approximation, the scattering matrix for the system has been formulated by the time-dependent mode-match method. Some interesting properties...

We theoretically investigate the electron transport properties for a semiconductor quantum wire containing a single finite-size attractive impurity under an external terahertz electromagnetic field illumination in the ballistic limit. Within the effective mass free-electron approximation, the scattering matrix for the system has been formulated by...

We theoretically study the conducting electronic contribution to the cohesive force in a metallic nanowire irradiated under a transversely polarized external electromagnetic field at low temperatures and in the ballistic regime. In the framework of the free-electron model, we have obtained a time-dependent two-level electronic wavefunction by means...

We theoretically study the electronic transport of a straight quantum wire partly irradiated under an external terahertz electromagnetic field at low temperature. Using the free-electron model and the scattering matrix approach, we demonstrate that although the electrons in a ballistic quantum wire only suffer from lateral collisions with photons,...