Guang-hua Guo

Strain-induced gap transition and anisotropic Dirac-like cones in monolayer and bilayer phosphorene

The electronic properties of two-dimensional monolayer and bilayer phosphorene subjected to uniaxial and biaxial strains have been investigated using first-principles calculations based on density functional theory. Strain engineering has obvious influence on the electronic properties of monolayer and bilayer phosphorene. By comparison, we find that biaxial strain is more effective in tuning the band gap than uniaxial strain. Interestingly, we observe the emergence of Dirac-like cones by the application of zigzag tensile strain in the monolayer and bilayer systems. For bilayer phosphorene, we induce the anisotropic Dirac-like dispersion by the application of appropriate armchair or biaxial compressive strain. Our results present very interesting possibilities for engineering the electronic properties of phosphorene and pave a way for tuning the band gap of future electronic and optoelectronic devices.

Strain induced topological phase transitions in monolayer honeycomb structures of group-V binary compounds

We present first-principles calculations of electronic structures of a class of two-dimensional (2D) honeycomb structures of group-V binary compounds. Our results show these new 2D materials are stable semiconductors with direct or indirect band gaps. The band gap can be tuned by applying lattice strain. During their stretchable regime, they all exhibit metal-indirect gap semiconductor-direct gap semiconductor-topological insulator (TI) transitions with increasing strain from negative (compressive) to positive (tensile) values. The topological phase transition results from the band inversion at the Γ point which is due to the evolution of bonding and anti-bonding states under lattice strain.

Domain wall structure transition during magnetization reversal process in magnetic nanowires

The analytical micromagnetics and numerical simulations were used to investigate the domain wall structure during the magnetization reversal in nanowires. Micromagnetic analysis shows that the domain wall structure is mainly determined by the competition between the demagnetization energy and exchange energy. The wall with vortex magnetization structure in cross-section is energetically more favorable for wires with large diameter. With the reduction of diameter the exchange energy increases. At a critical diameter the vortex structure can not be sustained and the transition from vortex wall to transverse wall occurs. The critical diameters for this transition are about 40 nm for Ni wire and 20 nm for Fe wire, respectively. A series of micromagnetic simulations on the cone-shaped wire confirm the analytical results. The simulations also show that during the reversal process the vortex domain wall moves much faster than the transverse one.

Strain engineering band gap, effective mass and anisotropic Dirac-like cone in monolayer arsenene

The electronic properties of two-dimensional puckered arsenene have been investigated using first-principles calculations. The effective mass of electrons exhibits highly anisotropic dispersion in intrinsic puckered arsenene. Futhermore, we find that out-of-plane strain is effective in tuning the band gap, as the material undergoes the transition into a metal from an indirect gap semiconductor. Remarkably, we observe the emergence of Dirac-like cone with in-plane strain. Strain modulates not only the band gap of monolayer arsenene, but also the effective mass. Our results present possibilities for engineering the electronic properties of two-dimensional puckered arsenene and pave a way for tuning carrier mobility of future electronic devices.

Spin-wave propagation in domain wall magnonic crystal

We present a new type of magnonic crystal consisting of a series of periodically distributed magnetic domain walls in a uniform strip. When spin waves propagate in such a structure, allowed and forbidden bands are formed due to translation symmetry and scattering of the spin waves at the domain wall boundaries caused by the dynamic stray field in the domain wall region. The control of the bandgap position in frequency and its width by the period of magnonic crystal and the domain wall width is investigated. It is found that the bandgap position decreases monotonously with the increase of the period or domain wall width, while the bandgap width displays an oscillated behavior. The origin of the oscillation of the bandgap width is discussed. This work may provide a new way of designing reconfigurable magnonic devices.

Electronic structures and magnetism of SrFeO2 under pressure: a first-principles study.

We have studied the electronic structures and magnetism of SrFeO2 under pressure by first-principles calculations in the framework of density functional theory (DFT) with GGA+U and HSE06 hybrid functionals, respectively. The pressure-induced spin transition from S = 2 to S = 1 and the antiferromagnetic-ferromagnetic (AFM-FM) transition observed in experiment are well reproduced by taking the site repulsion U and its pressure dependence into account. The electronic structure and its change with the pressure can be qualitatively understood in an ionic model together with the hybridization effects between the Fe 3d and O 2p states. It is found that the pressure leads to a change in Fe 3d electronic configuration from (d(z(2)))(2)(d(xz)d(yz))(2)(d(xy))(1)(d(x(2)-y(2)))(1) under ambient conditions to (d(z(2)))(2)(d(xz)d(yz))(3)(d(xy))(1)(d(x(2)-y(2)))(0) at high pressure. As a result, the spin state transits from S = 2 to S = 1 and both the antiferromagnetic intralayer Fe-O-Fe superexchange interaction and the interlayer Fe-Fe direction exchange coupling at ambient pressure become ferromagnetic at high pressure according to the Goodenough-Kanamori (G-K) rules. Additionally, our calculations predict another spin transition from S = 1 to S = 0 at pressures of about 220 GPa.

An analytical approach to the interaction of a propagating spin wave and a Bloch wall

The spin wave propagation and the spin-wave induced domain wall motion in a nanostrip with a Bloch domain wall are studied. The spin-wave dispersion relation and the transmission coefficients across the wall are derived analytically. A one-dimensional model for the domain wall motion is constructed. It is found that the spin wave can drive the wall to move either in the same direction or in the opposite direction to that of spin-wave propagation depending on the transmission coefficient. The transmitted magnons drag the wall moving backward without inertia by the adiabatic and nonadiabatic spin-transfer torques, while the reflected magnons push the wall moving forward by the linear momentum transfer torque.

Irreversible magnetic exchange-spring processes in antiferromagnetic exchange-coupled bilayer systems

The demagnetization processes of antiferromagnetic exchange-coupled hard-soft bilayer structures have been studied using a one-dimensional atomic chain model, taking into account the anisotropies of both hard and soft layers. It is found that for very thin soft layers, magnetization/demagnetization involves typical reversible exchange-spring behavior. However as the thickness t of the soft layer is increased, there is a crossover point tc, after which the exchange spring becomes irreversible. The value of the critical thickness tc is determined inter alia by the magnetic anisotropy of the soft layer. However, an important feature of the irreversible exchange spring is the formation of a large angle domain wall, realized immediately at the bending field transition.

Magnetocrystalline anisotropy and spin reorientation transition of DyMn6Sn6 compound

Abstract The spin-reorientation transition of intermetallic compound DyMn 6 Sn 6 was investigated by applying the molecular field theory. The temperature dependence of easy magnetization direction of compound and the magnetic moment directions of Dy and Mn ions were theoretically calculated and they have good agreement with the experimental data. In the framework of single ion model, the temperature dependence of magnetocrystalline anisotropic constants K 1 R and K 2 R of Dy ion were also calculated. The results show that the fourth-order crystal field parameter, B 0 4 , and the corresponding second-order magnetocrystalline anisotropic constant, K 2 R , of Dy ion must be taken into account in order to explain the spin-reorientation transition satisfactorily. The competition between K 2 R and K 1 R plays a key role in the spin-reorientation transition of DyMn 6 Sn 6 .

Low contact resistance in solid electrolyte-gated ZnO field-effect transistors with ferromagnetic contacts

Understanding the role of contacts and interfaces between ferromagnetic metals and semiconductors is a critical step for spin injection and transport. Here, high performance solid electrolyte gated ZnO field-effect-transistors (FETs) with ferromagnetic Co contacts as the source and drain electrodes were demonstrated. Solid electrolyte gating provides a large electric field in the FETs that leads to an ohmic contact between the Co electrode and the ZnO film. The contact resistance can be tuned by the gate voltage and reduced to 66 Ω cm. Compared with FETs using a traditional SiO2 dielectric, an improved transistor performance is achieved with a current on/off ratio of 106 and a field-effect mobility of 5.24 cm2 V−1 s−1. The magnetoresistance calculation based on a spin diffusion model indicates that the on-state contact resistance of the solid electrolyte gated FETs falls in the optimal range for the injection and detection of spin-polarized charge carriers. These results reveal that the electrolyte gating allows for engineering the contacts for nanoelectronic and spintronic devices.