The anisotropic transport properties of the three-terminal ballistic junction based onα-T3lattice.

The three-terminal ballistic junction (TBJ) has promising applications in nanoelectronics. We investigate the transport properties of aα-T3-based TBJ, where two typical configurations are considered, i.e. the A- and Z-TBJ. It is found that both A- and Z-TBJ exhibit transmission anisotropy, and the transmission of the A-TBJ has stronger anisotropy than that of the Z-TBJ. The amplitude of the rectification coefficient is smaller than that of phosphorene TBJ, but larger than that of graphene TBJ. When the symmetrical input is applied, the output voltage curve exhibits symmetric behavior. While in the case of asymmetric input, the symmetric behavior is broken, and the maximum value of the output voltage can reach a positive value. Interestingly, the voltage output shows a dramatic nonlinear response which may be useful for the voltage diode application with a push-pull input voltage. In addition, the heat fluxes of the asymmetric input are much smaller than those of the symmetric input. The maximum value of the heat flux under the symmetric input exceeds twice of that under the asymmetric input. Our results are useful to design nanoelectronic devices based onα-T3TBJ.

Strain effect on electronic structure and transport properties of zigzagα-T3nanoribbons: a mean-field theoretical study.

Theα-T3lattice, a minimal model that presents flat bands, has sparked much interest in research but the finite-size effect and interaction has been rarely involved. Here we theoretically study the electronic structure and transport properties of zigzag-edgeα-T3nanoribbons (ZαT3NRs) with and without uniaxial strain, where the exemplary widthsN= 40 and 41 for two series are considered. By adopting the mean-field Hubbard model combined with the nonequilibrium Green's function method, we show that the spin-degenerate dispersionless flat band at the Fermi energy for the pristine ribbons is split into spin-up and -down flat bands under electron-electron Coulomb interaction. Specifically, the two bands are shifted toward in an opposite direction and away from the Fermi energy, which leads to an energy gap opening in the case ofα≠ 1. All three series of ZαT3NRs with widthN= 3n, 3n+ 1, 3n+ 2 (wherenis a positive integer) exhibit an energy gap. This differs from the simple tight-binding calculations without considering electron-electron Coulomb interaction, for which the gap is always zero in the case ofN= 3n+ 1. Here, the origin of the energy gap forN= 3n+ 1 arises from Coulomb repulsion between electrons. Importantly, the energy gap can be effectively manipulated by an uniaxial strain and Coulomb interaction ifα≠ 1. The gap linearly increases (decreases) when a tensile (compressive) strain increases, and it also monotonously increases as enhancing Coulomb interaction. Interestingly, a ground state of antiferromagnetic to ferromagnetic transition occurs whenαincreases from 0.8 to 1, leading to a semiconductor to metallic transition. Besides, theα-, strain- and interaction-dependent conductance is also explored. The findings here may be of importance in the band gap engineering and electromechanical applications ofα-T3nanoribbon-based devices.

Even-odd effect of spin-dependent transport and thermoelectric properties for ferromagnetic zigzag phosphorene nanoribbons under an electric field.

We study the spin-dependent transport and thermoelectric properties of ferromagnetic zigzag-edge phosphorene nanoribbon with N chains (N-ZPNR) modulated by a perpendicular electric field (PEF). By adopting the Hubbard model Hamiltonian combined with the self-consistent calculation, and using the nonequilibrium Green's function method, we obtain the band structure, spin-dependent transmission coefficients and Seebeck coefficients. It is demonstrated that the bands are split with spin-up and -down ones for both 12- and 13-ZPNR in the ferromagnetic state, and both ZPNRs show spin-semiconducting behavior with the spin gap of value 1 eV. Interestingly, there is an even-odd effect on the spin-dependent transport and thermoelectric properties when a PEF is applied, which originates from the structure symmetry of N-ZPNR and can result in different behavior of edge states for even and odd cases. The electric field lifts the degeneracy of the two edge bands for both spin-up and -down channels in 12-ZPNR, however, it only bends the spin-up edge bands and shifts the spin-down ones upward without splitting in 13-ZPNR. In addition, we find that the spin Seebeck coefficients exhibit wide plateaus and their maximum values can reach unprecedented giant 5 mV K-1 for both ZPNRs. The spin Seebeck coefficients for both ZPNRs decrease gradually as temperature increasing. As the electric field increasing, the value of the plateau in the spin Seebeck coefficient for 12-ZPNR decreases gradually, however, the plateau almost has no variation for 13-ZPNR in this case. This different responses of spin Seebeck coefficients to the PEF between even and odd cases originate from the different edge state responses to the electric field, and we provide the explanation from the corresponding spin-up and spin-down Seebeck coefficients. Our results reveal that ferromagnetic ZPNRs are perfectly spin-polarized, and are promising candidates for spin caloritronics with high efficiency.

Magnetite ultrafine particles/porous reduced graphene oxide in situ grown onto Ni foam as a binder-free electrode for supercapacitors.

Here, we report a simple and green electrochemical route to fabricate a porous network of a Fe3O4 nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite supported on a nickel-foam substrate, which is directly used as a binder-free charge storage electrode. Through this method, pristine Fe3O4 NPs/Ni, p-rGO/Ni and Fe3O4 NPs@p-rGO/Ni electrodes are fabricated and compared. In the fabricated Fe3O4 NPs@p-rGO/Ni electrode, the porous rGO sheets served as a conductive network to facilitate the collection and transportation of electrons during the charge/discharge cycles, improving the conductivity of magnetite NPs and providing a larger specific surface area. As a result, the Fe3O4 NPs@p-rGO/Ni exhibited a specific capacitance of 1323 F g-1 at 0.5 A g-1 and 79% capacitance retention when the current density is increased 20 times, where the Fe3O4 NPs/Ni electrode showed low specific capacitance of 357 F g-1 and 43% capacity retention. Furthermore, the composite electrode kept 95.1% and 86.7% of its initial capacitances at the current densities of 1 and 4 A g-1, respectively, which were higher than those of a Fe3O4/NF electrode at similar loads (i.e. 80.4% and 65.9% capacitance retentions at 1 and 4 A g-1, respectively). These beneficial effects proved the synergistic contribution between p-rGO and Fe3O4. Hence, such ultrafine magnetite particles grown onto a porous reduced GO network directly imprinted onto a Ni substrate could be a promising candidate for high performance energy storage aims.

Controllable Valley Polarization Using Silicene Double Line Defects Due to Rashba Spin-Orbit Coupling.

We theoretically investigate the valley polarization in silicene with two parallel line defects due to Rashba spin-orbit coupling (RSOC). It is found that as long as RSOC exceeds the intrinsic spin-orbit coupling (SOC), the transmission coefficients of the two valleys oscillate with the same periodicity and intensity, which consists of wide transmission peaks and zero-transmission plateaus. However, in the presence of a perpendicular electric field, the oscillation periodicity of the first valley increases, whereas that of the second valley shortens, generating the corresponding wide peak-zero plateau regions, where perfect valley polarization can be achieved. Moreover, the valley polarizability can be changed from 1 to -1 by controlling the strength of the electric field. Our findings establish a different route for generating valley-polarized current by purely electrical means and open the door for interesting applications of semiconductor valleytronics.

Spin-dependent Seebeck effects in a graphene superlattice p-n junction with different shapes.

We theoretically calculate the spin-dependent transmission probability and spin Seebeck coefficient for a zigzag-edge graphene nanoribbon p-n junction with periodically attached stubs under a perpendicular magnetic field and a ferromagnetic insulator. By using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that the spin-dependent transmission probability and spin Seebeck coefficient for two types of superlattices can be modulated by the potential drop, the magnetization strength, the number of periods of the superlattice, the strength of the perpendicular magnetic field, and the Anderson disorder strength. Interestingly, a metal to semiconductor transition occurs as the number of the superlattice for a crossed superlattice p-n junction increases, and its spin Seebeck coefficient is much larger than that for the T-shaped one around the zero Fermi energy. Furthermore, the spin Seebeck coefficient for crossed systems can be much pronounced and their maximum absolute value can reach 528 µV [Formula: see text] by choosing optimized parameters. Besides, the spin Seebeck coefficient for crossed p-n junction is strongly enhanced around the zero Fermi energy for a weak magnetic field. Our results provide theoretical references for modulating the thermoelectric properties of a graphene superlattice p-n junction by tuning its geometric structure and physical parameters.

Symmetry-dependent spin-charge transport and thermopower through a ZSiNR-based FM/normal/FM junction.

We investigate the spin-dependent transport and spin thermopower for a zigzag silicene nanoribbon (ZSiNR) with two ends covered by ferromagnets (FMs) under the modulation of a perpendicular electric field, where we take the 6- and 7-ZSiNR to exemplify the effect of the even- and odd-N ZSiNRs, respectively. By using the nonequilibrium Green's function approach, it is demonstrated that a ZSiNR-based FM/normal/FM junction still shows an interesting symmetry-dependent property although the σ mirror plane is absent for any ZSiNR due to the buckled structure of silicene. The junction with even- or odd-N ZSiNR has very different transport and thermopower behavior, which is attributed to the different parity of π and [Formula: see text] band wavefunctions under the c 2 symmetry operation with respect to the centre axis between two edges, and is linked to the unique symmetry of the band structure for the ribbon. As a result, the magnetoresistance (MR) for the 6-ZSiNR junction with a 100% plateau around zero electron energy is observed, but the plateau is absent for the 7-ZSiNR one. In addition, the spin thermopower also displays the even-odd behaviour. The 6-ZSiNR junction is found to possess superior thermospin performance compared with the 7-ZSiNR one, and its spin thermopower can be improved by one order of magnitude in the absence of an electric field. As the strength of the field increases, the spin thermopower for the 6-ZSiNR junction dramatically decreases, while it notably enhances for the 7-ZSiNR one. Interestingly, the spin thermopower for both junctions is strongly dependent on the strength of magnetisation in FM, and it can be very pronounced with a maximum absolute value of 200 µV K(-1)by the optimisation of the parameters. However, with the increase in temperature, the spin thermopower for the 6-ZSiNR junction decreases, but the situation for the 7-ZSiNR one is opposite. Finally, the spin figure of merit for the 6-ZSiNR junction is found to be four orders of magnitude larger than that for the 7-ZSiNR one. This even-odd effect is common for N-SiNR, and the result can be regarded as an advance in the understanding of the characteristics of silicene and may be valuable for experimentally designing spin valve and heat spintronic devices based on silicene.

Spin-filtering and rectification effects in a Z-shaped boron nitride nanoribbon junction.

A Z-shaped junction constructed by a few-nanometer-long armchair-edged boron nitride nanoribbon (ABNNR) sandwiched between two semi-infinite zigzag-edged BNNR electrodes with different hydrogen-passivated edge treatment is proposed, and its spin-dependent electronic transport is studied by ab initio calculations. It is found that a short ABNNR exhibits metallic behavior and can be used as a conduction channel. Interestingly, the spin-filtering and rectification effects exist in the junctions without any edge passivation or with boron-edge passivation. The analysis on the projected density of states and spatial distribution of molecular projected self-consistent Hamiltonian eigenstates gives an insight into the observed results for the system. Our results suggest that a BNNR-based nanodevices with spin-filtering and rectification effects may be synthesized from an hexagonal boron nitride sheet by properly tailoring and edge passivation.

Spin-dependent transport for armchair-edge graphene nanoribbons between ferromagnetic leads.

We theoretically investigate the spin-dependent transport for the system of an armchair-edge graphene nanoribbon (AGNR) between two ferromagnetic (FM) leads with arbitrary polarization directions at low temperatures, where a magnetic insulator is deposited on the AGNR to induce an exchange splitting between spin-up and -down carriers. By using the standard nonequilibrium Green's function (NGF) technique, it is demonstrated that the spin-resolved transport property for the system depends sensitively on both the width of AGNR and the polarization strength of FM leads. The tunneling magnetoresistance (TMR) around zero bias voltage possesses a pronounced plateau structure for a system with semiconducting 7-AGNR or metallic 8-AGNR in the absence of exchange splitting, but this plateau structure for the 8-AGNR system is remarkably broader than that for the 7-AGNR one. Interestingly, an increase of the exchange splitting Δ suppresses the amplitude of the structure for the 7-AGNR system. However, the TMR is much enhanced for the 8-AGNR system under a bias amplitude comparable to the splitting strength. Further, the current-induced spin-transfer torque (STT) for the 7-AGNR system is systematically larger than that for the 8-AGNR one. The findings here suggest the design of GNR-based spintronic devices by using a metallic AGNR, but it is more favorable to fabricate a current-controlled magnetic memory element by using a semiconducting AGNR.

Magnetotransport and current-induced spin transfer torque in a ferromagnetically contacted graphene.

We theoretically investigate the spin-dependent transport through a graphene sheet between two ferromagnetic (FM) leads with arbitrary polarization directions at low temperatures, where a magnetic insulator is deposited on the graphene to induce an exchange splitting between spin-up and spin-down carriers. By using standard nonequilibrium Green's function (NGF) techniques, it is demonstrated that the density of states (DOS) decreases for spin-up and increases for spin-down when the polarization strength of the two leads in parallel alignment increases. For the electron energy around the exchange splitting, the DOS for both spin-up and spin-down channels is independent of the polarization. In contrast, the conductance increases for spin-up but decreases for spin-down with an increase of the polarization. Interestingly, the magnitude of tunneling magnetoresistance (TMR) can be dramatically suppressed with the increase of the exchange splitting in graphene. Furthermore, the current-induced spin transfer torque (STT) dependence on the relative angle θ between the magnetic moments of the two leads shows a sine-like behavior and is enhanced with an increase of the polarization and/or the bias voltage. We attribute these spin-resolved effects to the breaking of the insulator-type properties of graphene with an exchange splitting between spin-up and spin-down carriers.