Benhu Zhou

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 Greens 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-dependent Seebeck effects in a graphene nanoribbon coupled to two square lattice ferromagnetic leads

We theoretically investigate spin-dependent Seebeck effects for a system consisting of a narrow graphene nanoribbon (GNR) contacted to square lattice ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) were considered. Compared with our previous work with two-dimensional honeycomb-lattice FM leads, a more realistic model of two-dimensional square-lattice FM electrodes is adopted here. Using the nonequilibrium Greens function method combining with the tight-binding Hamiltonian, it is demonstrated that both the charge Seebeck coefficient SC and the spin-dependent Seebeck coefficient SS strongly depend on the geometrical contact between the GNR and the leads. In our previous work, SC for a semiconducting 15-AGNR system near the Dirac point is two orders of magnitude larger than that of a metallic 17-AGNR system. However, SC is the same order of magnitude for both metallic 17-AGNR and semiconducting 15-...

Seebeck effects in a graphene nanoribbon coupled to two ferromagnetic leads

We theoretically investigate the Seebeck effects for the system of a narrow graphene nanoribbon between two ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) have been considered. By using the nonequilibrium Greens function method combining with the tight-binding Hamiltonian, it is demonstrated that, the Seebeck coefficients are sensitive to the chirality and width of the nanoribbon in the absence of magnetic field. Compared with 22-ZGNR and metallic 17-AGNR systems, semiconducting 15-AGNR system is found to posses superior thermoelectric performance, its Seebeck coefficients can be improved by two orders of magnitude. Meanwhile, the Seebeck coefficients for both 22-ZGNR and metallic 17-AGNR systems are the same order as that of graphene system. Furthermore, the Seebeck coefficients are strongly dependent on the magnetization M as well as magnetic configuration of the two FM leads. Particularly, t...

Thermoelectric effect in a graphene sheet connected to ferromagnetic leads

We theoretically investigate the thermoelectric property of a graphene sheet coupled to two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Greens function combining with the tight-binding Hamiltonian, it is demonstrated that the thermopower weakly depends on the polarization strength as well as on magnetic configuration of the leads. On the contrary, the electronic contribution to the thermal conductance is sensitive to both the polarization strength and the relative alignment of magnetization directions. Furthermore, the thermopower shows a linear dependence on temperature and an inverse dependence on the Fermi energy in the graphene, which agrees well the recent experimental measurements.

Thermoelectric transport for a quantum wire side-coupled to a graphene sheet between ferromagnetic leads

We theoretically investigate the thermoelectric transport through a quantum wire (QW) side-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Greens function combining with the tight-binding Hamiltonian, it is demonstrated that both the thermopower and the electronic contribution to the thermal conductance develop an oscillating behavior with resonances and antiresonances due to constructive and destructive interferences in the system, respectively. Interestingly, the thermopower changes its sign for even- or odd-number of atoms in the wire, and the thermal conductance is always positive with an even-odd behavior at zero energy level position of the quantum wire. Moreover, the thermopower and the thermal conductance are weakly dependent on the wire–graphene coupling strength as well as the relative magnetic configurations of leads. On the contrary, they are both strongly dependent on the temperature and the polar...

Controllable fully spin-polarized transport in a ferromagnetically doped topological insulator junction

We investigate the energy band structure and the spin-dependent transport for a normal/ferromagnetic/normal two-dimension topological insulator (TI) junction. By diagonalizing Hamiltonian for the system, the band structure shows that the edge states on two sides are coupled resulting in a gap opening due to the transverse spatial confinement. Further, the exchange field induced by magnetic impurities can also modulate the band structure with two spin degenerate bands splitting. By using the nonequilibrium Greens function method, the dependence of spin-dependent conductance and spin-polarization on the Fermi energy, the exchange field strength and the ferromagnetic TI (FTI) length are also analyzed, respectively. Interestingly, the degenerate conductance plateaus for spin-up and -down channels are broken, and both the conductances are suppressed by magnetic impurities due to the time-reversal symmetry broken and inelastic scattering. The spin-dependent conductance shows different behaviors when the Fermi ...