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To investigate the effects of the optical field and the Fermi velocity on the transport properties of spin and valley electrons, we impose a normal/ferromagnetic/normal (N/F/N) quantum structure based on the monolayer WSe 2. The results indicate that there is a strong switch effect for spin- and valley-related electrons. When left-handed off-resonant circularly polarized light is irradiated in the intermediate ferromagnetic region, 100 % polarization for K valley electrons can be achieved in the entire effective energy spectrum of the optical field. Meanwhile, 100 % polarization of the K ′ valley can also be gained with the right-handed off-resonant circularly polarized light in the junction. Moreover, the perfect polarization of spin-up electrons can be obtained when the ferromagnetic exchange field is applied to the structure. Additionally, the Fermi velocity barrier also changes the energy band of the studied material, which makes the spin and valley transport increase with the increase of the velocity barrier but does not produce spin or valley polarizations. These interesting results clarify that the optical field and the Fermi velocity both make a contribution to the modulation of spin electrons for the two valleys and provide a useful method for the design of novel spintronic and valleytronic devices.

We investigate the asymmetric effect in a spin-filter device, which is based on the Rashba spin-orbit coupling effect and uses a nonmagnetic tunneling diode. The structural asymmetry is introduced by unequivalence of the two quantum wells in the spin-filter device. It is found that the structural asymmetry can greatly change spin-filtering efficiency. For some asymmetric spin-filter structures, one can see spin-dependent enhancement in the transmission. Moreover, the current density can increase or decrease greatly depending on the degree of the structural asymmetry.

Based on the group velocity concept and the two-band model, we investigated tunneling properties of spin-polarized electrons traversing ferromagnetic/insulator (semiconductor) double junctions under the influence of an external electric field. The tunneling time and the transmission coefficient, as well as the spin polarization, were calculated and examined. Effects of the electric field and quantum size are also considered. The results indicate that the tunneling time strongly depends on the spin orientation of tunneling electrons. In a wide range of incident energy, spin-down electrons spend a longer time tunneling through the structure than spin-up ones, and the difference of the tunneling time Δτ between electrons with opposite spin orientation is very sensitive to the incident energy in the lower energy region. Moreover, the variation of Δτ with the increasing of the incident energy shows pronounced oscillations for certain applied bias and structural size. The results also indicate that spin polariz...

In this work, we have explored the influence of an external electric field on the spin and valley transport of massive electrons in a graphene system with spin-orbit coupling. Both the strength and width of the spin- and valley-polarization are greatly dependent on the external electric field. As the external electric field increases, the spin/valley polarization can be enhanced, even up to 100%. In addition, the presence of a gap resulting from the interplay of massive electrons and spin-orbit coupling can occur in the direction of the spin polarization being changed. Without the gap, spin-down electrons can be filtered at the low-energy Fermi level. However, with the gap, the effect is just the opposite; spin-up electrons are filtered. These findings may open an avenue for the electric control of valley and spin transport in graphene-based electronic devices.

We investigate the role played by the zero-field conduction band offset in spin-dependent transport through a ZnSe/Zn1−xMnxSe heterostructure with a single paramagnetic layer. It is shown that the zero-field band offset can strongly affect features of spin-polarized transport: the spin polarization is greatly weakened for the negative zero-field offset while in the positive case it is drastically enhanced. The reason is that the polarization is determined by the discrepancy between the effective potential for spin-up electrons and that for spin-down ones. Furthermore, the magnitude of the effective potential as well as its configuration are conduction-band-offset dependent and field tunable.

We investigate spin-dependent transport through a band-gap-matched ZnSe/Zn1−xMnxSe heterostructure with a single paramagnetic layer under influence of both an electric and a magnetic field. It is shown that the magnetic-field tunable potential and its symmetry in this system can be significantly modulated by the electric field, which results in obvious dependence of the magnitude of the electronic spin polarization on the electric field magnitude. The results obtained in this work might shed light on the development of new electron-polarized devices.

Recent studies indicated that under zero bias there is no spin polarization in an antiparallel magnetic-electric barrier structure, where double δ-function magnetic fields point in the opposite direction. Our research demonstrates that an external electric field can make such a hybrid structure spin polarized, however, the degree of spin polarization is smaller than 5% for the GaAs system. It is also shown that the electric barrier can greatly suppress the current density and affect the degree of spin polarization. The results can be explained by the variation of the effective potential induced by the electric field.

Using the effective-mass approximation and Floquet theory, we theoretically investigate the terahertz photon-assisted transport through a ZnSe/Zn1−xMnxSeheterostructure under an external magnetic field, an electric field, and a spatially homogeneous oscillatory field. The results show that both amplitude and frequency of the oscillatory field can accurately manipulate the magnitude of the spin-dependent transmission probability and the positions of the Fano-type resonance due to photon absorption and emission processes. Transmission resonances can be enhanced to optimal resonances or drastically suppressed for spin-down electrons tunneling through the heterostructure and for spin-up ones tunneling through the same structure, resonances can also be enhanced or suppressed, but the intensity is less than the spin-down ones. Furthermore, it is important to note that transmission suppression can be clearly seen from both the spin-down component and the spin-up component of the current density at low magnetic field; at the larger magnetic field, however, the spin-down component is suppressed, and the spin-up component is enhanced. These interesting properties may provide an alternative method to develop multi-parameter modulation electron-polarized devices.

Spin thermoelectric effects in ferromagnetic (FM) germanene are theoretically investigated by using the nonequilibrium Green's function method. It is found that the spin Seebeck effect can be generated by temperature bias ΔT when a FM germanene is considered in the central region. However, the obtained spin resolved Seebeck coefficients is quite low with maximum value of Sσ≃700μV/K. The spin Seebeck effect is shown to increase enormously in different energy states with the assistance of electric field or stub structure. By modulating the geometric parameters of stub, the spin thermopower Ss has distinct peak values in the bulk states. Moreover, varying the Fermi energy within the bulk gap by the gate, Ss can be significantly enhanced by increasing the strength of electric field. The spin thermopower obtained by each method is predicted to be 2500μV/K, which is more than 300% larger relative to the case without electric field or stub. In addition, the magnitude and sign of spin thermopower can be manipulat...

We propose a scheme to measure the lifetime and energy splitting of a pair of Majorana bound states at the ends of a superconducting nanowire by using the shot noise in a dynamical channel blockade system. A quantum dot is coupled to one end of the wire and connected with two electron reservoirs. It is found that a finite Majorana energy splitting tends to produce a super-Poissonian shot noise, while Majorana relaxation process relieves the dynamical channel blockade and suppresses the noise Fano factor. When the dot energy level locates in the middle of the gap of topological superconductor, the Fano factor is independent on Majorana lifetime and Majorana energy splitting is thus extracted. For a finite energy splitting, we could evaluate the Majorana relaxation rate from the suppression of Fano factor. Under a realistic condition, the expected resolution of Majorana energy splitting and its relaxation rate calculated from our model are about 1μeV and 0.01−1μeV, respectively.

The resonance splitting and wave-vector filtering for electron tunneling through magnetic superlattices are investigated theoretically. Two kinds of magnetic superlattices are examined. One is a periodic arrangement of identical magnetic barriers while the other is periodically juxtaposed with two different magnetic barriers. In general, one resonant domain in the former splits into two resonant domains in the latter. It is confirmed that both the resonance splitting and wave-vector filtering strongly depend on the structure of the magnetic superlattices. The numerical results indicate that the magnetic superlattice, which is a periodic arrangement of two different magnetic barriers, possesses stronger wave-vector filtering.

Spin Seebeck effect in a germanene p-n junction is studied by using the nonequilibrium Green's function method combined with the tight-binding Hamiltonian. We find that the thermal bias ΔT can generate spin thermopower when a local exchange field is applied on one edge of the germanene nano-ribbon. The magnitude of the spin thermopower can be modulated by the potential drop across the two terminals of the p-n junction. When the value of the potential drop is smaller than the spin-orbit interaction strength, the spin thermopower is enhanced by two orders of magnitude larger as compared to the case of zero p-n voltage. Optimal temperature corresponding to maximum spin thermopower is insensitive to the potential drop. In the p-n region, maximum spin thermopower can be obtained at relatively higher temperatures. When the value of the potential drop is larger than that of the spin-orbit interaction, however, the spin Seebeck effect decays rapidly with increasing potential drop or temperature. By optimizing the structure parameters, the magnitude of the spin thermopower can be remarkably enhanced due to the coexistence of the exchange field and the potential drop.

In this work, with the completely asymmetric terahertz (THz) irradiation, electron transport through a two-level InAs quantum dot is theoretically discussed. The Coulomb interaction in this system is compared with the energy difference between the two energy levels and is finite contrasted with the Microwave Field system. We investigate the average current trend with the change of Coulomb interaction and the THz irradiation strength. Our results reveal that there exists the photon-electron pump effect at the Coulomb interaction related energy level, and demonstrate Fano-like resonance in the low THz field strength due to the presence of the photon induced excited state resonance. We also find that the Fano-like resonance can be suppressed with increasing the temperature.

Resonant tunneling through symmetric and asymmetric double barriers based on monolayer graphene at non-normal incidence is investigated. Due to the evanescent modes inside the barrier, the transmission, as a function of the incident energy, has a gap which can be tuned by the height of the barrier and the incident angle of the electrons. In terms of the coupling between the barriers and the well in the symmetric double barriers, several line-type resonance peaks with a unity value appear in the transmission gap, and the number of the resonance peaks is closely related to the incident angle, the height, and the width of the barrier. The resonant conditions in the transmission gap are derived and discussed. However, the line-type resonance peaks are greatly suppressed through the asymmetric double barriers. The authors demonstrate that line-type peaks possess potential applications in resonant tunneling devices and energy filters.

Quantum transport properties in fractal magnetically modulated structures are studied by the transfer-matrix method. It is found that the transmission spectra depend sensitively not only on the incident energy and the direction of the wave vector but also on the stage of the fractal structures. Resonance splitting, enhancement, and position shift of the resonance peaks under different magnetic modulation are observed at four different fractal stages, and the relationship between the conductance in the fractal structure and magnetic modulation is also revealed. The results indicate the spectra of the transmission can be considered as fingerprints for the fractal structures, which show the subtle correspondence between magnetic structures and transport behaviors.

The authors have investigated the dwell time of electrons tunneling through nonuniform magnetically modulated graphene monolayer. Two types of models, i.e., the square magnetic barrier and the δ-function magnetic barrier, are introduced to simulate the magnetic modulation realized by depositing nanoscale ferromagnetic stripes on top of the graphene monolayer. It is found that both the dwell time and the transmission probability show remarkable anisotropy that varies in different magnetically modulated configurations. Particularly, when the electrons tunnel through the graphene monolayer modulated by two antiparallelly aligned ferromagnetic stripes, the corresponding transmission probability exhibits angularly symmetric property, whereas the dwell time does not. Moreover, there exists great discrepancy of the dwell time between in the Klein tunneling region and in the resonant tunneling region, where each region corresponds to the perfect transmission peaks.

The current shot noise behavior in resonant tunneling step-barrier structures is investigated based on the standard scattering approach. The relations between the shot noise and the applied bias, the Fermi energies, as well as the structural parameters are revealed. By making a comparison of resonant tunneling among single square-barrier structures, step-barrier structures, and double-barrier structures, the distinct curve shape and extremely large suppression of the shot noise are revealed in the step-barrier structures. It is shown that in the step-barrier structures, a valley of the shot noise occurs at the bias where the current peak locates and the Fano factor drops down to minimum at the valley bottom. It is also found that the shot noise suppression is larger for larger chemical potentials and maximal suppression with the Fano factor close to 0.05 appears at a particular structure configuration. These results are helpful to improve the signal-to-noise ratio of quantum devices based on resonant-tunn...

Spin-polarized transport properties have been investigated in diluted-magnetic-semiconductor/semiconductor quantum wires. We stress the effects introduced by the structural configuration and geometric parameters as well as the external magnetic field. It is found that the symmetric quantum wire shows quite different spin-dependent transport characteristics from the asymmetric one. It is also found that the spin-up component of the conductance can be considerably suppressed when the magnetic field is large enough, and thus nearly 100% spin polarization can be obtained in such kind of quantum wire. Moreover, one can effectively modulate the spin-dependent conductance in the quantum wire structure by varying the geometric parameters and the Fermi energy.

We investigate spin-filtering effect in multilayered ferromagnetic (F)/semiconductor (S) heterostructures within the Landauer framework of ballistic transport. Spin-dependent transmission and polarization are calculated and analyzed for different magnetizations of three ferromagnetic layers in a F∕S∕F∕F structure proposed in this work. The results indicate that in such a multilayered configuration and when the magnetizations of the middle and the right ferromagnetic layers are antiparallel, the transmission for spin-up and spin-down electrons can be separated, which is quite different from the transport properties in the F∕S∕F structure, where electrons of different spin orientations have exactly the same contributions to transmission if the magnetic moments of the two ferromagnetic layers are antiparallel. It is also shown that the F∕S∕F∕F structure can have big values of the polarization than the F∕S∕F structure. The quantum size effect of the length of the middle ferromagnetic layer and that of the sem...

We have investigated the ballistic spin-polarized transport through a diluted magnetic semiconductor heterostructure with the inclusion of a nonmagnetic barrier. It is found that at suitable magnetic fields, the output current of the system exhibits a nearly 100% spin polarization and large values under the forward bias, while under the reverse bias it is small and shows a weak polarization. Such current and spin-filtering dual diode functions are robust under the change of the nonmagnetic barrier width.