A pure spin-current injector of semiconductor quantum dots with Andreev reflection and Rashba spin-orbit coupling

We propose a four-terminal device consisting of two parallel quantum dots with Rashba spin-orbit interaction （RSOI）, coupled to two side superconductor leads and two common ferromagnetic leads, respectively. The two ferromagnetic leads and two quantum dots form a ring threaded by Aharonov-Bohm （AB） flux. This device possesses normal quasiparticle transmission between the two ferromagnetic leads, and normal and crossed Andreev reflections providing conductive holes. For the appropriate spin polarization of the ferromagnetic leads, RSOI and AB flux, the pure spin-up （or spin-down） current without net charge current in the right lead, which is due to the equal numbers of electrons and holes with the same spin-polarization moving along the same direction, can be obtained by adjusting the gate voltage, which may be used in practice as a pure spin-current injector.

Pure spin-current diode based on interacting quantum dot tunneling junction

A magnetic field-controlled spin-current diode is theoretically proposed,which consists of a junction with an interacting quantum dot sandwiched between a pair of nonmagnetic electrodes.By applying a spin bias VS across the junction,a pure spin current can be obtained in a certain gate voltage regime,regardless of whether the Coulomb repulsion energy exists.More interestingly,if we applied an external magnetic field on the quantum dot,we observed a clear asymmetry in the spectrum of spin current IS as a function of spin bias,while the charge current always decays to zero in the Coulomb blockade regime.Such asymmetry in the current profile suggests a spin diode-like behavior with respect to the spin bias,while the net charge through the device is almost zero.Different from the traditional charge current diode,this design can change the polarity direction and rectifying ability by adjusting the external magnetic field,which is very convenient.This device scheme can be compatible with current technologies and has potential applications in spintronics or quantum processing.

Spin pumping through magnetic impurity effect

We propose a simple adiabatic quantum spin pump to generate pure spin current. The spin pump is driven by an ac gate voltage and a time-dependent magnetic impurity potential. It is found that the total pumped spin per cycle exhibits oscillations, whose magnitude decays exponentially with changing strength of the impurity potential. The proposed method may be useful for spintronic applications.

Photogalvanic effect induced charge and spin photocurrent in group-V monolayer systems

Photogalvanic effect(PGE)occurs in materials with non-centrosymmetric structures when irradiated by linearly or circularly polarized light.Here,using non-equilibrium Green’s function combined with density functional theory(NEGF-DFT),we investigated the linear photogalvanic effect(LPGE)in monolayers of group-V elements(As,Sb,and Bi)by first-principles calculations.First,by designing a two-probe structure based on the group-V elements,we found a giant anisotropy photoresponse of As between the armchair and zigzag directions.Then,we analyzed Sb and Bi’s charge and spin photocurrent characteristics when considering the spin-orbit coupling(SOC)effect.It is found that when the polarization direction of linearly polarized light is parallel or perpendicular to the transport direction(θ=0°or 90°),the spin up and spin down photoresponse in the armchair direction has the same magnitude and direction,leading to the generation of net charge current.However,in the zigzag direction,the spin up and spin down photoresponse have the same magnitude with opposite directions,leading to the generation of pure spin current.Furthermore,it is understood by analyzing the bulk spin photovoltaic(BSPV)coefficient from the symmetry point of view.Finally,we found that the net charge current generated in the armchair direction and the pure spin current generated in the zigzag direction can be further tuned with the increase of the material’s buckling height.Our results highlight that these group-V monolayers are promising candidates for novel functional materials,which will provide a broad prospect for the realization of ultrathin ferroelectric devices in optoelectronics due to their spontaneous polarization characteristics and high Curie temperature.

Arbitrary magnetic field modulations to a semiconductor pump with two types of spin-orbit couplings

Arbitrary magnetic field modulations to the semiconductor pump with both the Rashba and Dresselhaus spin-orbit couplings （SOC） are studied. The pump is driven by double time-dependent delta potentials, which are formed in the interfaces between the semiconductor region and two normal leads. Based on the Floquet scattering approach, our calculations show that various currents can be pumped by couplings of the magnetic fields and the SOCs. Pure spin currents modulated by the arbitrary magnetic fields are discussed in detail.