Spin transport in a chain of polygonal quantum rings with Dresselhaus spin-orbit coupling

Using a transfer matrix method, we investigate spin transport through a chain of polygonal rings with Dresselhaus spin-orbit coupling（DSOC）. The spin conductance is dependent on the number of sides in the polygons. When DSOC is considered in a chain which also has Rashba spin-orbit coupling（RSOC） of the same magnitude, the total conductance is the same as that for the same chain with no SOC. However, when the two types of SOC have different values, there results a unique anisotropic conductance.

Quantum spin transport through Aharonov-Bohm ring with a tangent magnetic field

Quantum spin transport in a mesoscopic Aharonov-Bohm ring with two leads subject to a magnetic field with circular configuration is investigated by means of one-dimensional quantum waveguide theory. Within the framework of Landauer-Bfittiker formalism, the polarization direction of transmitted electrons can be controlled either by the AB magnetic flux or by the tangent magnetic field. In particular, the spin flips can be induced by hopping the AB magnetic flux or the tangent field.

Spin transport properties of a Dresselhaus-polygonal quantum ring

We propose a theoretical method to investigate the effect of the Dresselhaus spin–orbit coupling（DSOC） on the spin transport properties of a regular polygonal quantum ring with an arbitrary number of segments. We find that the DSOC can break the time reversal symmetry of the spin conductance in a polygonal ring and that this property can be used to reverse the spin direction of electrons in the polygon with the result that a pure spin up or pure spin down conductance can be obtained by exchanging the source and the drain. When the DSOC is considered in a polygonal ring with Rashba spin–orbit coupling（RSOC） with symmetric attachment of the leads, the total conductance is independent of the number of segments when both of the two types of spin–orbit coupling（SOC） have the same value. However, the interaction of the two types of SOC results in an anisotropic and shape-dependent conductance in a polygonal ring with asymmetric attachment of the leads. The method we proposed to solve for the spin conductance of a polygon can be generalized to the circular model.

Polarized spin transport in mesoscopic quantum rings with electron phonon and Rashba spin-orbit coupling

The influence of electron-phonon （EP） scattering on spin polarization of current output from a mesoscopic ring with Rashba spin-orbit （SO） interaction is numerically investigated. There are three leads connecting to the ring at different positionsl unpolarized current is injected to one of them, and the other two are output channels with different bias voltages. The spin polarization of current in the outgoing leads shows oscillations as a function of EP coupling strength owing to the quantum interference of EP states in the ring region. As temperature increases, the oscillations are evidently suppressed, implying decoherence of the EP states. The simulation shows that the magnitude of polarized current is sensitive to the location of the lead. The polarized current depends on the connecting position of the lead in a complicated way due to the spin-sensitive quantum interference effects caused by different phases accumulated by transmitting electrons with opposite spin states along different paths.

Coherent Spin Transport Through a Six-Coordinate FeN6 Spin-Crossover Complex with Two Di erent Spin Configurations

Due to the magnetic bistability, single-molecule spin-crossover (SCO) complexes have been considered to be the most promising building blocks for molecular spintronic devices. Here, we explore the SCO behavior and coherent spin transport properties of a six-coordinate FeN6 complex with the low-spin (LS) and high-spin (HS) states by performing extensive first-principles calculations combined with non-equilibrium Green’s function technique. Theoretical results show that the LS$HS spin transition via changing the metal-ligand bond lengths can be realized by external stimuli, such as under light radiation in experiments. According to the calculated zero-bias transmission coefficients and density of states as well as the I-V curves under small bias voltages of FeN6 SCO complex with the LS and HS states sandwiched between two Au electrodes, we find that the examined molecular junction can act as a molecular switch, tuning from the OFF (LS) state to the ON (HS) state. Moreover, the spin-down electrons govern the current of the HS molecular junction, and this observed perfect spin-filtering effect is not sensitive to the detailed anchoring structure. These theoretical findings highlight this examined six-coordinate FeN6 SCO complex for potential applications in molecular spintronics.

Spin transport in antiferromagnetic insulators

Electrical spin,which is the key element of spintronics,has been regarded as a powerful substitute for the electrical charge in the next generation of information technology,in which spin plays the role of the carrier of information and/or energy in a similar way to the electrical charge in electronics.Spin-transport phenomena in different materials are central topics of spintronics.Unlike electrical charge,spin transport does not depend on electron motion,particularly spin can be transported in insulators without accompanying Joule heating.Therefore,insulators are considered to be ideal materials for spin conductors,in which magnetic insulators are the most compelling systems.Recently,we experimentally studied and theoretically discussed spin transport in various antiferromagnetic systems and identified spin susceptibility and the Néel vector as the most important factors for spin transport in antiferromagnetic systems.Herein,we summarize our experimental results,physical nature,and puzzles unknown.Further challenges and potential applications are also discussed.

Photon-assisted electronic and spin transport through two T-shaped three-quantum-dot molecules embedded in an Aharonov-Bohm interferometer

We investigate the time-modulated electronic and spin transport properties through two T-shaped three-quantum-dot molecules embedded in an Aharonov-Bohm（A-B） interferometer. By using the Keldysh non-equilibrium Green＇s function technique, the photon-assisted spin-dependent average current is analyzed. The T-shaped three-quantum-dot molecule A-B interferometer exhibits excellent controllability in the average current resonance spectra by adjusting the interdot coupling strength, Rashba spin-orbit coupling strength, magnetic flux, and amplitude of the time-dependent external field.Efficient spin filtering and multiple electron-photon pump functions are exploited in the multi-quantum-dot molecule A-B interferometer by a time-modulated external field.

Molecular design for enhanced spin transport in molecular semiconductors

Molecular semiconductors(MSCs),characterized by a longer spin lifetime than most of other materials due to their weak spin relaxation mechanisms,especially at room temperature,together with their abundant chemical tailorability and flexibility,are regarded as promising candidates for spintronic applications.Molecular spintronics,as an emerging subject that utilizes the unique properties of MSCs to study spin-dependent phenomena and properties,has attracted wide attention.In molecular spintronic devices,MSCs play the role as medium for information transport,process,and storage,in which the efficient spin inject–transport process is the prerequisite.Herein,we focus mainly on summarizing and discussing the recent advances in theoretical principles towards spin transport of MSCs in terms of the injection of spin-polarized carriers through the ferromagnetic metal/MSC interface and the subsequent transport within the MSC layer.Based on the theoretical progress,we cautiously present targeted design strategies of MSCs that contribute to the optimization of spin-transport efficiency and give favorable approaches to exploring accessional possibilities of spintronic materials.Finally,challenges and prospects regarding current spin transport are also presented,aiming to promote the development and application of the rosy and energetic field of molecular spintronics.

Universal behaviors of magnon-mediated spin transport in disordered nonmagnetic metalferromagnetic insulator heterostructures

We numerically investigate magnon-mediated spin transport through nonmagnetic metal/ferromagnetic insulator(NM/FI)heterostructures in the presence of Anderson disorder,and discover universal behaviors of the spin conductance in both one-dimensional(1D)and 2D systems.In the localized regime,the variance of logarithmic spin conductanceσ2(lnGγ)shows a universal linear scaling with its average(lnGγ),independent of Fermi energy,temperature,and system size in both 1D and 2D cases.In 2D,the competition between disorder-enhanced density of states at the NM/FI interface and disorder-suppressed spin transport leads to a non-monotonic dependence of average spin conductance on the disorder strength.As a result,in the metallic regime,average spin conductance is enhanced by disorder,and a new linear scaling between spin conductance fluctuation rms(GT)and average spin conductance GT is revealed which is universal at large system width.These universal scaling behaviors suggest that spin transport mediated by magnon in disordered 2D NM/FI systems belongs to a new universality class,different from that of charge conductance in 2D normal metal systems.

Recent advances in spin transport in organic semiconductors

The spin relaxation time is long in organic semiconductors because of the weak spin-orbit and hyperfine interactions,leading to intensive study on spin transport in organic semiconductors.The rapid progress towards utilizing spin degree of freedom in organic electronic devices is occurring.While the spin injection,transport and detection in organic semiconductors are demonstrated,the fundamental physics of these phenomena remains unclear.This paper highlights recent progress that has been made,focusing primarily on present experimental work.

Monte-Carlo simulation studies of the effect of temperature and diameter variation on spin transport in Ⅱ–Ⅵ semiconductor nanowires

We have analyzed the spin transport behaviour of four II-VI semiconductor nanowires by simulating spin polarized transport using a semi-classical Monte-Carlo approach. The different scattering mechanisms con- sidered are acoustic phonon scattering, surface roughness scattering, polar optical phonon scattering, and spin flip scattering. The II-VI materials used in our study are CdS, CdSe, ZnO and ZnS. The spin transport behaviour is first studied by varying the temperature （4-500 K） at a fixed diameter of 10 nm and also by varying the diameter （8-12 nm） at a fixed temperature of 300 K. For II-VI compounds, the dominant mechanism is for spin relaxation; D＇yakonovPerel and Elliot Yafet have been actively employed in the first order model to simulate the spin transport. The dependence of the spin relaxation length （SRL） on the diameter and temperature has been analyzed.

Quantum Spin Transport in Rashba Spin-Orbit-Coupled Graphene Nanoflakes

We study the spin-resolved transport in a two-terminal graphene nanoflake device with a Rashba spinorbit coupling region in the center of the device. The Green's function method is applied to the system and the spin transmission probability and the spin polarization in x, y, and z directions are calculated. It is found that the components of the spin polarization are antisymmetric functions of Fermi energy, which oscillate and decay to the zero with increasing the energy for all values of the Rashba strength. It is shown that by tuning the Rashba strength via a gate voltage and/or changing the size of the system, it is possible to control the sign and magnitude of the spin polarization. The system represented here is a typical candidate for full electrical spintronic devices based on the carbon materials that are used for spin filtration.

Organic-ferromagnetic hetero-structures with spin transport properties and fundamental physical effects

Organic spintronics refers to control spin dependent transport through organic materials.In the last two decades,extraordinary development has been achieved for organic-spintronics.A series of theoretical and experimental studies have been done to reveal the mechanisms of spin dependent transport properties.The theoretical analysis is based on the non-equilibrium Green's function formalism provides a mathematical framework for solving the transmission coefficients in the Landauer formula from atomistic first principles without any phenomenological parameters.In this article,we provide a brief theoretical review on organic spintronics devices and device physics therein.

Monte Carlo simulation of in-plane spin-polarized transport in GaAs/GaAlAs quantum well in the three-subband approximation

We develop a Monte Carlo （MC） tool incorporated with the three-subband approximation model to investigate the in-plane spln-polarized transport in GaAs/GaAlAs quantum well. Using the tool, the effects of the electron occupation of higher subbands and the intersuhband scattering on the spin dephasing have been studied. Compared with the corresponding results of the simple one-snbband approximation model, the spin dephasing length is reduced four times under 0.125 kV/cm of driving electric field at 300K by the MC tool incorporated with the three-subband approximation model, indicating that the three-subbarld approximation model predicts significantly shorter spin dephasing length with temperature increasing. Our simulation results suggest that the effects of the electron occupation of higher subbands and the intersubband scattering on the spln-dependent transport of GaAs 2-dhuensional electron gas need to be considered when the driving electric field exceeds the moderate value and the lattice temperature is above 100K. The simulation by using the MC tool incorporated with the three-subband approximation model also indicates that, under a eertain driving electric field and lattice temperature, larger channel widths cause spins to be depolarized faster. Ranges of the three components of the spins are different for three different injected spin polarizations due to the anisotropy of spin-orbit interaction.

Spin-dependent transport for a two-dimensional electron gas with magnetic barriers

The spin-dependent conductance and magnetoresistance ratio （MRR） for a semiconductor heterostructures consisting of two magnetic barriers with different height and space have been investigated by the transfer-matrix method. It is shown that the splitting of the conductance for parallel and antiparallel magnetization configurations results in tremendous spin-dependent MRR, and the maximal MRRs reach 5300% and 3800% for the magnetic barrier spaces W = 81.3 and 243.9 nm, respectively. The obtained spin-filtering transport property of nanostructures with magnetic barriers may be useful to magnetic-barrier-based spintronics.

Pure spin polarized transport based on Rashba spin orbit interaction through the Aharonov Bohm interferometer embodied four-quantum-dot ring

The spin-polarized linear conductance spectrum and current–voltage characteristics in a four-quantum-dot ring embodied into Aharonov–Bohm （AB） interferometer are investigated theoretically by considering a local Rashba spin–orbit interaction. It shows that the spin-polarized linear conductance and the corresponding spin polarization are each a function of magnetic flux phase at zero bias voltage with a period of 2π, and that Hubbard U cannot influence the electron transport properties in this case. When adjusting appropriately the structural parameter of inter-dot coupling and dot-lead coupling strength, the electronic spin polarization can reach a maximum value. Furthermore, by adjusting the bias voltages applied to the leads, the spin-up and spin-down currents move in opposite directions and pure spin current exists in the configuration space in appropriate situations. Based on the numerical results, such a model can be applied to the design of a spin filter device.

Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling

Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling （SOC） are investigated analytically using quantum networks and the transport matrix metLod. The results show that conduc- tances remain at exactly the same values when the output leads are located at axisymmetric positions. However, for the nonaxisymmetrical case, there is a phase difference between the upper and lower arm, which leads to zero conductances appearing periodically. An isotropy of the conductance is destroyed by the Rashba SOC effect in the axisymmetric case. In addition, the position of zero conductance is regulated with the strength of the Rashba SOC.

Spin Caloritronic Transport of 1,3,5-Triphenylverdazyl Radical

We investigate theoretically the spin caloritronic transport properties of a stable 1,3,5-triphenylverdazyl （TPV） radical sandwiched between Au electrodes through different connection fashions. Obvious spin Seebeck effect can be observed in the para-eonnection fashion. Furthermore, a pure spin current and a completely spin-polarized current can be realized by tuning the gate voltage. Furthermore, a 100% spin polarization without the need of gate voltage can be obtained in the meta-conneetion fashion. These results demonstrate that TPV radical is a promising material for spin caloritronic and spintronic applications.

Spin-polarized transport through the T-shaped double quantum dots

Using the Keldysh nonequilibrium Green function and equation-of-motion technique, this paper investigates the spin-polarized transport properties of the T-shaped double quantum dots （DQD） coupled to two ferromagnetic leads. There are both Fano effect and Kondo effect in the system, and due to their mutual interaction, the density of states, the current, and the differential conductance of the system depend sensitively on the spin-polarized strength. Thus the obtained results show that this system is provided with excellent spin filtering property, which indicates that this system may be a candidate for spin valve transistors in the spintronics.

Spin Caloritronic Transport of (2×1) Reconstructed Zigzag MoS_2 Nanoribbons

Using first-principles density functional theory combined with nonequilibrium Green＇s function method, we inves-tigate the spin caloritronic transport properties of （2×1） reconstructed zigzag MoS2 nanoribbons. These systems can exhibit obvious spin Seebeck effect. Furthermore, by tuning the external magnetic field, a thermal giant magnetoresistance up to 10^4% can be achieved. These spin caloritronic transport properties are understood in terms of spin-resolved transmission spectra, band structures, and the symmetry analyses of energy bands around the Fermi level.