Rashba spin−orbit coupling effect on tunneling time in semiconductor spintronic junctions

Based on the transfer-matrix method and the group velocity concept, we investigate theoretically the spin tunneling time through single and double barriers of diluted magnetic semiconductor structures in the presence of the Rashba spin?orbit coupling (RSOC) effect. The calculation of transmission probability is based on an effective mass quantum-mechanical approach in the presence of an external magnetic field applied along the growth direction of the junction. The results show that the RSOC has great different influence on spin-dependent tunneling time of electrons with spin up and spin down in these structures. We also study the effect of zero-field conduction band offset on spin-dependent transmission properties. It is found that in the presence of RSOC and a positive zero-field conduction band offset as high as 10 meV, the results show a high degree of spin polarization and spin separation in the tunneling time in the considered system and this aspect may be utilized in designing new spin filter devices.     

Effects of a ferromagnetic metal stripe and a Schottky metal stripe on the electron transport in a nanostructure

In this paper, the spin-dependent electron transport in a two-dimensional electron gas (2DEG) modulated by a ferromagnetic (FM) metal stripe and a Schottky metal (SM) stripe is in detail studied. It is found that the position and the width of the SM stripe as well as the incident energy of electron play an important role on the spin polarization. It is also found that the spin polarization is obviously dependent on the electric-barrier height induced by an applied voltage to the SM stripe and such a device can be used as a voltage-tunable electron-spin filter.     

Two steps bulk-surface functionalization of nanoporous alumina by methyl and vinyl-silane adsorption. Evidence for oxide surface highly reactive sites creation

Functionalization of a novel nanoporous monolithic alumina synthesized from amalgam is investigated. The structure is studied by X-ray diffraction, BET, MEB and IR spectroscopy, before and after chemical functionalization by trimethylethoxy silane adsorption and annealing at high temperature. These treatments retain both monolith microstructure and nanostructure while strongly improving material mechanical properties. Allyldimethoxysilane and alcohol adsorption on the annealed samples, proves that highly reactive sites are available for further polymer grafting, as demonstrated by a significant shift of allyldimethoxysilane ν_SiH to 2,215 cm⁻¹ and adsorbed acetate formation. Simple quantum computations on model systems support this conclusion. Chemical processes reported in this paper, allow a nanostructured alumina monoliths functionalization to optimize ceramics-polymer bonds, and to tune new hybrid biomaterial properties.     

Effect of Ferromagnetic Metal Stripe and Strained Barrier on Electron Transport Characteristics in a Graphene

In this paper, the effect of ferromagnetic metal stripe and the strained barrier on the valley-dependent transport characteristics of electrons is studied in a graphene nanostructure. The numerical results show that a large valley polarization can be obtained in such a graphene, and the valley-dependent transport characteristics can be well controlled by changing the strength of the magnetic field induced by the ferromagnetic metal stripe, the width of the ferromagnetic metal stripe and the position of the strained barrier. Therefore, the valley polarization can be modulated by controlling the ferromagnetic metal stripe and the strained barrier. This work can promote the research and development of the new valleytronic devices, and then meet the practical application needs.

     

Electric Field Effects on Spin Polarized Transport in FM/NMS and FM/NMS/FM Structures in Interaction Approximation

In this article, we have solved spin-dependent drift-diffusion equations analytically by considering a spin selective barrier between the magnet and semiconductor layer in interaction approximation and also taking into account the correlation and exchange effects. We have studied the electric field effects on the spin polarized transport in the ferromagnetic/nonmagnetic semiconductor (FM/NMS) and FM/NMS/FM structures in a degenerate regime. We have shown by increasing the conductivity of semiconductor up to ferromagnetic conductivity, semiconductor effective resistance becomes smaller and the spin injection efficiency will be increased. Also, the electric field enhances spin polarization density. Furthermore, in injection structures with interfacial barriers, the electric field enhances spin polarization considerably. In fact, the spin selective interfacial barrier acts as a spin filter, which permits electrons with a particular spin direction ↑(↓) pass through the interface. In addition, the calculated results in interacting approximation show that spin injection is increased. Finally, it is found that in FM/NMS/FM structures at low-field regime, the width of the semiconductor has the important role in spin transport.     

自旋注入对有机半导体极化子电导的影响

在有机半导体自旋电子器件中,自旋从铁磁极注入到有机半导体后,自旋相上的极化子和自旋向下的极化子有不同的态密度,从而产生不同的电导.利用自旋漂移一扩散方程通过自洽计算得到了铁磁/有机半导体自旋注入结构中极化子自旋相关的电导和电流的自旋极化率.计算结果表明,极化子电导的自旋相关性是自旋注入引起的,和电流的自旋极化率密切相关;在自旋注入发生后,有机半导体内不同位置上极化子自旋态密度不同,由此产生的极化子电导也不相同,极化予电导是位置的函数.另外还发现,外电场会增强有机半导体电流的自旋极化率.     

Electrical Spin Injection in a Ferromagnetic Metal/Insulator/Semiconductor Tunnel Heterostructure

We demonstrate experimentally the electrical spin injection from a ferromagnetic metal/tunnel barrier contact into a semiconductor III–V heterostructure. The injected electrons have an in-plane spin orientation. We show that by applying an oblique external magnetic field this spin orientation can be manipulated within the semiconductor, and a nonzero perpendicular spin component arises. This perpendicular component can be easily monitored by optical means (circular polarization of the emitted light). In a CoFe/AlO _x /(Al,Ga)As/GaAs heterostructure we observe injected spin polarization in access of 9% at 80 K and optimized structures have recently shown spin injection up to room temperature.     

Ballistic electron transport in a nanostructure under the influence of magnetic barriers and δ-doping

The electron transport properties are theoretically investigated in a nanostructure under the influence of magnetic barriers and the δ-doping. The resulting transmission and spin polarization are strongly dependent on the weight of δ-doping. It is shown that the large spin polarization can be achieved due to the remarkable discrepancy in the transmission for electrons with opposite spin orientations. It is also shown that both the transmission probability and the spin polarization periodically change with the increase of L. These interesting features will be more helpful for developing new types of spintronic devices.     

Revealing the Cooperative Relationship between Spin,Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge‐transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light‐emitting diodes are reported. Magneto‐photoluminescence studies reveal that the triplet‐to‐singlet conversion in exciplexes involves an artificially generated spin‐orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge‐transfer occurs with forming electric dipoles (D^+?→A^??), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔE_ST) in 9,9′‐diphenyl‐9H,9′H‐3,3′‐bicarbazole (BCzPh):3′,3′″,3′″″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(([1,1′‐biphenyl]‐3‐carbonitrile)) (CN‐T2T) (ΔE_ST = 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔE_ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge‐transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE_ST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+?→A^??) size, enhanced SOC, increased orbital polarization, and decreased ΔE_ST can simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion.     

Ballistic electron transport in a nanostructure under the influence of magnetic barriers and δ-doping

The electron transport properties are theoretically investigated in a nanostructure under the influence of magnetic barriers and the δ-doping. The resulting transmission and spin polarization are strongly dependent on the weight of δ-doping. It is shown that the large spin polarization can be achieved due to the remarkable discrepancy in the transmission for electrons with opposite spin orientations. It is also shown that both the transmission probability and the spin polarization periodically change with the increase of L. These interesting features will be more helpful for developing new types of spintronic devices.     

Magnetically tunable spin filtering in semiconductor nanowires

We investigate resonant transmission and spin filtering in symmetric semiconductor nanowires (SSNs), where Rashba spin-orbit coupling (SOC) is symmetrically distributed by applying external electric field. It is shown that electronic bandgap structure has been formed, and the width of the bandgap can be enlarged by increasing the strength of SOC. Resonant transmission has been found in the electronic bandgap, which is characterized by perfect transmission peak. Interestingly, by introducing a weak magnetic modulation, the transmission spectra of spin-up and spin-down electrons are separated. With increasing the length of the centre segment in the SSN, multiple spin-dependent perfect transmission peaks appear in the bandgap. The resonant energy and the number of modes of resonant transmission therein can be manipulated. Around resonant energy, high spin-polarization is observed, and fully spin-polarized conductance is obtained in this SSN. Our investigations achieve potential applications in spin filters.     

Non-equilibrium Spin Currents in Systems with Striped Rashba Spin-Orbit Coupling

It is shown that non-homogeneity in the spin-orbit coupling (SOC) arising from structure inversion asymmetry gives rise to a finite intrinsic spin Hall conductivity in contrast with the corresponding case of a homogeneous SOC in the disordered two-dimensional electron gas. In particular, we examine the inhomogeneity arising from a striped modulation of the Rashba type spin-orbit coupling. A corresponding structure could be realized at oxide interfaces with periodic top gating or in semiconductor heterostructures.     

Local Definition of Spin Polarization in a Semiconductor by Micro-scale Current Loops

We present an approach to electrical control of the spin polarization in a diluted magnetic semiconductor (DMS) structure. A variable magnetic field induced by a micro-scale current loop magnetizes the Mn²⁺ ions in a CdMnTe/CdMgTe DMS quantum well, which via the sp-d exchange interaction polarizes photo-generated electron-hole pairs confined in the well. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The current-induced magnetic field and the current-generated heating of the spin system are quantitatively extracted by micro magneto-luminescence measurements.     

Spin characterization and control over the regime of radiation-induced zero-resistance states

Over the regime of the radiation-induced zero-resistance states and associated oscillatory magnetoresistance, we propose a low-magnetic-field analog of quantum-Hall-limit techniques for the electrical detection of electron spin and nuclear magnetic resonance, dynamical nuclear polarization via electron spin resonance, and electrical characterization of the nuclear spin polarization via the Overhauser shift. In addition, beats observed in the radiation-induced oscillatory magnetoresistance are developed into a method to measure and control the zero-field spin splitting due to the Bychkov-Rashba and bulk inversion asymmetry terms in the high-mobility GaAs-AlGaAs system.     

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts. Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.     

Hybrid nanostructure heterojunction solar cells fabricated using vertically aligned ZnO nanotubes grown on reduced graphene oxide

Using reduced graphene oxide (rGO) films as the transparent conductive coating, inorganic/organic hybrid nanostructure heterojunction photovoltaic devices have been fabricated through hydrothermal synthesis of vertically aligned ZnO nanorods (ZnO-NRs) and nanotubes (ZnO-NTs) on rGO films followed by the spin casting of a poly(3-hexylthiophene) (P3HT) film. The data show that larger interfacial area in ZnO-NT/P3HT composites improves the exciton dissociation and the higher electrode conductance of rGO films helps the power output. This study offers an alternative to manufacturing nanostructure heterojunction solar cells at low temperatures using potentially low cost materials.     

Spin injection in spin FETs using a step-doping profile

We investigate the effect of a step-doping profile on the spin injection from a ferromagnetic metal contact into a semiconductor quantum well in spin field-effect transistors using a Monte Carlo model. The considered scheme uses a heavily doped layer at the metal-semiconductor interface to vary the Schottky barrier shape and enhance the tunneling current. It is found that spin flux (spin current density) is enhanced proportionally to the total current, and the variation of current spin polarization does not exceed 20%.     

Determining spatial modes of lasers with spatial coherence measurements

We explain a technique that extracts both the structure and the modal weights of spatial modes of lasers by analyzing the spatial coherence of the beam. This is the first time, to our knowledge, that an experimental method is being used to measure arbitrary forms of the spatial modes. We applied this method to an edge-emitting Fabry-Perot semiconductor laser with a stripe width of 5 mum and extracted fundamental and first-order lateral modes with relative power weights of 96.2% and 3.8%. There was a single transverse mode.     

Transient Spectroscopy of Optical Spin Injection in ZnMnSe/ZnCdSe Quantum Structures

We show, by time-resolved magneto-photoluminescence (PL) spectroscopy in combination with selective laser excitation, that optical polarization of the ZnCdSe spin detector induced by spin injection from the ZnMnSe spin injector persists over a much longer time scale than the lifetime of the ZnMnSe excitons. This finding provides compelling experimental evidence that the dominant mechanism for the observed spin injection in the ZnMnSe/ZnCdSe structures should not be due to injection of the excitonic spins of the diluted magnetic semiconductor (DMS). It is rather due to e.g. a delayed spin injection arising from tunneling of individual carriers or/and trapped spins in ZnMnSe.