Strong tuning of Rashba spin-orbit interaction in single InAs nanowires

A key concept in the emerging field of spintronics is the gate voltage or electric field control of spin precession via the effective magnetic field generated by the Rashba spin-orbit interaction. Here, we demonstrate the generation and tuning of electric field induced Rashba spin-orbit interaction in InAs nanowires where a strong electric field is created by either a double gate or a solid electrolyte surrounding gate. In particular, the electrolyte gating enables 6-fold tuning of Rashba coefficient and nearly 3 orders of magnitude tuning of spin relaxation time within only 1 V of gate bias. Such a dramatic tuning of spin-orbit interaction in nanowires may have implications in nanowire-based spintronic devices.     

Influence of Barrier Material on Spin Splitting Due to Inversion Asymmetry in Heterostructures

Influence of barrier material on the spin splitting of conduction subbands in heterostructures because of structure inversion asymmetry (Bychkov–Rashba splitting) is studied. The spin splitting at a vanishing magnetic field is calculated for two heterostructures: InAs/SiO₂ and InAs/In_0.8Al_0.2As, having the same well material InAs but very different barrier materials. It is demonstrated that the barrier material strongly influences the spin splitting of the ground conduction subband in InAs. The spin splittings for both heterostructures are computed as functions of electron density, we obtain the splitting in InAs/SiO₂ almost twice larger than that in InAs/In_0.8Al_0.2As. The influence of spin-dependent part of the boundary conditions on the spin spin splitting is studied and it is shown that for considered heterostructures it changes the splitting up to 25% of its value. It is emphasized that the Bychkov–Rashba spin splitting is not proportional to the average electric field in heterostructure.     

Rashba effect in strained InGaAs/InP quantum wire structures

We investigated the effect of the Rashba spin–orbit coupling in two-dimensional electron gases and quasi one-dimensional wire structures based on a strained InGaAs/InP heterostructure. For the two-dimensional electron gas structure it is demonstrated that the Rashba effect can be controlled by using a gate electrode. By a detailed discussion it is shown that our heterostructure can be employed for a spin transistor based on the Rashba effect [Appl. Phys. Lett. 56 (1990) 665]. The Rashba effect in quantum wire structures is studied by means of magnetotransport measurements. As for the two-dimensional case characteristic beating patterns were found for wire structures having a width down to 600 nm. Our results clearly show that Rashba spin–orbit coupling can directly be observed in quasi one-dimensional structures.     

Enhancement of Rashba Spin–Orbit Interaction Due to Wave Function Engineering

We have demonstrated an enhancement of Rashba spin–orbit interaction (SOI) in In_0.53Ga_0.47As/In_0.7Ga_0.3As/In_0.53Ga_0.47As double-step structure in comparison with In_0.53Ga_0.47As normal quantum well. In the double-step structure, high electron probability density is located on the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface to enhance the interface contribution of Rashba SOI. The double-step structure is designed based on k⋅p formalism considering field and interface contributions separately. The Rashba parameter α calculated by the k⋅p formalism shows good agreement with the experimental value by analyzing weak antilocalization. The large carrier density dependence of α is due to the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface contribution as well as the energy-band bending in the In_0.7Ga_0.3As quantum well. The results of this study suggest that the precise control of interface and field contributions in Rashba SOI will make its application to semiconductor spintronics.     

InAs Nanowire Devices with Strong Gate Tunability:Fundamental Electron Transport Properties and Application Prospects:A Review

The high electron mobility has granted indium arsenide(InAs) nanowires(NWs) as an important class of nanomaterials for high performance electronics such as field-effect transistors(FETs).We reviewed recent progresses on the studies of quantum coherence,gate tunable one-dimensional(1D) confinement and spin orbit interaction(SOI) in InAs NW based electronic and thermoelectric transport devices.We also demonstrated gas sensing response of InAs NW FETs and elucidated the mechanism via a gating experiment.By using InAs NWs as an example,these fundamental transport studies have shed important lights on the potential thermoelectric,spintronic and gas sensing applications of semiconductor NWs where the 1D confinement,SOI or surface states effects are exploited.     

Quantum transport analysis and narrow-gap heterojunction growth for Rashba-type spintronics devices

Research results of spintronics based on spin-orbit (SO) interaction in non-magnetic semiconductor hetero-junctions obtained recently have been described. Works are based on the two-dimensional electron gases (2DEGs) confined at compound semiconductor narrow band-gap hetero-interface. Due to the electric field originated from the confining potential asymmetry, the 2DEG often yields strong SO interaction which could reveal under no magnetic field. This type of SO interaction (Rashba interaction) can be controlled by the applied gate voltage and hence the field effect transistor (FET) utilizing this principle has so far been proposed and discussed extensively. We describe two recent results in this paper: First is molecular beam epitaxy (MBE) growth of novel narrow-gap modulation-doped heterojunction, InGaSb/InAlSb material system which possibly reveals high quality electronic properties as well as very strong Rashba SO coupling. Recently we indeed obtained the sample with a very large SO coupling constant of ~40×10⁻¹² eVm which is almost comparable to the best value obtained in the former InGaAs/InAlAs systems. Second is relating to the control of Rashba SO interaction in long wires with side gates. As a result of careful analysis about the dependencies of the SO coupling constant on the gate voltage, we confirmed the side-gate control of the Rashba effect for the first time, which could be a promising result to develop the spin-FET based quantum-bit devices.     

Spin-Dephasing Anisotropy for Electrons in a Diffusive Quasi-1D GaAs Wire

We present a numerical study of dephasing of electron spin ensembles in a diffusive quasi-one-dimensional GaAs wire due to the D’yakonov–Perel’ spin-dephasing mechanism. For widths of the wire below the spin precession length and for equal strength of Rashba and linear Dresselhaus spin–orbit fields a strong suppression of spin-dephasing is found. This suppression of spin-dephasing shows a strong dependence on the wire orientation with respect to the crystal lattice. The relevance for realistic cases is evaluated by studying how this effect degrades for deviating strength of Rashba and linear Dresselhaus fields, and with the inclusion of the cubic Dresselhaus term.     

Rashba Precession in Quantum Wires

The length over which electron spins reverse direction due to the Rashba effect when injected with an initial polarization along the axes of a quantum wire is investigated theoretically. A soft wall confinement of the wire renormalizes the spin–orbit parameter (and the effective mass) stronger than hard walls. Electron–electron interactions enhance the Rashba precession while evidence is found that the coupling between transport channels may suppress it.     

Rashba Effect in Functional Spintronic Devices

Exploiting spin transport increases the functionality of electronic devices and enables such devices to overcome physical limitations related to speed and power. Utilizing the Rashba effect at the interface of heterostructures provides promising opportunities toward the development of high-performance devices because it enables electrical control of the spin information. Herein, the focus is mainly on progress related to the two most compelling devices that exploit the Rashba effect: spin transistors and spin–orbit torque devices. For spin field-effect transistors, the gate-voltage manipulation of the Rashba effect and subsequent control of the spin precession are discussed, including for all-electric spin field-effect transistors. For spin–orbit torque devices, recent theories and experiments on interface-generated spin current are discussed. The future directions of manipulating the Rashba effect to realize fully integrated spin logic and memory devices are also discussed.     

Rashba Spin–Orbit Interaction and Its Applications to Spin-Interference Effect and Spin-Filter Device

The Rashba spin–orbit interaction in InGaAs quantum wells (QW) is studied using the weak antilocalization analysis as a function of the structural inversion asymmetry (SIA). We have observed a clear cross-over from positive to negative magnetoresistance near zero-magnetic field by controlling the degree of the SIA in the QWs. This is a strong evidence of a zero-field spin splitting that is induced by the Rashba effect. The spin-interference effect in a gate-controlled mesoscopic Aharonov–Bohm ring structure is investigated in the presence of Rashba spin–orbit interaction. The oscillatory behavior appearing in ensemble averaged Fourier spectrum of h/2e oscillations as a function of gate voltage is possibly because of the Aharonov–Casher type interference. We propose a spin-filter device based on the Rashba effect using a nonmagnetic resonant tunneling diode structure. Detailed calculation using InAIAs/InGaAs heterostructures shows that the spin-filtering efficiency exceeds 99.9%.     

Ten Years of Spin Hall Effect

In this short review I survey the theory of the spin Hall effect in doped semiconductors and metals in the light of recent experiments on both kinds of materials. After a brief introduction to different types of spin–orbit coupling in solids, I describe in detail the three conceptually distinct mechanisms that are known to contribute to the spin Hall effect, namely “skew-scattering”, “side-jump”, and “intrinsic mechanism”. The skew-scattering mechanism is shown to be dominant in certain clean two-dimensional semiconductors in which one component of the spin is conserved. In such systems the side-jump mechanism is sub-dominant, but universal in form, and can become dominant if the electron mobility is reduced by changing the temperature. Both skew-scattering and side-jump contributions are generally reduced by spin precession, and skew-scattering is completely suppressed in the linear Rashba model in the absence of magnetic field. Different models of spin–orbit coupling can, however, sustain an intrinsic spin Hall effect. A brief summary of the present experimental situation concludes the review.     

Switching voltage, dynamic power dissipation and on-to-off conductance ratio of a spin field effect transistor

The metal-insulator-semiconductor (MIS) and high electron mobility transistor (HEMT) implementations of the spin field effect transistor (SpinFET) proposed by Datta and Das are considered. In both configurations, the SpinFET's switching voltage (for switching on or off) and power dissipation are found to be larger than those of the traditional MISFET or HEMT if the channel length is les 90 nm. This is a consequence of the fact that spin orbit interaction strengths in semiconductors are too weak to impart any significant advantage to the SpinFET. The issue of non-ideal spin injection and detection at the source and drain contacts is also considered. The SpinFET's on-to-off conductance ratio rapidly degrades with decreasing spin injection/detection efficiency, dropping from infinity (for a one-dimensional channel) to as low as ~9.5, if the spin injection/detection efficiency drops from 100% to 90%. The transconductance has a quadratic dependence on the spin injection efficiency. These analyses are valid at arbitrary temperatures.     

Tunable spin-orbit interaction in trilayer graphene exemplified in electric-double-layer transistors

Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin-orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L(φ) and spin relaxation length L(so) of carriers can be effectively modulated with gate bias. As a direct result, SOI-induced weak antilocalization (WAL), together with a crossover from WAL to weak localization (WL), was observed at near-zero magnetic field. Interestingly, among existing localization models, only the Iordanskii-Lyanda-Geller-Pikus theory can successfully reproduce the obtained magnetoconductance well, serving as evidence for gate tuning of the weak but distinct SOI in graphene. Realization of SOI and its large tunability in the trilayer graphene EDLTs provides us with a possibility to electrically manipulate spin precession in graphene systems without ferromagnetics.     

Electric field control of spin rotation in bilayer graphene

The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin field-effect transistors. An alternative solution can be found in magnetic doping of graphene or, as discussed in the present work, in exploiting the proximity effect between graphene and ferromagnetic oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction, that acts as an effective Zeeman field for electrons in graphene, inducing a spin precession around the magnetization axis of the FO. Here we show that in an appropriately designed double-gate field-effect transistor, with a bilayer graphene channel and FO used as a gate dielectric, spin-precession of carriers can be turned ON and OFF with the application of a differential voltage to the gates. This feature is directly probed in the spin-resolved conductance of the bilayer.     

Giant Rashba-type spin splitting in bulk BiTeI

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.     

Probing Entanglement via Rashba-Induced Shot Noise Oscillations

We have recently calculated shot noise for entangled and spin-polarized electrons in novel beam-splitter geometries with a local Rashba orbit–spin (s-o) interaction in the incoming leads. This interaction allows for a gate-controlled rotation of the incoming electron spins. Here we present an alternate simpler route to the shot noise calculation in the above work and focus on only electron pairs. Shot noise for these shows continuous bunching and antibunching behaviors. In addition, entangled and unentangled triplets yield distinctive shot noise oscillations. Besides allowing for a direct way to identify triplet and singlet states, these oscillations can be used to extract s-o coupling constants through noise measurements. Incoming leads with spin–orbit interband mixing give rise to an additional modulation of the current noise. This extra rotation allows the design of a spin transistor with enhanced spin control.     

Improved subthreshold slope in an InAs nanowire heterostructure field-effect transistor

An n-type InAs/InAsP heterostructure nanowire field-effect transistor has been fabricated and compared with a homogeneous InAs field-effect transistor. For the same device geometry, by introduction of the heterostructure, the threshold voltage is shifted 4 V, the maximum current on-off ratio is enhanced by a factor of 10,000, and the subthreshold swing is lowered by a factor 4 compared to the homogeneous transistor. At the same time, the drive current remains constant for a fixed gate overdrive. A single nanowire heterostructure transistor has a transconductance of 5 muA/V at a low source-drain voltage of 0.3 V. For the homogeneous InAs transistor, we deduced a high electron mobility of 1500 cm2/Vs.     

Electrically tuned spin-orbit interaction in an InAs self-assembled quantum dot

Electrical control over electron spin is a prerequisite for spintronics spin-based quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spin-orbit interaction in two-dimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spin-orbit interaction in the range 50-150 μeV while maintaining the electron occupation of a single self-assembled InAs quantum dot. We determine the spin-orbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.     

Comparison of Performance of n- and p-Type Spin Transistors With Conventional Transistors

A spintronic device that has stimulated much research interest is the Datta–Das spin transistor. The mechanism behind it called the Rashba effect is that an applied voltage gives rise to a spin splitting. We propose ways to optimize this effect. The relevant spin splitting in k-space is predicted to increase with electric field at a rate that is more than two orders of magnitude larger for holes than for electrons. Furthermore, the almost negligible lattice-mismatch between GaAs and AlGaAs can be used to further enhance the advantage of hole-based spin transistors. Compared to present transistors we conclude that electron-based spin transistors will have problems to become competitive but hole-based ones are much more promising.

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The Role of the Spin–Orbit Couplings and Optical Phonons on the Anisotropic Resistivity

We have investigated the influence of the electron–phonon interaction on magnetoelectric properties and spin-related transport effects of a two dimensional electron gas in the presence of the spin–orbit couplings. We have employed a semiclassical method that has been extended to include the anisotropic effects of band structure in the presence of the spin–orbit couplings. We found that resistivity and anisotropic resistance (AR) can be controlled by Rashba spin–orbit coupling.