Conductance and Thermopower of a Quantum Dot with Fano–Rashba Effect

We investigate the conductance and thermopower of a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano–Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We obtain an important enhancement of the thermopower due to the Fano–Rashba effect.     

The Rashba Effect Within the Coherent Scattering Formalism with Applications to Electron Quantum Optics

The influence of spin–orbit coupling in electron quantum optics experiments is investigated within the framework of the Landauer–Büttiker coherent scattering formalism. We begin with a brief review of our electron quantum optics toolbox: an electron intensity interferometer (Hanbury Brown and Twiss-type experiment), an electron collision analyzer (Hong–Ou–Mandel-type experiment), and a proposed Bell state analyzer. These experiments are performed or proposed in two-dimensional electron gas systems and, therefore, may be influenced by the Rashba spin–orbit coupling. To quantify this effect, we define the creation/annihilation operators for the stationary states of the Rashba spin–orbit coupling Hamiltonian and use them to derive the current operator within the Landauer–Büttiker formalism. The current is expressed as it is in the standard spin-independent case, but with the spin label replaced by a new label that we call the spin–orbit coupling label. The spin–orbit coupling effects can then be represented in a scattering matrix that relates the spin–orbit coupling stationary states in different leads. We apply this new formalism to the case of a four-port beamsplitter, and it is shown to mix states with different spin–orbit coupling labels in a manner that depends on the angle between the leads. A noise measurement after the collision of spin-polarized electrons at an electron beamsplitter provides a new experimental means to measure the Rashba parameter . It is also shown that the degree of electron bunching in an entangled-electron collision experiment is reduced by the spin–orbit coupling according the beamsplitter lead angle.     

Spin-Dependent Electron Interferometers

We have studied theoretically the interplay between quantum interference and spin-orbit coupling in electron interferometers realized in semiconductor nanostructures. The correlation between spin quantization axis and propagation direction for electrons with fixed energy results in the appearance of spin-dependent interference fringes. We apply the spin-resolved scattering formalism to calculate transport in electronic versions of Youngs double-slit interferometer and the Mach-Zehnder interferometer. The possibility to tune spin-orbit coupling of the Rashba type using external gate voltages makes it possible to use such interferometer setups for magnet-less spintronics and quantum information processing.     

Spin-Dependent Electron Interferometers

We have studied theoretically the interplay between quantum interference and spin-orbit coupling in electron interferometers realized in semiconductor nanostructures. The correlation between spin quantization axis and propagation direction for electrons with fixed energy results in the appearance of spin-dependent interference fringes. We apply the spin-resolved scattering formalism to calculate transport in electronic versions of Youngs double-slit interferometer and the Mach-Zehnder interferometer. The possibility to tune spin-orbit coupling of the Rashba type using external gate voltages makes it possible to use such interferometer setups for magnet-less spintronics and quantum information processing.     

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.     

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.     

Spin filtering and quantum phase transition in double quantum dots attached to spin-polarized leads

We study the spin filtering and quantum phase transition (QPT) in double quantum dots attached to spin-polarized leads. For spin-independent leads, we observe a Kosterlitz-Thouless transition between the local triplet and doublet. For spin-polarized leads, the above QPT becomes first order, and Kondo splitting, gate-controlled spin reversal and a perfect spin filtering are observed. The breaking of spin-rotation SU(2) symmetry and the interdot transport mediated by the conduction electron are responsible for the fully spin-polarized conductance. Because spin-polarized leads suppress the Kondo effect, in order to obtain a large conductance with perfect spin filtering, one should choose leads with small spin polarization, such as Rashba spin-orbital coupling leads.     

Rashba Effect on the Effective Mass of the Polaron in a Parabolic Quantum Well

We investigate the influence of the Rashba effect on the properties of the polaron in a parabolic quantum well within the improved linear combination operator method. We discuss the relations among the polaron effective mass ratio with the vibrational frequency, the electron–phonon coupling strength, the velocity of the polaron and the Rashba parameter, respectively. Due to Rashba effect, the effective mass ratio occurs splitting. The effective mass ratio is an increase function of the vibrational frequency and the coupling strength. The change tendencies of the spin-up splitting effective mass ratio and the spin-down splitting effective mass ratio with the velocity and the Rashba parameter are opposite.     

Novel quantum interference effects in transport through molecular radicals

We investigate electronic transport through molecular radicals and predict a correlation-induced transmission node arising from destructive interference between transport contributions from different charge states of the molecule. This quantum interference effect has no single-particle analog and cannot be described by effective single-particle theories. Large errors in the thermoelectric properties and nonlinear current-voltage response of molecular radical junctions are introduced when the complementary wave and particle aspects of the electron are not properly treated. A method to accurately calculate the low-energy transport through a radical-based junction using an Anderson model is given.     

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.     

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%.     

Magnetotransport Calculations for Two-Dimensional Electrons with Spin-Orbit Interaction

We present calculations of the magnetoconductivity in a two-dimensional electron system including the Rashba spin-orbit interaction. Essential for these calculations is an extension of the self-consistent Born approximation which takes into account the electron spin degree of freedom. The calculated magnetoconductivity exhibits, besides the beating in the Shubnikov-de Haas oscillations, a modulation related to the spin-orbit induced crossings of Landau levels, as a consequence of spin-conserving scattering between spin-orbit coupled states.     

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.     

Shot noise in resonant tunneling through an interacting quantum dot with intradot spin-flip scattering

We present theoretical investigation of the zero-frequency shot-noise spectra in electron tunneling through an interacting quantum dot connected to two ferromagnetic leads with the possibility of spin-flip scattering between the two spin states by means of the recently developed bias-voltage and temperature-dependent quantum rate equations. For this purpose, a generalization of the traditional generation-recombination approach is made for properly taking into account the coherent superposition of electronic states, i.e., the nondiagonal density matrix elements. Our numerical calculations find that the Fano factor increases with increasing the polarization of the two leads, but decreases with increasing the intradot spin-flip scattering.     

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.     

Quantum-interference-controlled molecular electronics

Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with ab initio electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular pi-states and contact sigma-states. It is masked by sigma tunneling in small molecular rings with Au leads, such as benzene, due to strong pi-sigma hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize quantum interference effect (QIE) transistors.     

Multiscale model of electronic behavior and localization in stretched dry DNA

When the DNA double helix is subjected to external forces it can stretch elastically to elongations reaching 100% of its natural length. These distortions, imposed at the mesoscopic or macroscopic scales, have a dramatic effect on electronic properties at the atomic scale and on electrical transport along DNA. Accordingly, a multiscale approach is necessary to capture the electronic behavior of the stretched DNA helix. To construct such a model, we begin with accurate density-functional-theory calculations for electronic states in DNA bases and base pairs in various relative configurations encountered in the equilibrium and stretched forms. These results are complemented by semi-empirical quantum mechanical calculations for the states of a small size [18 base pair poly(CG)–poly(CG)] dry, neutral DNA sequence, using previously published models for stretched DNA. The calculated electronic states are then used to parametrize an effective tight-binding model that can describe electron hopping in the presence of environmental effects, such as the presence of stray water molecules on the backbone or structural features of the substrate. These effects introduce disorder in the model hamiltonian which leads to electron localization. The localization length is smaller by several orders of magnitude in stretched DNA relative to that in the unstretched structure.     

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.     

Spin Currents and Intrinsic Spin-Hall Effect in Low Dimensional Systems

We consider the spin-Hall effect in two-dimensional electron systems (2DES) with Rashba and Dresselhaus spin-orbit couplings (SO). We find nonzero diagonal spin conductivity provided that Dresselhaus coupling is finite. In addition we consider the influence of disorder on spin conductivities in these systems. By comparing with the exact diagonalization method the finite quasiparticle lifetime (Born) approximation we argue that the latter one seems to be sufficient to describe the case when spin-orbit coupling is stronger or comparable with disorder strength. Furthermore, in the framework of Landauer–Buttiker formalism we show that the transverse and diagonal spin conductivities in Rashba plus Dresselhaus SO systems are robust against disorder for finite size systems.