Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Quantum Hall effect in the field-induced spin density wave states

The Hall conductance is investigated in the field-induced spin density wave phases of the quasi-one-dimensional system. The role of the order parameters in the quantization of the Hall effect is shown in the framework of the topological number of the wave functions in the momentum space. A possible mechanism of the sign changes of the Hall conductance observed in the experiments is proposed in this framework.     

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.     

Channel Width Dependence of Spin Polarized Transports in NiFe/InGaAs Hybrid Two-Terminal Structures

We investigated spin polarized transports in NiFe/InGaAs hybrid two-terminal structures at 1.5K as well as their channel width dependence. The two-terminal structures were fabricated in order to neglect the local Hall effect (LHE) by fringe fields of NiFe contacts. First, we measured magneto-resistance (MR) characteristics of the samples under vertical magnetic fields, and obtained clear oscillations indicating the ohmic formation at NiFe/InGaAs interfaces. Next, we measured spin valve (SV) properties under parallel magnetic fields, and successfully observed clear SV peaks without LHE hysterisis loops. Furthermore, we also confirmed unique behavior of SV peaks depending on the channel width. Such dependence also indicates spin injection/detection through NiFe/InGaAs interfaces.     

Spin Wave Excitation,Detection, and Utilization in the Organic-Based Magnet,V(TCNE)x (TCNE = Tetracyanoethylene)

Spin waves, quantized as magnons, have low energy loss and magnetic damping, which are critical for devices based on spin-wave propagation needed for information processing devices. The organic-based magnet [V(TCNE)_x; TCNE = tetracyanoethylene; x ≈ 2] has shown an extremely low magnetic damping comparable to, for example, yttrium iron garnet (YIG). The excitation, detection, and utilization of coherent and non-coherent spin waves on various modes in V(TCNE)_x is demonstrated and show that the angular momentum carried by microwave-excited coherent spin waves in a V(TCNE)_x film can be transferred into an adjacent Pt layer via spin pumping and detected using the inverse spin Hall effect. The spin pumping efficiency can be tuned by choosing different excited spin wave modes in the V(TCNE)_x film. In addition, it is shown that non-coherent spin waves in a V(TCNE)_x film, excited thermally via the spin Seebeck effect, can also be used as spin pumping source that generates an electrical signal in Pt with a sign change in accordance with the magnetization switching of the V(TCNE)_x. Combining coherent and non-coherent spin wave detection, the spin pumping efficiency can be thermally controlled, and new insight is gained for the spintronic applications of spin wave modes in organic-based magnets.     

Lattice and spin bipolarons in metal oxides and doped fullerenes

We extrapolate the BCS theory to the strong electron-phonon and (or) electron-spin fluctuation interaction and show that in the strong-coupling limit the ground state is a charged Bose liquid of lattice and (or) spin bipolarons. Kinetic and thermodynamic properties of charged bosons on a lattice in the normal and superconducting states are discussed, and some evidence for the model is given from NMR, neutron scattering, near-infrared absorption, Hall effect, resistivity, thermal conductivity, isotope effect, heat capacity, and critical magnetic fields of high-T _c oxides. The maximum attainableT _c is estimated to be in the region of the transition from the Fermi liquid to a charged Bose liquid (polaronic superconductivity). The proposed theory is not restricted by low dimensionality and might be applied to cubic oxides such as the old BaPbBiO and to alkali-doped C₆₀ as well.We thank D. Khmelnitskii, W. Liang, J. Loram, M. Pepper, E. Salje, and J. Wheattey for helpful stimulating discussions, and J. Cooper, A. Carrington, and A. Mackenzie for extensive experimental data and discussion. A. Bratkovsky has been instrumental in elaborating the temperature dependence of the infrared absorption and the electrical conductivity. One of us (A.S.A.) appreciates the financial support from the Leverhulme Trust.     

Excitation and Amplification of Spin Waves by Spin–Orbit Torque

The emerging field of nanomagnonics utilizes high‐frequency waves of magnetization—spin waves—for the transmission and processing of information on the nanoscale. The advent of spin‐transfer torque has spurred significant advances in nanomagnonics, by enabling highly efficient local spin wave generation in magnonic nanodevices. Furthermore, the recent emergence of spin‐orbitronics, which utilizes spin–orbit interaction as the source of spin torque, has provided a unique ability to exert spin torque over spatially extended areas of magnonic structures, enabling enhanced spin wave transmission. Here, it is experimentally demonstrated that these advances can be efficiently combined. The same spin–orbit torque mechanism is utilized for the generation of propagating spin waves, and for the long‐range enhancement of their propagation, in a single integrated nanomagnonic device. The demonstrated system exhibits a controllable directional asymmetry of spin wave emission, which is highly beneficial for applications in nonreciprocal magnonic logic and neuromorphic computing.     

Spin Electronics: From Concentrated to Diluted Magnetic Semiconductors and Beyond

This presentation will review physical properties, relevant to spintronics, of concentrated magnetic semiconductors. Examples from the open literature and current research utilizing thin film EuS will be used as illustrations. New research on potential spin injection materials and spin detection will also be described. This latter work, carried out at MARTECH, the Center for Materials Research and Technology, demonstrates that carriers from the half-metal CrO₂ may be injected across an insulator without loss of (100%) spin polarization. It also features the development of a Hall gradiometer capable of detecting as few as 10⁵ spins.     

Nonequilibrium Magnetic Polarization and Spin Currents Controlled by Gate Voltage in a Ferromagnetic Single-Electron Transistor

Spin-dependent electronic transport in a single-electron transistor with ferromagnetic external electrodes and nonmagnetic central part (island) is analyzed theoretically in the sequential tunneling and cotunneling regimes. Nonequilibrium magnetic polarization of the island due to spin accumulation (spin splitting of the chemical potential) is taken into account. The accumulation takes place when the spin relaxation time on the island is sufficiently long. The crossover from slow to fast spin relaxation limits is also analyzed. Magnetic polarization of the island and spin polarization of the flowing current are examined as a function of the gate and transport voltages.     

Organic Spin‐Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering

Since the first observation of the spin‐valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin‐polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin‐flip mechanisms in organic semiconductors and the role of hybrid metal–organic interfaces in spin injection. Recent findings suggest that organic single crystals can provide spin‐transport media with much less structural disorder relative to organic thin films, thus reducing momentum scattering. Additionally, modification of the band energetics, morphology, and even spin magnetic moment at the metal–organic interface by interface engineering can greatly impact the efficiency of spin‐polarized carrier injection. Here, progress on efficient spin‐polarized carrier injection into organic semiconductors from ferromagnetic metals by using various interface engineering techniques is presented, such as inserting a metallic interlayer, a molecular self‐assembled monolayer (SAM), and a ballistic carrier emitter. In addition, efforts to realize long spin transport in single‐crystalline organic semiconductors are discussed. The focus here is on understanding and maximizing spin‐polarized carrier injection and transport in organic semiconductors and insight is provided for the realization of emerging organic spintronics technologies.     

Precision Tests of a Quantum Hall Effect Device DC Equivalent Circuit Using Double-Series and Triple-Series Connections

Precision tests verify the dc equivalent circuit used by Ricketts and Kemeny to describe a quantum Hall effect device in terms of electrical circuit elements. The tests employ the use of cryogenic current comparators and the double-series and triple-series connection techniques of Delahaye. Verification of the dc equivalent circuit in double-series and triple-series connections is a necessary step in developing the ac quantum Hall effect as an intrinsic standard of resistance.     

Spin drift–diffusion transport and its applications in semiconductors

We study theoretically the propagation and distribution of electron spin density in semiconductors within the drift–diffusion model in an external electric field. From the solution of the spin drift–diffusion equation, we derive the expressions for spin currents in the down-stream (DS) and up-stream (US) directions. We find that drift and diffusion currents contribute to the spin current and there is an electric field, called the drift–diffusion crossover field, where the drift and diffusion mechanisms contribute equally to the spin current in the DS direction, and that the spin current in the US direction vanishes when the electric field is very large. We calculate the drift–diffusion crossover field and show that the intrinsic spin diffusion length in a semiconductor can be determined directly from it if the temperature, electron density and both the temperature and electron density, respectively, are known for nondegenerate, highly degenerate and degenerate systems. The results will be useful in obtaining transport properties of the electron’s spin in semiconductors, the essential information for spintronic technology.     

Novel Aspects of Spin-Polarized Transport and Spin Dynamics

There is a renewed interest to study spin-polarized transport and spin dynamics in various electronic materials. Motivation to examine the spin degrees of freedom (mostly in electrons, but also in holes and nuclei) comes from various sources: ranging from novel applications which are either not feasible or ineffective with conventional electronics, to using the spin-dependent phenomena for exploring fundamental properties of solid-state systems. Taken in a broader context, term spintronics is addressing various aspects of these efforts and stimulating new interactions between different subfields of condensed matter physics. Recent advances in material fabrication made it possible to introduce the nonequilibrium spin in novel class of systems, including ferromagnetic semiconductors, high temperature superconductors, and carbon nanotubes—which leads to a question of how could such a spin be utilized. For this purpose it is important to extend the understanding of spin-polarized transport and spin dynamics to consider inhomogeneous systems, various heterostructures, and the role of interfaces. This article presents some views on novel aspects of spin-polarized transport and spin dynamics (referring also to the topics which were addressed at the conference Spintronics 2001) and suggests possible future research directions.     

Electrons in Magnetic Structures with Domain Walls: Accumulation of Spin, Charge, and Transport Properties

We present the results of our theoretical analysis of magnetic, electric, and transport properties of domain walls in ferromagnets. The results were obtained within the semiclassical approximation and are valid for smooth domain walls. Taking into account Coulomb interaction between electrons, we calculated spin and charge accumulation at the wall. Local conductivity due to scattering from impurities located in the region of the domain wall was also calculated.     

Berry Phase and Topological Spin Transport in the Chiral d-Density Wave State

In this paper we demonstrate the possibility of dissipationless spin transport in the chiral d-density wave state, by the sole application of a uniform Zeeman field gradient. The occurrence of these spontaneous spin currents is attributed to the parity (℘) and time-reversal ( ) violation induced by the density wave order parameter. We calculate the spin Hall conductance and reveal its intimate relation to the Berry phase which is generated when the Zeeman field is applied adiabatically. Finally, we demonstrate that in the zero temperature and doping case, the spin Hall conductance is quantized as it becomes a topological invariant.      

Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to their strong quenching of spin relaxation processes that are promoted by carrier and exciton motions. Unfortunately, the spin injection efficiency in such nanostructures is very low and the exact physical mechanism of the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer that immediately surround the QDs and QMSs. These localized excitons in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We attribute the microscopic origin of the severe spin loss observed during spin injection to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We determined that the AEI of the injected excitons and, thus, the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs. This symmetry largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures. Furthermore, they provide a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in spintronic device applications utilizing semiconductor QDs.

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Charge Expulsion, Spin Meissner Effect, and Charge Inhomogeneity in Superconductors

Superconductivity occurs in systems that have a lot of negative charge: the highly negatively charged (CuO₂)⁼ planes in the cuprates, negatively charged (FeAs)⁻ planes in the iron arsenides, and negatively charged B ⁻ planes in magnesium diboride. And, in the nearly filled (with negative electrons) bands of almost all superconductors, as evidenced by their positive Hall coefficient in the normal state. No explanation for this charge asymmetry is provided by the conventional theory of superconductivity, within which the sign of electric charge plays no role. Instead, the sign of the charge carriers plays a key role in the theory of hole superconductivity, according to which metals become superconducting because they are driven to expel negative charge (electrons) from their interior. This is why NIS tunneling spectra are asymmetric, with larger current for negatively biased samples. The theory also offers a compelling explanation of the Meissner effect: as electrons are expelled towards the surface in the presence of a magnetic field, the Lorentz force imparts them with azimuthal velocity, thus generating the surface Meissner current that screens the interior magnetic field. In type II superconductors, the Lorentz force acting on expelled electrons that do not reach the surface gives rise to the azimuthal velocity of the vortex currents. In the absence of applied magnetic field, expelled electrons still acquire azimuthal velocity, due to the spin–orbit interaction, in opposite direction for spin-up and spin-down electrons: the “Spin Meissner effect.” This results in a macroscopic spin current flowing near the surface of superconductors in the absence of applied fields, of magnitude equal to the critical charge current (in appropriate units). Charge expulsion also gives rise to an interior outward-pointing electric field and to excess negative charge near the surface. In strongly type II superconductors this physics should give rise to charge inhomogeneity and spin currents throughout the interior of the superconductor, to large sensitivity to (non-magnetic) disorder and to a strong tendency to phase separation.     

Hall effect on MHD flow and heat transfer over a stretching sheet with variable thickness

We investigate the MHD flow and heat transfer of an electrically conducting fluid over a stretching sheet with variable thickness. The wall temperature and the wall velocity are assumed to vary. The effects of external magnetic field along the sheet and the Hall currents are considered. The governing equations are solved numerically using an implicit finite difference scheme. The obtained numerical results are compared with the available results in the literature for some special cases and the results are found to be in very good agreement. The effects of the physical parameters on the velocity and temperature fields are presented graphically and analyzed. The effect of the Hall current gives rise to a cross flow. Moreover, the Hall current and the magnetic field have strong effect on the flow and heat transfer characteristics, i.e., shear stress and the Nusselt number.     

Effects of Hall current and two temperatures in transversely isotropic magnetothermoelastic with and without energy dissipation due to ramp-type heat

The present article is concerned with the investigation of disturbances in a homogeneous transversely isotropic thermoelastic rotating medium with two temperatures, in the presence of the combined effects of Hall currents and magnetic field. The formulation is applied to the thermoelasticity theories developed by Green-Naghdi theories of type-II and type-III. Laplace and Fourier transform techniques are applied to solve the problem. The analytical expressions of displacements, stress components, temperature change, and current density components are obtained in the transformed domain. A numerical inversion technique has been applied to obtain the results in the physical domain. Numerical simulated results are depicted graphically to show the effect of Hall current and two temperatures on resulting quantities. Some special cases are also deduced from the present investigation.