Electric Field Induced Suppression of the Magnetoresistance of Dilute Magnetic Semiconductor Trilayers

In semiconductor spintronic devices, the semiconductor is usually lightly doped and nondegenerate, and moderate electric fields can dominate the carrier motion. We recently derived a drift-diffusion equation for spin polarization in the semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics and identified a high-field diffusive regime that has no analog in metals. Here high fields are argued to substantially reduce the magnetoresistance observable in a recent experiment on magnetic-semiconductor–nonmagnetic-semiconductor–magnetic-semiconductor trilayers.     

Spin Injection: A Survey and Review

The study of the transport and relaxation of spin-polarized carriers in the solid state began about 30 years ago. Tunneling spectroscopy was applied to ferromagnet–insulator–superconductor junctions to demonstrate the polarization of interfacial currents. The use of a ferromagnetic material as an injector and/or detector of polarized carriers has since become a valuable tool, and spin injection has been applied to nonmagnetic metals, superconductors, and semiconductors. The spin injection phenomenology is reviewed in the context of two topics of continuing importance for basic and applied research: (i) the transmission of polarized carriers across ferromagnet/nonmagnetic material interfaces and (ii) carrier spin relaxation inside the nonmagnetic material.     

Spin Relaxation in a Two-Dimensional Electron Gas in Si Quantum Wells

Electron spin resonance of two-dimensional (2D) electron gas in Si/SiGe quantum wells allows to evaluate both the longitudinal and the dephasing spin relaxation time. Diakonov–Perel (DP) relaxation, caused by Bychkov–Rashba (BR) spin orbit coupling, occurs to be the dominant mechanism in high mobility samples. For low mobility the Elliott–Yaffet mechanism dominates the longitudinal spin relaxation. When the BR effect is small, inhomogeneous broadening caused by potential fluctuations is seen. We compare spin relaxation of the 2D electron gas in Si and in GaAs quantum wells with respect to applications of these materials in spintronics.     

Exciton Spin Manipulation in ZnMnSe-Based Structures

Strong effect of structural design on spin functionality is observed in quantum structures based on II–VI semiconductors. Spin switching is realized when using a thin layer of Zn^0.95Mn^0.05Se diluted magnetic semiconductor (DMS) as a spin manipulator. This is evident from the polarization of photoluminescence related to a spin detector (an adjacent nonmagnetic quantum well (QW)) measured under the resonant excitation of the spin-up and spin-down states of the DMS, which is identical in value but opposite in sign. The achieved spin switching is suggested to reflect fast carrier diffusion from the DMS due to the absence of an energy barrier between the upper spin state of the DMS layer and the QW. On the other hand, the spin alignment is accomplished in the tunneling structures where the presence of the energy barrier inserted between a spin manipulator (i.e., a ZnMnSe/CdSe DMS superlattice) and a spin detector ensures a slow escape rate from the DMS layer.     

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.     

Spin Rabi flopping in the photocurrent of a polymer light-emitting diode

Electron spin is fundamental in electrical and optical properties of organic electronic devices. Despite recent interest in spin mixing and spin transport in organic semiconductors, the actual spin coherence times in these materials have remained elusive. Measurements of spin coherence provide impartial insight into spin relaxation mechanisms, which is significant in view of recent models of spin-dependent transport and recombination involving high levels of spin mixing. We demonstrate coherent manipulation of spins in an organic light-emitting diode (OLED), using nanosecond pulsed electrically detected electron spin resonance to drive singlet-triplet spin Rabi oscillations. By measuring the change in photovoltaic response due to spin-dependent recombination, we demonstrate spin control of electronic transport and thus directly observe spin coherence over 0.5 s. This surprisingly slow spin dephasing underlines that spin mixing is not responsible for magnetoresistance in OLEDs. The long coherence times and the spin manipulation demonstrated are crucially important for expanding the impact of organic spintronics.     

Spin-Polarized Bipolar Transport and Its Applications

In spin-polarized bipolar transport both electrons and holes in doped semiconductors contribute to spin–charge coupling. The current conversion between the minority (as referred to carriers and not spin) and majority carriers leads to novel spintronic schemes if nonequilibrium spin is present. Most striking phenomena occur inhomogeneously doped magnetic p–n junctions, where the presence of nonequilibrium spin at the depletion layer leads to the spin-voltaic effect: Electric current flows without external bias, powered only by spin. The spin-voltaic effect manifests itself in giant magnetoresistance of magnetic p–n junctions, where the relative change of the magnitude of electric current upon reversing magnetic field can be more than 1000%. The paper reviews nonmagnetic and magnetic spin-polarized p–n junctions, formulates the essentials of spin-polarized bipolar transport as carrier recombination and spin relaxation limited drift and diffusion, and discusses specific device schemes of spin-polarized solar cells and magnetic diodes.     

Intrinsic Decay of Spin Currents: The Spin Coulomb Drag Effect

We review the properties of spin Coulomb drag, which describes the effects of the friction arising between different spin-polarized carrier populations when they travel with different average velocities. We compare this effect with the ordinary Coulomb drag between separate slabs underlining some important differences related to the form of the Coulomb interaction in the two cases. We show that the spin-transresistivity, a measure of the spin Coulomb drag effect, can become as high as 10^–2–10^–3 cm in three dimensions and of the order of several kiloohms in two dimensions. We finally underline that, in some realistic systems, the spin transresistivity can become comparable to the usual Drude resistivity.     

D''yakonov–Perel'' Spin Relaxation Controlled by Electron–Electron Scattering

The D'yakonov–Perel' mechanism of spin relaxation is connected with the spin splitting of the electron dispersion curve in crystals lacking a center of symmetry. In a two-dimensional noncentrosymmetric system, e.g. quantum well or heterojunction, the spin splitting is a linear function of k, at least for small values of k. We demonstrate that the spin relaxation time _s due to the spin splitting is controlled not only by momentum relaxation processes as widely accepted but also by electron–electron collisions which have no effect on the electron mobility. In order to calculate the time _s taking into account the electron–electron scattering, we have solved the two-dimensional kinetic equation for the electron spin density matrix. The result has been compared with that obtained assuming the momentum scattering to occur due to elastic scattering of electrons by ionized impurities. We have also extended the quasi-elastic approximation to describe the electron–electron collision integral for a spin-polarized three-dimensional electron gas.     

Filtering Spin with Tunnel-Coupled Hole Quantum Wires

Creating spin-polarized currents in nonmagnetic semiconductors is one of the key prerequisites for realizing spintronics devices. We have shown previously that the k-linear Rashba spin splitting present in two-dimensional (2D) electron systems can be utilized in a momentum-selective tunneling geometry to design a spin filter without using magnetic fields or ferromagnetic contacts. Motivated by the fact that spin–orbit effects are typically much stronger in 2D hole systems, we consider quantum wires formed by additional confinement of the lowest (heavy-hole) 2D valence subband. Its k ³-type Rashba term gives rise to a k-linear spin splitting for holes in the quantum wire. Implementation of the spin-filter design is then analogous to the electron case but, in the hole system, requires less momentum selectivity and should therefore be easier to realize.     

Hanle Effect Measurements of Spin Lifetime in Zn0.4Cd0.6Se Epilayers Grown on InP

We use the Hanle effect to study spin relaxation in Zn_xCd_1–xSe epilayers grown on lattice-matched InP substrates. We study three samples with a fixed composition (x = 0.4) and with varying levels of n-doping, as well as an undoped sample with x = 0.5. Our measurements show that the spin relaxation time changes non-monotonically as a function of carrier density, with a maximum transverse spin lifetime of ~10.5 ns at low temperatures for a sample doped near the metal–insulator transition.     

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.     

The Mechanism of High-Tc Superconductivity: “Nonlinear” Superconductivity

The main purpose of the paper is to present an overview of the current situation in the development of the understanding of the mechanism of high-T _c superconductivity which arises due to moderately strong, nonlinear electron–phonon interactions and due to spin fluctuations. The former are responsible for electron pairing, and the latter mediate the phase coherence. In addition, a key experiment for superconductivity in cuprates is proposed.     

Transport and Optical Properties of Diluted Magnetic Semiconductors

We investigated the transport and optical properties of diluted magnetic semiconductors theoretically by using a simple model where carriers move in a single band. In this model the carrier feels a nonmagnetic potential at a magnetic impurity site, and its spin interacts with the localized spins of the magnetic impurities through exchange interactions. The electronic states of a carrier were calculated by using the coherent potential approximation (CPA). The magnetism was investigated by minimizing the free-energy and the electrical conductivity was calculated by using the Kubo formula. We examined the results in several typical cases which correspond to (Ga_1–xMn_x)As with x = 0.05.     

Analysis of Response Functions of a Spin-Polarized Electron Gas

We present a spin analysis of local-field corrections and various susceptibility functions of a spin-polarized electron gas (SPEG). With a use of spin-resolved pair correlation functions of the SPEG, including the carrier correlations, we evaluate the local-field correction within a generalized random-phase approximation (RPA) and examine spin-polarization dependences of various susceptibility functions generalized in terms of spin-dependent local-field corrections. A pronounced maximum in spin-resolved local-field correction is observed and the location of the peaks are found to depend strongly on the values of spin polarization. For a system with vanishing spin polarization, the charge–spin mixed susceptibilities vanish. However, in the SPEG of finite spin polarization, the mixed susceptibilities become finite and are as important as the usual charge–charge, and spin–spin susceptibilities.     

Carrier-Density Dependence of Faraday Rotation and Spin Splitting in Cd1?xMnxTe

We employ spin-quantum-beat spectroscopy to investigate the carrier-density dependence of the spin-precession frequency and the magnitude of the Faraday rotation of Cd_1–x Mn _x Te samples at fixed magnetic field. We find an onset of saturation of the Faraday rotation at carrier densities as low as 4× 10¹⁶ cm^–3 and attribute it to electrons (not holes which dominate in other types of experiments). The spin splitting at fixed magnetic field remains density dependent down to 3 × 10¹⁵ cm^–3 (the lowest density accessible in our measurements) which suggests a direct influence of the carrier-density on the sp–d exchange not mediated by screening effects.     

Anisotropic Spin Splitting and Spin Relaxation in AlGaAs/GaAs Quantum Structures

Spin relaxation due to the D'yakonov–Perel' mechanism is intimately related with the spin splitting of the electronic states. We calculate the spin relaxation rates from anisotropic spin splittings of electron subbands in n-(001)-AlGaAs/GaAs quantum structures obtained in a self-consistent multiband approach. The giant anisotropy of spin relaxation rates found for different spin components in the (001) plane can be ascribed to a mutual compensation of terms because of the asymmetry of the bulk crystal and the quantum well structure.     

Charge–Spin Coupling at a Ferromagnet–Nonmagnet Interface

The topic of electrical spin injection from a ferromagnetic to a nonmagnetic material is presently attracting great interest and attention. A thermodynamic study of spin injection across a ferromagnetic–nonmagnetic material interface is presented. Using an entropy production calculation, the linear dynamic equations for interfacial transport of charge, heat, and spin magnetic moment are derived. A general equation for the fractional polarization of injected current is developed by matching boundary conditions at the interface. Polarization efficiency is sensitive to the intrinsic interface resistance, and to the resisivities and spin diffusion lengths of both materials. The physics of nonequilibrium spin diffusion across the interface is discussed, and the limiting case where resistance mismatch is important is identified. Example systems of interest are spin injection from a ferromagnetic metal to a nonmagnetic metal and from a ferromagnetic metal to a semiconductor. Charge–spin coupling and spin diffusion in one dimension, compared with higher dimension, are also discussed.     

Evaluation of charge mobility in organic materials: from localized to delocalized descriptions at a first-principles level

The carrier mobility for carbon electronic materials is an important parameter for optoelectronics. We report here some recently developed theoretical tools to predict the mobility without any free parameters. Carrier scatterings with phonons and traps are the key factors in evaluating the mobility. We consider three major scattering regimes: i) where the molecular internal vibration severely induces charge self-trapping and, thus, the hopping mechanism dominates; ii) where both intermolecular and intramolecular scatterings come to play roles, so the Holstein-Peierls polaron model is applied; and, iii) where charge is well delocalized with coherence length comparable with acoustic phonon wavelength, so that a deformation potential approach is more appropriate. We develop computational methods at the first-principles level for the three different cases that have extensive potential application in rationalizing material design.     

Noise of Spin-Polarized Currents at a Beam Splitter with Local Spin–Orbit Interaction

An electronic beam splitter with a local Rashba spin–orbit coupling can serve as a detector for spin-polarized currents. The spin–orbit coupling plays the role of a tunable spin rotator and can be controlled via a gate electrode on top of the conductor. We use spin-resolved scattering theory to calculate the zero-temperature current fluctuations (shot noise) for such a four-terminal device and show that the shot noise is proportional to the spin polarization of the source. Moreover, we analyze the effect of spin–orbit-induced intersubband coupling, leading to an additional spin rotation.