Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.

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Optical emission from a charge-tunable quantum ring

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field

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Semiconductor Spintronics

A magnetic field can lift the spin degeneracy of electrons. This Zeeman effect is an important route to generating the spin polarization required for spintronics. It is now shown that such polarization can also be achieved without the need for magnetism. The unique crystal symmetry of tungsten selenide creates a Zeeman-like effect when a monolayer of the material is exposed to an external electric field.

Zeeman-type spin splitting controlled by an electric field

A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin–orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing1 and spintronics2,3, forming the basis of various device proposals4,5,6,7,8. For example, the concept of spin field-effect transistors4,5 is based on spin precession due to the spin–orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to ∼30 MHz.

http://www.nanoclub.rwth-aachen.de/Kato_Nature_2004_.pdf

Coherent spin manipulation without magnetic fields in strained semiconductors

The results of magnetoconductivity measurements in ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells are presented. The observed magnetoconductivity appears due to the quantum interference, which lead to the weak localization effect. It is established that the details of the weak localization are controlled by the spin splitting of electron spectra. A theory is developed that takes into account both linear and cubic in electron wave-vector terms in spin splitting, which arise due to the lack of inversion center in the crystal, as well as the linear terms that appear when the well itself is asymmetric. It is established that, unlike spin-relaxation rate, contributions of different terms into magnetoconductivity are not additive. It is demonstrated that in the interval of electron densities under investigation [(0.98-1.85)\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ ] all three contributions are comparable and have to be taken into account to achieve a good agreement between the theory and experiment. The results obtained from comparison of the experiment and the theory have allowed us to determine what mechanisms dominate the spin-relaxation in quantum wells and to improve the accuracy of determination of spin-splitting parameters in ${\mathit{A}}_{3}$${\mathit{B}}_{5}$ crystals and two-dimensional structures. \textcopyright{} 1996 The American Physical Society.

Weak antilocalization and spin precession in quantum wells.

The quantum interference corrections to the conductivity are calculated for an electron gas in asymmetric quantum wells in a magnetic field. The theory takes into account two different types of the spin splitting of the conduction band: the Dresselhaus terms, both linear and cubic in the wave vector, and the Rashba term, linear in wave vector. It is shown that the contributions of these terms into magnetoconductivity are not additive, as it was traditionally assumed. While the contributions of all terms of the conduction-band splitting into the D'yakonov-Perel' spin relaxation rate are additive, in the conductivity the two linear terms cancel each other, and, when they are equal, in the absence of the cubic terms the conduction-band spin splitting does not show up in the magnetoconductivity at all. The theory agrees well with experimental results and enables one to determine experimentally parameters of the spin-orbit splitting of the conduction band.

Conduction-band spin splitting and negative magnetoresistance in A3B5 heterostructures

Sequential single-electron charging is observed in InAs self-assembled quantum dots using capacitance spectroscopy. In this system, the Coulomb energy is smaller than the interlevel energy spacings due to the quantum confinement and both effects can be separately identified. A theoretical model is proposed for this system and the capacitance experiments were devised in order to experimentally observe the effects of Coulomb interaction between electrons on the dots. The effects of inter- and intradot Coulomb interaction have been observed in the capacitance spectra. A good agreement between the proposed model and experiment is achieved.

Single-electron charging and Coulomb interaction in InAs self-assembled quantum dot arrays

We investigate both theoretically and experimentally the spin-orbit effects on the weak localization in a (110) GaAs two-dimensional electron gas. We analyze the role of two different terms in the spin splitting of the conduction band: the Dresselhaus terms, which arise due to the lack of inversion center in the bulk GaAs, and the Rashba terms, which are caused by the asymmetry of the quantum well. It is shown that in ${\mathrm{A}}_{3}$${\mathrm{B}}_{5}$ quantum wells the magnetoresistance due to the weak localization depends qualitatively on the orientation of the well. In particular, it is demonstrated that the (110) geometry has a distinctive feature that in the absence of the Rashba terms the ``antilocalization'' effect, i.e., the positive magnetoresistance, does not exist. Calculation of the weak antilocalization magnetoresistance is found to be in excellent agreement with experiments.

Spin splitting and weak localization in (110) GaAs/ Al x Ga 1 − x As quantum wells

The screening behavior of a two-dimensional electron gas (2DEG) subjected to a periodic lattice of scattering centers or «antidots» is investigated. We have obtained an exact solution for the nonlinear screening problem of a 2DEG containing a single antidot and we have worked out a Monte Carlo simulation method to solve the case of a periodic antidot lattice. We also provide general theoretical predictions concerning the depletion phenomenon in this system. The shrinkage of the effective antidot size with increasing carrier density of decreasing periodicity of the lattice has been calculated and compared with experimental results

F. G. Pikus

Papers

Weak antilocalization and spin precession in quantum wells.

Conduction-band spin splitting and negative magnetoresistance in A3B5 heterostructures

Single-electron charging and Coulomb interaction in InAs self-assembled quantum dot arrays

Spin splitting and weak localization in (110) GaAs/ Al x Ga 1 − x As quantum wells

Effective size of scattering centers in a two-dimensional electron gas.