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

This book presents the conceptual framework underlying the atomistic theory of matter, emphasizing those aspects that relate to current flow. This includes some of the most advanced concepts of non-equilibrium quantum statistical mechanics. No prior acquaintance with quantum mechanics is assumed. Chapter 1 provides a description of quantum transport in elementary terms accessible to a beginner. The book then works its way from hydrogen to nanostructures, with extensive coverage of current flow. The final chapter summarizes the equations for quantum transport with illustrative examples showing how conductors evolve from the atomic to the ohmic regime as they get larger. Many numerical examples are used to provide concrete illustrations and the corresponding Matlab codes can be downloaded from the web. Videostreamed lectures, keyed to specific sections of the book, are also available through the web. This book is primarily aimed at senior and graduate students.

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Quantum Transport: Atom to Transistor

In 1984, Bychkov and Rashba introduced a simple form of spin-orbit coupling to explain the peculiarities of electron spin resonance in two-dimensional semiconductors. Over the past 30 years, Rashba spin-orbit coupling has inspired a vast number of predictions, discoveries and innovative concepts far beyond semiconductors. The past decade has been particularly creative, with the realizations of manipulating spin orientation by moving electrons in space, controlling electron trajectories using spin as a steering wheel, and the discovery of new topological classes of materials. This progress has reinvigorated the interest of physicists and materials scientists in the development of inversion asymmetric structures, ranging from layered graphene-like materials to cold atoms. This Review discusses relevant recent and ongoing realizations of Rashba physics in condensed matter.

New perspectives for Rashba spin–orbit coupling

Journal of Physics Condensed Matter: Preface

The behaviour of traditional electronic devices can be understood in terms of the classical diffusive motion of electrons. As the size of a device becomes comparable to the electron coherence length, however, quantum interference between electron waves becomes increasingly important, leading to dramatic changes in device properties. This classical-to-quantum transition in device behaviour suggests the possibility for nanometer-sized electronic elements that make use of quantum coherence. Molecular electronic devices are promising candidates for realizing such device elements because the electronic motion in molecules is inherently quantum mechanical and it can be modified by well defined chemistry. Here we describe an example of a coherent molecular electronic device whose behaviour is explicitly dependent on quantum interference between propagating electron waves-a Fabry-Perot electron resonator based on individual single-walled carbon nanotubes with near-perfect ohmic contacts to electrodes. In these devices, the nanotubes act as coherent electron waveguides, with the resonant cavity formed between the two nanotube-electrode interfaces. We use a theoretical model based on the multichannel Landauer-Buttiker formalism to analyse the device characteristics and find that coupling between the two propagating modes of the nanotubes caused by electron scattering at the nanotube-electrode interfaces is important.

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Fabry - Perot interference in a nanotube electron waveguide

Organic semiconductors that are π-conjugated are emerging as an important platform for ‘spintronics’, which purports to harness the spin degree of freedom of a charge carrier to store, process and/or communicate information1. Here, we report the study of an organic nanowire spin valve device, 50 nm in diameter, consisting of a trilayer of ferromagnetic cobalt, an organic, Alq3, and ferromagnetic nickel. The measured spin relaxation time in the organic is found to be exceptionally long—between a few milliseconds and a second—and it is relatively temperature independent up to 100 K. Our experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott–Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.

Observation of extremely long spin relaxation times in an organic nanowire spin valve

The Early History of Spin Spin The Bohr Planetary Model and Space Quantization The Birth of "Spin" The Stern-Gerlach Experiment The Advent of Spintronics The Quantum Mechanics of Spin Pauli Spin Matrices The Pauli Equation and Spinors More on the Pauli Equation Extending the Pauli Equation - the Dirac Equation The Time Independent Dirac Equation Appendix The Bloch Sphere The Spinor and the "Qubit" The Bloch Sphere Concept Evolution of a Spinor Spin-1/2 Particle in a Constant Magnetic Field: Larmor Precession Preparing to Derive the Rabi Formula The Rabi Formula The Density Matrix The Density Matrix Concept: Case of a Pure State Properties of the Density Matrix Pure Versus Mixed State Concept of the Bloch Ball Time Evolution of the Density Matrix: Case of Mixed State The Relaxation Times T1 and T2 and the Bloch Equations Spin Orbit Interaction Spin Orbit Interaction in a Solid Magneto-Electric Sub-Bands in Quantum Confined Structures in the Presence of Spin-Orbit Interaction Dispersion Relations of Spin Resolved Magneto-Electric Subbands and Eigenspinors in a Two-Dimensional Electron Gas in the Presence of Spin-Orbit Interaction Dispersion Relations of Spin Resolved Magneto-Electric Subbands and Eigenspinors in a One-Dimensional Electron Gas in the Presence of Spin-Orbit Interaction Magnetic Field Perpendicular to Wire Axis and the Electric Field Causing Rashba Effect Eigenenergies of Spin Resolved Subbands and Eigenspinors in a Quantum Dot in the Presence of Spin-Orbit Interaction Why Are the Dispersion Relations Important? The Three Types of Hall Effect Spin Relaxation Spin Relaxation Mechanisms Spin relaxation in a quantum dot Is the Effective Magnetic Field due to Spin-Orbit Interaction Proportional to v or k? The Spin Galvanic Effect Exchange Interaction Identical Particles and the Pauli Exclusion Principle Hartree and Hartree-Fock Approximations The Role of Exchange in Ferromagnetism The Heisenberg Hamiltonian Spin Transport in Solids The Drift-Diffusion Model The Semiclassical Model Concluding Remarks Passive Spintronic Devices and Related Concepts Spin Valve Spin Injection Efficiency Hysteresis in Spin Valve Magnetoresistance Giant Magnetoresistance Spin Accumulation Spin Injection Across a Ferromagnet/Metal Interface Spin Injection in a Ferromagnet/Semiconductor/Ferromagnet Spin Valve Spin Extraction at a Ferromagnetic Contact/Semiconductor Interface Hybrid Spintronics Spin based transistors Spin Field Effect Transistors (SPINFET) Device Performance of SPINFETs Power Dissipation Estimates Other Types of SPINFETs The Importance of the Spin Injection Efficiency Transconductance, Gain, Bandwidth and Isolation Spin Bipolar Junction Transistors (SBJT) GMR-based Transistors Concluding Remarks Monolithic Spintronics Monolithic Spintronics Reading and Writing Single Spin Single Spin Logic Energy Dissipation Issues Comparison Between Hybrid and Monolithic Spintronics Concluding Remarks Quantum Computing with Spins The Quantum Inverter Can the NAND Gate Be Switched Without Dissipating Energy? Universal Reversible Gate: The Toffoli-Fredkin Gate A-Matrix Quantum Gates Qubits Superposition States Quantum Parallelism Universal Quantum Gates A 2-Qubit "Spintronic" Universal Quantum Gate Conclusion A Quantum Mechanics Primer Blackbody Radiation and Quantization of Electromagnetic Energy The Concept of the Photon Wave-Particle Duality and the De Broglie Wavelength Postulates of Quantum Mechanics Some Elements of Semiconductor Physics: Particular Applications in Nanostructures The Rayleigh-Ritz Variational Procedure The Transfer Matrix Formalism

Introduction to spintronics

The consideration of space charge in the analysis of resonant tunneling devices leads to a substantial modification of the current‐voltage relationship. The region of negative differential resistance (NDR) is shifted to a higher voltage, and broadened along the voltage axis. Moreover, the peak value of current prior to NDR is reduced, leading to a reduction in the predicted peak‐to‐valley ratio. An approach is presented to include space‐charge effects, and a recently fabricated GaAs‐AlxGa1−xAs structure is analyzed, to underscore the importance of a self‐consistent electrostatic potential in theoretical calculations.

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Importance of space-charge effects in resonant tunneling devices

Experimental evidence is presented showing that strong spin polarization in side-gated quantum point contacts can be achieved electrically, making these structures attractive for future spintronic applications.

All-electric quantum point contact spin-polarizer

The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin-orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin-orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact -- a short wire -- made from a semiconductor with high intrinsic spin-orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices.

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Marc Cahay

Papers

Observation of extremely long spin relaxation times in an organic nanowire spin valve

Introduction to spintronics

Importance of space-charge effects in resonant tunneling devices

All-electric quantum point contact spin-polarizer

All-Electric Quantum Point Contact Spin Polarizer