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

Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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Nanoscale thermal transport

We review recent advances in experimental methods for high spatial-resolution and high time-resolution thermometry, and the application of these and related methods for measurements of thermal transport in low-dimensional structures. Scanning thermal microscopy (SThM) achieves lateral resolutions of 50 nm and a measurement bandwidth of 100 kHz: SThM has been used to characterize differences in energy dissipation in single-wall and multi-wall carbon nanotubes. Picosecond thermoreflectance enables ultrahigh time-resolution in thermal diffusion experiments and characterization of heat flow across interfaces between materials; the thermal conductance G of interfaces between dissimilar materials spans a relatively small range, 20<G<200 MW m -2 K -1 near room temperature. Scanning thermoreflectance microscopy provides nanosecond time resolution and submicron lateral resolution needed for studies of heat transfer in microelectronic, optoelectronic and micromechanical systems. A fully-micromachined solid immersion lens has been demonstrated and achieves thermal-radiation imaging with lateral resolution at far below the diffraction limit, <2 μm. Microfabricated metal bridges using electrical resistance thermometry and joule heating give precise data for thermal conductivity of single crystal films, multilayer thin films, epitaxial superlattices, polycrystalline films, and interlayer dielectrics. The room temperature thermal conductivity of single crystal films of Si is strongly reduced for layer thickness below 100 nm. The through-thickness thermal conductivity of Si-Ge and GaAs-AlAs superlattices has recently been shown to be smaller than the conductivity of the corresponding alloy. The 3ω method has been recently extended to measurements of anisotropic conduction in polyimide and superlattices. Data for carbon nanotubes measured using micromachined and suspended heaters and thermometers indicate a conductivity near room temperature greater than diamond.

https://nanoheat.stanford.edu/sites/default/files/publications/A57.pdf

Thermometry and Thermal Transport in Micro/Nanoscale Solid-State Devices and Structures

The site-specific engineering of colloidal surfaces has provided a powerful approach to pushing the boundaries of today's materials research. The resulting surface-anisotropic and patchy particles have become the center of vital research areas, ranging from the need for large-scale fabrication techniques to exploring new applications of these materials. This Review summarizes patchy particle fabrication techniques, including but not limited to particle and nanosphere lithography and glancing-angle deposition. The variety of existing patchy particle fabrication techniques is revealed and the need for a scalable approach to high-volume patchy particle production is identified. Ongoing modeling efforts describing patchy particle interactions and properties are reviewed as potential predictive tools. Research endeavors that deal with the directed assembly of patchy particles in electric and magnetic fields, as well as with supraparticular assembly through chemical interactions, are discussed. The Review is concluded with a note on the future application of patchy particles as phoretic motors.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/marc.200900614

Fabrication, assembly, and application of patchy particles.

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

We propose a semiconductor structure that can filter electron spin without using external magnetic fields. The structure consists of a T-shaped quasi-one-dimensional channel and an electrode placed near the channel intersection to control the spin-orbit interaction for electrons. Our calculations demonstrate that the proposed device can redirect electrons with opposite spins from an unpolarized source to left and right output leads, respectively, and, thus, serve as a spin filter. When the incident electron energy is in resonance with the quasilocalized zero-dimensional states at the intersection, polarization of the transmitted fluxes approaches 100%.

T-shaped ballistic spin filter

We have investigated spatiotemporal kinetics of electron spin polarization in a semiconductor narrow two-dimensional (2D) strip and explored the ability to manipulate spin relaxation. Information about the conduction electron spin and mechanisms of spin rotation is incorporated into a Monte Carlo transport simulation program. A model problem, involving linear-in-$k$ splitting of the conduction band responsible for the D'yakonov-Perel' mechanism of spin relaxation in the zinc-blende semiconductors and heterostructures, is solved numerically to yield the decay of spin polarization of an electron ensemble in the 2D channel of finite width. For very wide channels, a conventional 2D value of spin relaxation is obtained. With decreasing channel width, the relaxation time increases rapidly by orders of magnitude. Surprisingly, the crossover point between 2D and quasi-1D behavior is found to be at tens of electron mean-free paths. Thus, classically wide channels can effectively suppress electron spin relaxation.

Progressive suppression of spin relaxation in two-dimensional channels of finite width

An entanglement-preserving photodetector converts photon polarisation to electron spin. Up and down spin must respond equally to oppositely polarised photons, creating a requirement for degenerate spin energies. g/sub e//spl sime/0, for electrons. The authors present a plot of g/sub e/ factor against lattice constant, analogous to bandgap against lattice constant, that can be used for g factor engineering of III-V alloys and quantum wells.

Electron g factor engineering in III-V semiconductors for quantum communications

The Zeeman splitting and the underlying $g$ factor for conduction-band electrons in $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells have been measured by spin-beat spectroscopy based on a time-resolved Kerr rotation technique. The experimental data are plotted as functions of the lowest band-to-band optical transition energy, i.e., the effective band gap of the quantum wells. The model calculations suggest that in the tracked range of transition energies $E$ from $1.52\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, the component of electron $g$ factor along the growth axis follows closely the universal dependence ${g}_{\ensuremath{\Vert}}(E)\ensuremath{\approx}0.445+3.38(E\ensuremath{-}1.519)\ensuremath{-}2.21{(E\ensuremath{-}1.519)}^{2}$ (with $E$ given in eV), and this universality also embraces ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ alloys. On the other hand, the in-plane $g$ factor component deviates in a systematic way from this universal curve, with the degree of deviation controlled by the structural anisotropy. The experimental data are in good agreement with the theoretical predictions.

Universal behavior of the electron g factor in Ga As ∕ Al x Ga 1 − x As quantum wells

A planar ballistic T-shaped structure with a ring resonator attached is shown to be highly effective in filtering electron spin from an unpolarized source into two output fluxes with opposite and practically pure spin polarizations. The operability of the proposed device relies on the peculiar spin-dependent transmission properties of the T-shaped connector in the presence of Rashba spin–orbit interaction as well as the difference in dynamic phase gain of the two alternative paths around the ring resonator through upper and lower branches for even and odd eigenmodes.

A. A. Kiselev

Papers

T-shaped ballistic spin filter

Progressive suppression of spin relaxation in two-dimensional channels of finite width

Electron g factor engineering in III-V semiconductors for quantum communications

Universal behavior of the electron g factor in Ga As ∕ Al x Ga 1 − x As quantum wells

T-shaped spin filter with a ring resonator