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

Photonic crystals are materials patterned with a periodicity in dielectric constant, which can create a range of 'forbidden' frequencies called a photonic bandgap. Photons with energies lying in the bandgap cannot propagate through the medium. This provides the opportunity to shape and mould the flow of light for photonic information technology.

Photonic crystals: putting a new twist on light

We describe a fully-vectorial, three-dimensional algorithm to compute the definite-frequency eigenstates of Maxwell's equations in arbitrary periodic dielectric structures, including systems with anisotropy (birefringence) or magnetic materials, using preconditioned block-iterative eigensolvers in a planewave basis. Favorable scaling with the system size and the number of computed bands is exhibited. We propose a new effective dielectric tensor for anisotropic structures, and demonstrate that O Delta x;2 convergence can be achieved even in systems with sharp material discontinuities. We show how it is possible to solve for interior eigenvalues, such as localized defect modes, without computing the many underlying eigenstates. Preconditioned conjugate-gradient Rayleigh-quotient minimization is compared with the Davidson method for eigensolution, and a number of iteration variants and preconditioners are characterized. Our implementation is freely available on the Web.

Block-iterative frequency-domain methods for Maxwell’s equations in a planewave basis

A fundamentally different type of optical waveguide structure is demonstrated, in which light is confined to the vicinity of a low-index region by a two-dimensional photonic band gap crystal. The waveguide consists of an extra air hole in an otherwise regular honeycomb pattern of holes running down the length of a fine silica glass fiber. Optical fibers based on this waveguide mechanism support guided modes with extraordinary properties.

https://science.sciencemag.org/content/sci/282/5393/1476.full.pdf

Photonic Band Gap Guidance in Optical Fibers

Size dependence of nanostructures: Impact of bond order deficiency

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.

Hexagonal photonic-band-gap structures

We present a design of two-dimensional periodic dielectric structures that gives rise to absolute band gaps common to E and H polarized waves. This structure is formed by an arrangement of parallel cylindrical rods centered at the vertices of regular hexagons. We show that two absolute band gaps exist for lattices of dielectric rods in air. Potential application of this structure at the submicrometer length scale is discussed.

Photonic band gaps in a two-dimensional graphite structure.

A tight-binding model is used to investigate confinement and shape effects on the optical absorption spectrum of CdSe nanocrystals up to 5 nm diam. The effects of size dispersion are studied and we show that to simulate the spectrum of a collection of nanocrystals it is not always sufficient to broaden inhomogeneously the mean diameter cluster spectrum. Our results are compared with calculations based on other methods and show good agreement with experimental data. We have also calculated the absorption spectra of CdSe crystallites with different morphologies: spherical clusters and oblate and prolate elliptic nanocrystals. We have performed a comparison of the energies and absorption bands as a function of the morphological changes and we present a detailed study of the modifications in the features of the optical spectra.

Confinement and shape effects on the optical spectra of small CdSe nanocrystals

The concept of generalized Wannier functions, adopted from the electronic theory of solids, is used to build a localized representation of electromagnetic waves in dielectric materials. For two-dimensional photonic crystals, we demonstrate the existence of such a localized state basis, and we establish an efficient computational method, allowing a tight-binding-like parameter free modelization of any dielectric structure deviating from periodicity. Numerical simulations of a T-shaped photonic crystal waveguide prove its ability to deal with large-scale systems and complex structures.

D. Bertho

Papers

Weak antilocalization and spin precession in quantum wells.

Hexagonal photonic-band-gap structures

Photonic band gaps in a two-dimensional graphite structure.

Confinement and shape effects on the optical spectra of small CdSe nanocrystals

Generalized Wannier function method for photonic crystals