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.

/pdf/spintronics-fundamentals-and-applications-2dx38n6phu.pdf

Spintronics: Fundamentals and applications

The optical constants of amorphous Ge are determined for the photon energies from 0.08 to 1.6 eV. From 0.08 to 0.5 eV, the absorption is due to k-conserving transitions of holes between the valence bands as in p-type crystals; the spin-orbit splitting is found to be 0.20 and 0.21 eV in non-annealed, and annealed samples respectively. The effective masses of the holes in the three bands are 0.49 m (respectively 0.43 m); 0.04 m, and 0.08 m. An absorption band is observed below the main absorption edge (at 300 °K the maximum of this band is at 0.86 eV); the absorption in this band increases with increasing temperature. This band is considered to be due to excitons bound to neutral acceptors, and these are presumably the same ones that play a decisive role in the transport properties and which are considered to be associated with vacancies. The absorption edge has the form: ω2ϵ2∼(hω−Eg)2 (Eg = 0.88 eV at 300 °K). This suggests that the optical transitions conserve energy but not k vector, and that the densities of states near the band extrema have the same energy-dependence as in crystalline Ge. A simple theory describing this situation is proposed, and comparison of it with the experimental results leads to an estimate of the localization of the conduction-band wavefunctions.

/pdf/optical-properties-and-electronic-structure-of-amorphous-4vuqzbf2cc.pdf

Optical Properties and Electronic Structure of Amorphous Germanium

We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

We show that the quantum spin Hall (QSH) effect, a state of matter with topological properties distinct from those of conventional insulators, can be realized in mercury telluride–cadmium telluride semiconductor quantum wells. When the thickness of the quantum well is varied, the electronic state changes from a normal to an “inverted” type at a critical thickness d c . We show that this transition is a topological quantum phase transition between a conventional insulating phase and a phase exhibiting the QSH effect with a single pair of helical edge states. We also discuss methods for experimental detection of the QSH effect.

Quantum Spin Hall Effect and Topological Phase Transition in HgTe Quantum Wells

Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.

Weyl and Dirac semimetals in three-dimensional solids

Band structure of indium antimonide

The theory of ``direct'' and ``phonon‐assisted'' tunneling is reviewed. Theoretical I–V characteristics are calculated using the constant field model. Generalizations to nonconstant field and more complicated band structure models are discussed briefly.

Theory of Tunneling

Zener tunneling in semiconductors

Energy band structure in p-type germanium and silicon

The density of states in highly impure semiconductors is studied using a semiclassical or Thomas-Fermi type approximation The "local" density of states is assumed proportional to ${(E\ensuremath{-}\mathcal{U})}^{\frac{1}{2}}$, where $\mathcal{U}$ is the local potential The problem then reduces to the calculation of the distribution function for the potential, which is found to be a Gaussian in the high-density limit It is clear that this approach predicts tails on both band edges which are identical except for a multiplying factor of ${({m}^{*})}^{\frac{3}{2}}$, the density-of-states mass The important contributions to the potential variation near the average potential are shown to arise from fluctuations in impurity clusters whose volume is of the order of the cube of the screening length For energies far below the average potential the important cluster sizes are much smaller than the screening length cubed Their size is determined by the kinetic energy of localization, an effect which is not accounted for by the ${(E\ensuremath{-}\mathcal{U})}^{\frac{1}{2}}$ assumption The Thomas-Fermi method involves a number of approximations, all of which are valid in the limit of high density The most serious approximation results from the improper treatment of the kinetic energy of localization which requires ${(n{{a}_{0}}^{*3})}^{\frac{1}{12}}\ensuremath{\gg}1$ for validity Because of this requirement, the method is never highly accurate in any attainable concentration range The effect of potential fluctuations on tunnel diode $I\ensuremath{-}V$ characteristics is also studied The results agree satisfactorily with experimental studies of silicon junctions by Logan et al

Evan O. Kane

Papers

Band structure of indium antimonide

Theory of Tunneling

Zener tunneling in semiconductors

Energy band structure in p-type germanium and silicon

Thomas-Fermi Approach to Impure Semiconductor Band Structure