In semiconductor spintronic devices, the semiconductor is usually lightly doped and nondegenerate, and moderate electric fields can dominate the carrier motion. We recently derived a driftdiffusion equation for spin polarization in the semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics and identified a high-field diffusive regime which has no analogue in metals. Here spin injection from a ferromagnet (FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying this spin drift-diffusion equation to several typical injection structures such as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field regime spin injection from a ferromagnet into a semiconductor is enhanced by several orders of magnitude. For injection structures with interfacial barriers, the electric field further enhances spin injection considerably. In FM/NS/FM structures high electric fields destroy the symmetry between the two magnets at low fields, where both magnets are equally important for spin injection, and spin injection becomes locally determined by the magnet from which carriers flow into the semiconductor. The field-induced spin injection enhancement should also be insensitive to the presence of a highly doped nonmagnetic semiconductor (NS + ) at the FM interface, thus FM/NS + /NS structures should also manifest efficient spin injection at high fields. Furthermore, high fields substantially reduce the magnetoresistance observable in a recent experiment on spin injection from magnetic semiconductors.

We present a model to describe electrical conductivity along the DNA double helix. In this model, DNA is considered as a one-dimensional disordered system, and electrons are transported via variable range hopping between localized states. Thermal structural fluctuations in DNA further localize electronic wave functions, giving rise to a temperature-dependent localization length. The model quantitatively explains the temperature dependence of the conductivity observed in the lambda phage DNA (lambda-DNA).

We derive a drift-diffusion equation for spin polarization in semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics. We identify a high-field diffusive regime which has no analogue in metals. In this regime there are two distinct spin diffusion lengths. Furthermore, spin injection from a ferromagnetic metal into a semiconductor is enhanced by several orders of magnitude and spins can be transported over distances much greater than the low-field spin diffusion length.PACS numbers: 72.25. Dc, 72.20.Ht, 72.25.Hg, 72.25.Mk. Semiconductor devices based on the control and manipulation of electron spin (semiconductor spintronics) have recently attracted considerable attention [1]. Spin transport and injection properties of semiconductors and heterostructures strongly constrain the design of new spintronic devices. In theoretical studies of spin transport and injection in semiconductors [2,3,4] the spin polarization is usually assumed to obey the same diffusion equation as in metals [5],where µ ↑(↓) is the electrochemical potential of up-spin (down-spin) electrons. In this diffusion equation, the electric field does not play any role, and spin polarization decays away on a length scale of L from an injection point. This is reasonable for metals because the electric field E is essentially screened. For semiconductor spintronic devices, however, the semiconductor often is lightly doped and nondegenerate, and moderate electric field can dominate the carrier motion. Equation (1) corresponds to neglecting drift in the more general driftdiffusion equation for the spin polarization,where n ↑ −n ↓ is the difference between up-spin and downspin electron densities and L (s) is the intrinsic spin diffusion length.If Eq. (1) holds, spin injection from a ferromagnetic metal to a semiconductor without a spin-selective interfacial barrier is virtually impossible due to the "conductivity mismatch", or more precisely, a mismatch between effective resistances in the metal (L (f ) /σ f ) and in the semiconductor (are the spin diffusion lengths for the ferromagnetic metal and the semiconductor, and σ f and σ s are conductivities for the two materials. Even for spin injection from ferromagnetic semiconductors,/σ s , and the spin polarization is much less than 99%, so the large spin injection percentages achieved from ZnMnSe [6,7] and GaMnAs [8] are difficult to understand via Eq. (1).Here we clarify the central role of the electric field on spin transport in semiconductors. We obtain the driftdiffusion equation (2) for the spin polarization in a semiconductor. Equation (2) consistently takes into account electric-field effects and nondegenerate electron statistics. We identify a high-field diffusive regime which has no analogue in metals. This regime occurs for field as small as 1 V/cm at low temperatures. Two distinct spin diffusion lengths now characterize spin motion, i.e., upstream (L u ) and down-stream (L d ) spin diffusion lengths, which can differ in orders of magnitude wi...

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