Adhesive factor/rabbit 2, a new fimbrial adhesin and a virulence factor from Escherichia coli O103, a serogroup enteropathogenic for rabbits

Enteropathogenic Escherichia coli-like E. coli strains belonging to serovar O103:K-:H2 and rhamnose-negative biotypes are highly pathogenic diarrhea-inducing strains for weaned European rabbits. We describe here the cloning and sequencing of the major subunit gene of a new fimbrial adhesin, adhesive factor/rabbit 2 (AF/R2), which confers on these strains the ability to attach to rabbit enterocytes and to HeLa cells in a diffuse manner and which is associated with in vivo virulence. The chromosomal operon that encodes functional AF/R2 has been cloned from strain B10. The major subunit gene afr2G, as well as an adjacent open reading frame, afr2H, has been sequenced. The Afr2G protein shows homologies with FaeG and ClpG, which are the respective major subunits of fimbrial adhesin K88 (F4) and afimbrial adhesin CS31A. Plasmid carrying the operon transcomplements an AF/R2-negative TnphoA mutant for its ability to express AF/R2. As a whole, AF/R2 is a new member of the E. coli K88 adhesin family which is associated with virulence and which may serve in the design of vaccines

Spin injection and accumulation in inhomogeneous semiconductors

We present a study of spin transport in charge and spin inhomogeneous semiconductor systems. In particular, we investigate the propagation of spin-polarized electrons through a boundary between two semiconductor regions with different doping concentrations. We use a theoretical and numerical method, presented in this paper, based on a self-consistent treatment of a two-component version of the Boltzmann transport equation. We show that space-charge effects strongly influence the spin transport properties, in particular giving rise to pronounced spin accumulation and spin density enhancement.Comment: 4 page

Spin-dependent resonant tunneling in ZnSe/ZnMnSe heterostructures

Using the transfer matrix method and the effective-mass approximation, the effect of resonant states on spin transport is studied in ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric and magnetic fields. The numerical results show that the ZnMnSe layers, which act as spin filters, polarize the electric currents. Variation of thickness of the central ZnSe layer shifts the resonant levels and exhibits an oscillatory behavior in spin current densities. It is also shown that the spin polarization of the tunneling current in geometrical asymmetry of the heterostructure where two ZnMnSe layers have different Mn concentrations, depends strongly on the thickness and the applied bias.Comment: 13 pages, 6 figure

Detection of electrical spin injection by light-emitting diodes in top- and side-emission configuration

Detection of the degree of circular polarization of the electroluminescence of a light-emitting diode fitted with a spin injecting contact (a spin-LED) allows for a direct determination of the spin polarization of the injected carriers. Here, we compare the detection efficiency of (Al,Ga)As spin-LEDs fitted with a (Zn,Be,Mn)Se spin injector in top- and side-emission configuration. In contrast with top emission, we cannot detect the electrical spin injection in side emission from analysing the degree of circular polarization of the electroluminescence. To reduce resonant optical pumping of quantum-well excitons in the side emission, we have analysed structures with mesa sizes as small as 1 micron.Comment: 15 pages with 3 figure

Experimental Investigation and Modeling of Growth Cone Adaptation in the Development of Topographic Projections (Experimentelle Untersuchung und Modellierung der Wachstumskegel-Adaption bei der Entwicklung topographischer Karten)

In the visual system, retinal ganglion cell growth cones (GCs) map topographically onto the optic tectum in the midbrain. Gradients of EphAs and ephrin-As along both, the retinal temporonasal and the tectal anteroposterior axes provide the chemoaffinity cues guiding retinal GCs to their topographically appropriate tectal destinations. Topographic precision is thought to rely on a faithful read-out of guidance cue concentrations, in this work, however, I show that GCs can adapt towards both cues

Quantum size effects on spin-tunneling time in a magnetic resonant tunneling diode

We study theoretically the quantum size effects of a magnetic resonant tunneling diode (RTD) with a (Zn,Mn)Se dilute magnetic semiconductor layer on the spin-tunneling time and the spin polarization of the electrons. The results show that the spin-tunneling times may oscillate and a great difference between the tunneling time of the electrons with opposite spin directions can be obtained depending on the system parameters. We also study the effect of structural asymmetry which is related to the difference in the thickness of the nonmagnetic layers. It is found that the structural asymmetry can greatly affect the traversal time and the spin polarization of the electrons tunneling through the magnetic RTD. The results indicate that, by choosing suitable values for the thickness of the layers, one can design a high speed and perfect spin-filter diode.Comment: 6 pages, 5 figure

Molecular-beam epitaxy of (Zn,Mn)Se on Si(100)

We have investigated the growth by molecular-beam epitaxy of the II-VI diluted magnetic semiconductor (Zn,Mn)Se on As-passivated Si(100) substrates. The growth start has been optimized by using low-temperature epitaxy. Surface properties were assessed by Nomarski and scanning electron microscopy. Optical properties of (Zn,Mn)Se have been studied by photoluminescence and a giant Zeeman splitting of up to 30 meV has been observed. Our observations indicate a high crystalline quality of the epitaxial films.Comment: To be published in Applied Physics Letter

Spin Current in Spin-Orbit Coupling Systems

We present a simple and pedagogical derivation of the spin current as the linear response to an external electric field for both Rashba and Luttinger spin-orbital coupling Hamiltonians. Except for the adiabatic approximation, our derivation is exact to the linear order of the electric field for both models. The spin current is a direct result of the difference in occupation levels between different bands. Moreover, we show a general topological spin current can be defined for a broad class of spin-orbit coupling systems

Spintronics and Quantum Computing: Switching Mechanisms for Qubits

Quantum computing and quantum communication are remarkable examples of new information processing technologies that arise from the coherent manipulation of spins in nanostructures. We review our theoretical proposal for using electron spins in quantum-confined nanostructures as qubits. We present single- and two-qubit gate mechanisms in laterally as well as vertically coupled quantum dots and discuss the possibility to couple spins in quantum dots via exchange or superexchange. In addition, we propose a new stationary wave switch, which allows to perform quantum operations with quantum dots or spin-1/2 molecules placed on a 1D or 2D lattice.Comment: 6 pages, 3 EPS figures, Latex, to appear in Physica E, proceedings of the PASP2000 conference on the physics and application of spin-related phenomena in semiconductors, Sendai, Japan, 200