Screening for extracellular protein - protein interactions in a novel yeast growth selection system

The recent sequencing of entire eukaryotic genomes revealed a great number of open reading frames (ORFs), most of which potentially code for proteins of unknown function. Identifying interaction partners may facilitate the functional characterisation of unknown gene products. Moreover, large-scale approaches to identify protein interactions may be used to untangle the complete interaction network of all the gene products expressed in a given organism, the so-called interactome. Currently, the yeast two-hybrid system is the most widely used genetic assay for large-scale detection of protein – protein interactions and it has successfully been applied to map the interactome of several organisms. However, since interactions in this system are detected in the nuclear environment, it does not account for the particular biochemical requirements of extracellular and integral membrane proteins. In the first part of this thesis, I describe a novel genetic growth selection system to detect interactions between extracellular and transmembrane proteins in a topologically extracellular compartment of Saccharomyces cerevisiae. In this system the proteins of interest are expressed as fusions to mutually complementing mutant derivatives of the yeast ER resident transmembrane receptor Ire1p. Interaction between the proteins of interest causes dimerisation of the Ire1p moieties allowing for complementation and activation of Ire1p. Active Ire1p in turn triggers a signalling cascade, which induces expression of selectable reporter genes in the nucleus and promotes growth under selective conditions. The feasibility of this system to monitor interactions between extracellular proteins was demonstrated by specific pairings of epitope and single-chain Fv (scFv) antibody fragments. In part two, I describe the application of this system in a screening procedure to select scFv antibody fragments that specifically bind to human Interleukin-13 (hIL13). In a first round, hIL-13 binders were selected from a scFv-antibody library by yeast growth under selective conditions. In order to improve their binding affinity, parts of the scFv fragments obtained from the primary screening were randomised by homologous recombination in yeast and subjected to growth selection under increased selective conditions

Spin flip scattering at Al surfaces

Non-local measurements are performed on a multi terminal device to $in-situ$ determine the spin diffusion length and in combination with resistivity measurements also the spin relaxation time in Al films. By varying the thickness of Al we determine the contribution to spin relaxation from surface scattering. From the temperature dependence of the spin diffusion length it is established that the spin relaxation is impurity dominated at low temperature. A comparison of the spin and momentum relaxation lengths for different thicknesses reveals that the spin flip scattering at the surfaces is weak compared to that within the bulk of the Al films.Comment: 11 pages, 5 figure

Spin accumulation probed in multiterminal lateral all-metallic devices

We study spin accumulation in an aluminium island, in which the injection of a spin current and the detection of the spin accumulation are done by means of four cobalt electrodes that connect to the island through transparent tunnel barriers. Although the four electrodes are designed as two electrode pairs of the same shape, they nonetheless all exhibit distinct switching fields. As a result the device can have several different magnetic configurations. From the measurements of the amplitude of the spin accumulation, we can identify these configurations, and using the diffusion equation for the spin imbalance, we extract the spin relaxation length $\lambda_\mathrm{sf} = 400 \pm 50$~nm and an interface spin current polarization $P = (10 \pm 1)$ at low temperature and $\lambda_\mathrm{sf} = 350 \pm 50$~nm, $P = (8 \pm 1)$ at room temperature

Enhanced spin accumulation in a superconductor

A lateral array of ferromagnetic tunnel junctions is used to inject and detect non-equilibrium quasi-particle spin distribution in a superconducting strip made of Al. The strip width and thickness is kept below the quasi particle spin diffusion length in Al. Non-local measurements in multiple parallel and antiparallel magnetic states of the detectors are used to in-situ determine the quasi-particle spin diffusion length. A very large increase in the spin accumulation in the superconducting state compared to that in the normal state is observed and is attributed to a diminishing of the quasi-particle population by opening of the gap below the transition temperature.Comment: 6 pages, 4 figures; accepted for publication in Journal of Applied Physic

Spin injection and relaxation in a mesoscopic superconductor

We study spin accumulation and spin relaxation in a superconducting nanowire. Spins are injected and detected by using a set of magnetic tunnel contact electrodes, closely spaced along the nanowire. We observe a giant enhancement of the spin accumulation of up to five orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements combined with our theoretical model, allow us to distinguish the individual spin flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.Comment: 5 pages, 5 figure

Film Edge Nonlocal Spin Valves

Spintronics is a new paradigm for integrated digital electronics. Recently established as a niche for nonvolatile magnetic random access memory (MRAM), it offers new functionality while demonstrating low power and high speed performance. However, to reach high density spintronic technology must make a transition to the nanometer scale. Prototype devices are presently made using a planar geometry and have an area determined by the lithographic feature size, currently about 100 nm. Here we present a new nonplanar geometry in which one lateral dimension is given by a film thickness, the order of 10 nm. With this new approach, cell sizes can shrink by an order of magnitude. The geometry is demonstrated with a nonlocal spin valve, where we study devices with an injector/detector separation much less than the spin diffusion length.Comment: 10 pages, 3 figure

Boltzmann Equations for Spin and Charge Relaxations in Superconductors

In a superconductor coupled with a ferromagnetic metal, spin and charge imbalances can be induced by injecting spin-polarized electron current from the ferromagnetic metal. We theoretically study a nonequilibrium distribution of quasiparticles in the presence of spin and charge imbalances. We show that four distribution functions are needed to characterize such a nonequilibrium situation, and derive a set of linearized Boltzmann equations for them by extending the argument by Schmid and Sch\"{o}n based on the quasiclassical Green's function method. Using the Boltzmann equations, we analyze the spin imbalance in a thin superconducting wire weakly coupled with a ferromagnetic electrode. The spin imbalance induces a shift $\delta\mu$ ($- \delta \mu$) of the chemical potential for up-spin (down-spin) quasiparticles. We discuss how $\delta \mu$ is relaxed by spin-orbit impurity scattering.Comment: 16 pages, 2 figure

Baryon chiral perturbation theory with virtual photons and leptons

We construct the general pion-nucleon SU(2) Lagrangian including both virtual photons and leptons for relativistic baryon chiral perturbation theory up to fourth order. We include the light leptons as explicit dynamical degrees of freedom by introducing new building blocks which represent these leptons.Comment: 11 page

Vector Meson Mixing and Charge Symmetry Violation

We discuss the consistency of the traditional vector meson dominance (VMD) model for photons coupling to matter, with the vanishing of vector meson-meson and meson-photon mixing self-energies at q^2=0. This vanishing of vector mixing has been demonstrated in the context of rho-omega mixing for a large class of effective theories. As a further constraint on such models, we here apply them to a study of photon-meson mixing and VMD. As an example we compare the predicted momentum dependence of one such model with a momentum-dependent version of VMD discussed by Sakurai in the 1960's. We find that it produces a result which is consistent with the traditional VMD phenomenology. We conclude that comparison with VMD phenomenology can provide a useful constraint on such models.Comment: 7 pages, uses epsfig.sty. Publication details added to title pag

Direct electronic measurement of the spin Hall effect

The generation, manipulation and detection of spin-polarized electrons in nanostructures define the main challenges of spin-based electronics[1]. Amongst the different approaches for spin generation and manipulation, spin-orbit coupling, which couples the spin of an electron to its momentum, is attracting considerable interest. In a spin-orbit-coupled system, a nonzero spin-current is predicted in a direction perpendicular to the applied electric field, giving rise to a "spin Hall effect"[2-4]. Consistent with this effect, electrically-induced spin polarization was recently detected by optical techniques at the edges of a semiconductor channel[5] and in two-dimensional electron gases in semiconductor heterostructures[6,7]. Here we report electrical measurements of the spin-Hall effect in a diffusive metallic conductor, using a ferromagnetic electrode in combination with a tunnel barrier to inject a spin-polarized current. In our devices, we observe an induced voltage that results exclusively from the conversion of the injected spin current into charge imbalance through the spin Hall effect. Such a voltage is proportional to the component of the injected spins that is perpendicular to the plane defined by the spin current direction and the voltage probes. These experiments reveal opportunities for efficient spin detection without the need for magnetic materials, which could lead to useful spintronics devices that integrate information processing and data storage.Comment: 5 pages, 4 figures. Accepted for publication in Nature (pending format approval