How spin-flip scattering influences the minigap in a S-N junction

A superconductor in contact with a normal metal modifies the behaviour of the electrons in the normal part (”proximity effect”). The electrons in the adjacent normal metal will exhibit superconducting properties like: • apparition of an energy gap in the density of states • modification of the conductance (strongly dependent on energy) • screening of a magnetic field The scale at which these properties can be observed is a so called ”mesoscopic scale”, characterised by the presence of a sufficient number of particles to use statistical methods but still showing a non negligible phase coherence effect. In this work, we will study the influence of inelastic and spin-flip scattering on the electron density of states in diffusive S-N junctions

Effects of magnetism in superconducting hybrid structures

In this thesis we study various effects of magnetism in proximity structures, composed of superconducting electrodes in contact with a normal metal. Magnetism can be present in the system through the Zeeman and the orbital coupling. Proximity structures offer in particular a unique opportunity to study the interplay between ferromagnetism and conventional superconductivity, which can hardly coexist in bulk samples. The orbital effect of an external magnetic field applied to a Josephson junction results in interference effects between local currents. In Chapter 1, we give an introduction to the main features of the proximity effect and to the theoretical formalism used throughout the thesis. In Chapter 2 we study the Josephson effect in a superconductor–ferromagnet–superconductor (SFS) junction with ferromagnetic domains of noncollinear magnetization. It is well known [1] that as a consequence of the exchange splitting of the Fermi level [2] the Cooper pair wave function shows damped oscillations in a ferromagnet, leading to the appearance of the so-called "π state" in SFS junctions [3]. In the π state, the superconducting order parameter is of opposite sign in the two S electrodes of the Josephson junction, and a spontaneous non-dissipative current can appear in a ring containing such a junction. As a model for our study of the influence of magnetic domains on the π state formation, we consider a diffusive junction with two ferromagnetic domains along the junction. We find analytically the critical current as a function of domain lengths and of the angle between the orientations of their magnetizations. Varying those parameters, the junction may undergo transitions between 0 and π phases. We find that the presence of domains reduces the range of junction lengths at which the π phase is observed. For the junction with two domains of the same length, the π phase totally disappears as soon as the misorientation angle exceeds π/2. We further comment on possible implications of our results for experimentally observable 0–π transitions in SFS junctions. Experimentally, π junctions are realized as thin films deposited in layers. In Chapter 3, we study therefore the influence of in-plane magnetic domains on the Josephson current. We find that the properties of the junction depend on the size of the domains relative to the magnetic coherence length. In the case of large domains, the junction exhibits transitions to the π state, similarly to a single-domain SFS junction. In the case of small domains, the magnetization effectively averages out, and the junction is always in the zero state, similarly to a superconductor–normal metal–superconductor (SNS) junction. In both those regimes, the influence of domain walls may be approximately described as an effective spin-flip scattering. We also study the inhomogeneous distribution of the local current density in the junction. Close to the 0–π transitions, the directions of the critical current may be opposite in the vicinity of the domain wall and in the middle of the domains. In Chapter 4, we discuss the orbital effects of an external magnetic field in a SNS junction. In the limit of a long junction, we find that the properties of such a system depend on the width of the junction relative to the length associated with the magnetic field. We compute the critical width separating the regime of pure decay (narrow junction) and the regime of damped oscillations (wide junction) of the critical current as a function of the magnetic flux through the junction. We find an exponential damping of the current, different from the well known Fraunhofer limit which corresponds to the limit of a tunnel junction. In the limit of a wide junction, the superconducting pair correlations and the critical current become localized near the border of the junction

Real-time pollen monitoring using digital holography

We present the first validation of the SwisensPoleno, currently the only operational automatic pollen mon-itoring system based on digital holography. The device pro-vides in-flight images of all coarse aerosols, and here wedevelop a two-step classification algorithm that uses theseimages to identify a range of pollen taxa. Deterministiccriteria based on the shape of the particle are applied toinitially distinguish between intact pollen grains and othercoarse particulate matter. This first level of discriminationidentifies pollen with an accuracy of 96 %. Thereafter, in-dividual pollen taxa are recognized using supervised learn-ing techniques. The algorithm is trained using data obtainedby inserting known pollen types into the device, and out ofeight pollen taxa six can be identified with an accuracy ofabove 90 %. In addition to the ability to correctly identifyaerosols, an automatic pollen monitoring system needs to beable to correctly determine particle concentrations. To fur-ther verify the device, controlled chamber experiments us-ing polystyrene latex beads were performed. This providedreference aerosols with traceable particle size and numberconcentrations in order to ensure particle size and samplingvolume were correctly characterized

Magnetic interference patterns in long disordered Josephson junctions

We study a diffusive superconductor - normal metal - superconductor (SNS) junction in an external magnetic field. In the limit of a long junction, we find that the form of the dependence of the Josephson current on the field and on the length of the junction depends on the ratio between the junction width and the length associated with the magnetic field. A certain critical ratio between these two length scales separates two different regimes. In narrow junctions, the critical current exhibits a pure decay as a function of the junction length or of the magnetic field. In wide junctions, the critical current exhibits damped oscillations as a function of the same parameters. This damped oscillating behavior differs from the Fraunhofer pattern typical for short or tunnel junctions. In wide and long junctions, superconducting pair correlations and supercurrent are localized along the edges of the junction.Comment: 9 pages, 4 figures, minor modifications corresponding to the published versio

Extracellular Blockade of K+ Channels by Tea: Results from Molecular Dynamics Simulations of the Kcsa Channel

TEA is a classical blocker of K+ channels. From mutagenesis studies, it has been shown that external blockade by TEA is strongly dependent upon the presence of aromatic residue at Shaker position 449 which is located near the extracellular entrance to the pore (Heginbotham, L., and R. MacKinnon. 1992. Neuron. 8:483–491). The data suggest that TEA interacts simultaneously with the aromatic residues of the four monomers. The determination of the 3-D structure of the KcsA channel using X-ray crystallography (Doyle, D.A., J.M. Cabral, R.A. Pfuetzner, A. Kuo, J.M. Gulbis, S.L. Cohen, B.T. Chait, and R. MacKinnon. 1998. Science. 280:69–77) has raised some issues that remain currently unresolved concerning the interpretation of these observations. In particular, the center of the Tyr82 side chains in KcsA (corresponding to position 449 in Shaker) forms a square of 11.8-Å side, a distance which is too large to allow simultaneous interactions of a TEA molecule with the four aromatic side chains. In this paper, the external blockade by TEA is explored by molecular dynamics simulations of an atomic model of KcsA in an explicit phospholipid bilayer with aqueous salt solution. It is observed, in qualitative accord with the experimental results, that TEA is stable when bound to the external side of the wild-type KcsA channel (with Tyr82), but is unstable when bound to a mutant channel in which the tyrosine residue has been substituted by a threonine. The free energy profile of TEA relative to the pore is calculated using umbrella sampling simulations to characterize quantitatively the extracellular blockade. It is found, in remarkable agreement with the experiment, that the TEA is more stably bound by 2.3 kcal/mol to the channel with four tyrosine residues. In the case of the wild-type KcsA channel, TEA (which has the shape of a flattened oblate spheroid) acts as an ideal plug blocking the pore. In contrast, it is considerably more off-centered and tilted in the case of the mutant channel. The enhanced stability conferred by the tyrosine residues does not arise from Π–cation interactions, but appears to be due to differences in the hydration structure of the TEA. Finally, it is shown that the experimentally observed voltage dependence of TEA block, which is traditionally interpreted in terms of the physical position of the TEA along the axis of the pore, must arise indirectly via coupling with the ions in the pore

Biomass selection by floods and related timescales. Part 2: Stochastic modeling

We analyze the results from a flume experiment presented in a companion paper by Perona et al. [1] exploring the selective action of floods on pioneer riparian vegetation. We study the way seedlings react to periodic flow disturbances in the early stages of their growth and propose a conjecture to describe their statistical distribution in the alluvial bare sediment. In order to access the regime of competition between growth and uprooting by the floods, the experiment focuses on the situation where the flooding frequency is comparable with the plant germination and growth rates and gives a detailed insight into the statistics of the riverbed and uprooted vegetation. We develop a stochastic description for the growth-uprooting process and use our model to analyze the collected data. Results confirm the conjecture that the flooding events remove young vegetation with characteristics (we focus on the distribution of the main root length) remaining constant over time and allow to define the scale over which vegetation becomes resilient to uprooting. (C) 2011 Elsevier Ltd. All rights reserved

Evidence and modelling biomass selection by floods in a riverbed

Riparian and in-bed vegetation growth and erosion dynamics are strongly coupled with river hydrologic and morphodynamic processes. Many field observations documented the engineering role of vegetation and its contribution to build, stabilize and control erosion and deposition on gravel bars. Yet unclear, is how do the interplay between the hydraulic time scale (arrival time of flood disturbances) and the vegetation germination and growth rates determines the statistical distribution of the surviving vegetation and of that being removed by flooding events. Soon after germination on exposed gravel bars, young vegetation can be removed by even moderate floods without substantial channel reworking, root anchoring being almost negligible at this stage of growth. Hence, understanding survival and uprooting dynamics of early germinated seedlings or rejuvinated woody debris is key to unravel their role in the future evolution of alluvial forms. In this paper, we provide experimental evidence and a model on how the statistics of young biomass up- rooted by flow depend on stream power and on the above- and below-ground characteristics of the biomass growing in the alluvial sediment. Particularly, we present results of recent laboratory experiments where periodic flow disturbances of constant magnitude have been run on sand flumes with vegetation growing in situ at a rate that is comparable to the interarrival time of disturbances. This allows to access the regime of competition between vegetation development and flood disturbances. While non-eroded plants continue to grow in the successive runs, flow erosion acts preferentially on plants that have a weaker root system. Eventually, the statistic of the uprooted biomass turns out to be time independent, leading to suppose the existence of a biomass selection mechanism operated by floods in alluvial floodplains. We then propose a simple time and state dependent stochastic model to quantitatively explain this conjecture. Eventually, our minimalist model can be solved analytically, and to a certain extent it helps shedding light on which components of the hydrologic and vegetation processes and related time scales control the erosion mechanism of early germinated vegetation

On the biomechanics of seedling anchorage

We propose a minimal model for the response of vegetation to pullout constraints at early development stage. We try to capture both the average mechanical properties of the root system and the stochastic component of the uprooting process of seedlings. We identify a minimal set of relevant physical components in the purpose of quantifying the uprooting process: length of the root fibres, elastic response of the fibres and adhesion between the roots and the soil matrix. We present for validation a dataset extracted from Edmaier et al. (under revision), accounting for 98 uprooting experiments using Avena sativa L. seedlings (common oat), growing in non-cohesive sediment under controlled conditions. The corresponding root system has a very simple architecture, with three root fibres of different lengths. The response of the system to the constraint is however complex: the stress-strain signal presents sudden jumps followed by partial elastic recoveries. The analysis of the jumps and partial recoveries gives an insight into the resilience of the system. The anchorage of less mature seedlings rapidly collapses after the peak force has been reached, while more mature seedlings usually recover from partial failures. We explore this crossover with our validation dataset. The type of seedlings we study has been used in flume experiments investigating the feedbacks between the vegetation and the river morphodynamics (see for example Perona et al. (2012)). An understanding of the characteristics of the uprooting curve (maximal uprooting force and total uprooting work) of such vegetation reveals the ability of seedlings to withstand environmental constraints in terms of duration or intensity (see Edmaier et al., under revision), and is therefore helpful for planning future experiments. REFERENCES - P. Perona, P. Molnar, B. Crouzy, E. Perucca, Z. Jiang, S. McLelland, D. Wüthrich, K. Edmaier, R. Francis, C. Camporeale, et al., Biomass selection by floods and related timescales: Part 1. Experimental observations, Advances in Water Resources 39 (2012) 85-96. - K. Edmaier, P. Burlando and P. Perona, Mechanisms of vegetation uprooting by flow in alluvial non-cohesive sediment. Hydrology And Earth System Sciences, vol. 15, p. 1615-1627, 2011. - K. Edmaier, B. Crouzy, P. Perona, Experimental characterization of root anchoring in non-cohesive sediment, Earth Surface Processes and Landforms (under revision)

Minigap in a SN junction with paramagnetic impurities

We study the effect of spin-flip scattering on the density of states in a long diffusive S-N junction with transparent interface. We calculate the critical value of the spin-flip scattering rate at which the minigap closes and give the dependence of the minigap on the spin-flip rate. For the system we consider, the minigap and the critical spin-flip rate have the scale of the Thouless energy, and not of the superconducting gap