Electron-energy bunching in laser-driven soft recollisions

We introduce soft recollisions in laser-matter interaction. They are characterized by the electron missing the ion upon recollision in contrast to the well-known head-on collisions responsible for high-harmonic generation or above-threshold ionization. We demonstrate analytically that soft recollisions can cause a bunching of photo-electron energies through which a series of low-energy peaks emerges in the electron yield along the laser polarization axis. This peak sequence is universal, it does not depend on the binding potential, and is found below an excess energy of one fifth of the ponderomotive energy.Comment: 4 pages, 3 figures, 1 tabl

Stabilization of microbial residues by co-precipitation with Fe and Al oxyhydroxides

It is now widely accepted that microbial residues are a significant source for soil organic matter (SOM) formation. This material must be stabilised in soil in order to persist. A potential mechanism for stabilisation of organic materials in soil is co-precipitation with metal oxyhydroxides (Fe and Al), which, however, may be affected by redox transitions. We thus evaluated the mineralisation of 14C-labelled bacterial residues (Escherichia coli cells and cell envelope fragments) and their co-precipitates with Fe or Al oxyhydroxide under different redox conditions in a laboratory incubation experiment. The co-precipitates or untreated microbial residues (control) were mixed with soil and incubated in sealed vessels under either fully aerobic or under oxygen-limited conditions for up to 345 days. To achieve oxygen limitation, incubation was conducted under an N2 atmosphere for the first 100 days. The redox potential was further decreased by waterlogging the samples (from day 100) and by substrate and nutrient additions (from day 290), to increase electron acceptor consumption by the soil microbes. Mineralisation of the microbial residues was quantified by liquid scintillation counting. The data were fitted to different types of models, depending on the experimental phase. Co-precipitation with Fe and Al oxyhydroxides decreased mineralisation of both intact cells and cell envelope fragments significantly, indicating strong protection of biomass and its fragments. Mineralisation of intact cells was slightly faster than that of cell envelope fragments, indicating higher recalcitrance of the latter material, which therefore may be enriched in SOM. Strongly reducing conditions resulted reductive dissolution of Fe oxyhydroxide and thus in a loss of the stabilising effect of the co-precipitation. We conclude that co-precipitation with and incrustation of organic material by Fe and Al oxyhydroxides provide significant stabilisation of microbial residues. However, environmental conditions, e.g. the redox potential, modify the extent of this stabilisation. Fitting the mineralisation data to the models indicated that initially mainly pool sizes were affected by the factors studied, whereas later in the experiment the rate constants were more sensitive. The results improved significantly our understanding how organic materials, in particular microbial residues, are stabilised in soil

Differentiable Kernels in Generalized Matrix Learning Vector Quantization

In the present paper we investigate the application of differentiable kernel for generalized matrix learning vector quantization as an alternative kernel-based classifier, which additionally provides classification dependent data visualization. We show that the concept of differentiable kernels allows a prototype description in the data space but equipped with the kernel metric. Moreover, using the visualization properties of the original matrix learning vector quantization we are able to optimize the class visualization by inherent visualization mapping learning also in this new kernel-metric data space

Microscale Modeling of Magnetoactive Composites Undergoing Large Deformations

This paper is concerned with the development of a material model for the constituents of a magnetoactive composite. Special attention is paid to magnetorheological elastomers which are synthesized from a soft polymeric matrix material with embedded magnetizable particles. Because the particles interact under an applied magnetic load, a coupled magneto-mechanical field problem has to be solved. The mechanical properties of the polymer matrix motivate the consideration of large deformations. We present the balance equations with boundary conditions and an appropriate material model. The corresponding boundary value problems are solved by the Finite-Element-Method. A weak numerical coupling scheme enables the staggered solution of two subproblems, the stationary magnetic and mechanical one. The coupling between both is realized by a surrounding iterative loop

Reversible magnetomechanical collapse: virtual touching and detachment of rigid inclusions in a soft elastic matrix

Soft elastic composite materials containing particulate rigid inclusions in a soft elastic matrix are candidates for developing soft actuators or tunable damping devices. The possibility to reversibly drive the rigid inclusions within such a composite together to a close-to-touching state by an external stimulus would offer important benefits. Then, a significant tuning of the mechanical properties could be achieved due to the resulting mechanical hardening. For a long time, it has been argued whether a virtual touching of the embedded magnetic particles with subsequent detachment can actually be observed in real materials, and if so, whether the process is reversible. Here, we present experimental results that demonstrate this phenomenon in reality. Our system consists of two paramagnetic nickel particles embedded at finite initial distance in a soft elastic polymeric gel matrix. Magnetization in an external magnetic field tunes the magnetic attraction between the particles and drives the process. We quantify the scenario by different theoretical tools, i.e., explicit analytical calculations in the framework of linear elasticity theory, a projection onto simplified dipole-spring models, as well as detailed finite-element simulations. From these different approaches, we conclude that in our case the cycle of virtual touching and detachment shows hysteretic behavior due to the mutual magnetization between the paramagnetic particles. Our results are important for the design and construction of reversibly tunable mechanical damping devices. Moreover, our projection on dipole-spring models allows the formal connection of our description to various related systems, e.g., magnetosome filaments in magnetotactic bacteria.Comment: 14 pages, 7 figure