Numerical Studies on the Magnetism of Fe-Ni-Mn Alloys in the Invar Region

By means of self-consistent semi-empirical LCAO calculations we study the itinerant magnetism of (Fe_{0.65}Ni_{0.35})_{1-y} Mn_y alloys for y between 0 and 0.22 at T=0 K, neglecting only the transverse spin components. We find that the magnetic behaviour is quite complicated on a local scale. In addition to ferromagnetic behaviour, also metastable spin-glass-like configurations are found. In the same approach, using a direct numerical calculation by the Kubo-Formalism without any fit parameters, we also calculate the electrical conductance in the magnetic state and find that the $y$-dependence observed in the experiments is well reproduced by our calculations, except of an overall factor of rougly 5, by which our resistivities are too large.Comment: 12 pages (Latex, to be applied 2 times) + 13 figures (eps-files

Relationship between solidification microstructure and hot cracking susceptibility for continuous casting of low-carbon and high-strength low-alloyed steels: A phase-field study

© The Minerals, Metals & Materials Society and ASM International 2013Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphase-field software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz-Drezet-Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior. © 2013 The Minerals, Metals & Materials Society and ASM International

Doping, density of states and conductivity in polypyrrole and poly(p-phenylene vinylene)

The evolution of the density of states (DOS) and conductivity as function of well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene) [OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6 have been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains non-metallic, while the latter crosses the metal-insulator transition. In both systems a similar almost linear increase in DOS as function of charges per unit volume c* has been observed from the electrochemical gated transistor data. In PPy-PF_6, when compared to doped OC_1C_10-PPV, the energy states filled at low doping are closer to the vacuum level; by the higher c* at high doping more energy states are available, which apparently enables the conduction to change to metallic. Although both systems on the insulating side show log(sigma) proportional to T^-1/4 as in variable range hopping, for highly doped PPy-PF_6 the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.Comment: 4 pages (incl. 6 figures) in Phys. Rev.

Model-based Comparison of Cell Density-dependent Cell Migration Strategies

Here, we investigate different cell density-dependent migration strategies. In particular, we consider strategies which differ in the precise regulation of transitions between resting and motile phenotypes. We develop a lattice-gas cellular automaton (LGCA) model for each migration strategy. Using a mean-field approximation we quantify the corresponding spreading dynamics at the cell population level. Our results allow for the prediction of cell population spreading based on experimentally accessible single cell migration parameters

On the structure of the energy distribution function in the hopping regime

The impact of the dispersion of the transport coefficients on the structure of the energy distribution function for charge carriers far from equilibrium has been investigated in effective-medium approximation for model densities of states. The investigations show that two regimes can be observed in energy relaxation processes. Below a characteristic temperature the structure of the energy distribution function is determined by the dispersion of the transport coefficients. Thermal energy diffusion is irrelevant in this regime. Above the characteristic temperature the structure of the energy distribution function is determined by energy diffusion. The characteristic temperature depends on the degree of disorder and increases with increasing disorder. Explicit expressions for the energy distribution function in both regimes are derived for a constant and an exponential density of states.Comment: 16 page

Revival of Silenced Echo and Quantum Memory for Light

We propose an original quantum memory protocol. It belongs to the class of rephasing processes and is closely related to two-pulse photon echo. It is known that the strong population inversion produced by the rephasing pulse prevents the plain two-pulse photon echo from serving as a quantum memory scheme. Indeed gain and spontaneous emission generate prohibitive noise. A second $\pi$-pulse can be used to simultaneously reverse the atomic phase and bring the atoms back into the ground state. Then a secondary echo is radiated from a non-inverted medium, avoiding contamination by gain and spontaneous emission noise. However, one must kill the primary echo, in order to preserve all the information for the secondary signal. In the present work, spatial phase mismatching is used to silence the standard two-pulse echo. An experimental demonstration is presented.Comment: 13 pages, 6 figure