Quantum tunneling of magnetization in metallic FeC ferrofluids

We have achieved the observance of the quantum tunneling of magnetization in a ferrofluid system constituted by small single domain particles of FeC in a nonmagnetic carrier fluid. The constancy of the coercive field below 1 K indicates that this temperature corresponds to the crossover from the thermal to the quantum regime.     

Ultrafine amorphous alloy particles

Different techniques for the preparation of amorphous alloy particles are reviewed. Special attention is made to the studies of the preparation and the physical properties of ultrafine amorphous iron-boron and iron-carbon alloy particles. The properties of the ultrafine alloy particles are in most cases similar to those of bulk amorphous alloys of the same composition, e.g., the dependence of the hyperfine parameters on temperature and composition as well as Curie temperatures are similar.     

A simplicial branch-and-bound algorithm for solving quadratically constrained quadratic programs

We propose a branch-and-bound algorithm for solving nonconvex quadratically-constrained quadratic programs. The algorithm is novel in that branching is done by partitioning the feasible region into the Cartesian product of two-dimensional triangles and rectangles. Explicit formulae for the convex and concave envelopes of bilinear functions over triangles and rectangles are derived and shown to be second-order cone representable. The usefulness of these new relaxations is demonstrated both theoretically and computationally.     

Valid inequalities for separable concave constraints with indicator variables

We study valid inequalities for optimization models that contain both binary indicator variables and separable concave constraints. These models reduce to a mixed-integer linear program (MILP) when the concave constraints are ignored, or to a nonconvex global optimization problem when the binary restrictions are ignored. In algorithms designed to solve these problems to global optimality, cutting planes to strengthen the relaxation are traditionally obtained using valid inequalities for the MILP only. We propose a technique to obtain valid inequalities that are based on both the MILP constraints and the concave constraints. We begin by characterizing the convex hull of a four-dimensional set consisting of a single binary indicator variable, a single concave constraint, and two linear inequalities. Using this analysis, we demonstrate how valid inequalities for the single node flow set and for the lot-sizing polyhedron can be “tilted” to give valid inequalities that also account for separable concave functions of the arc flows. We present computational results demonstrating the utility of the new inequalities for nonlinear transportation problems and for lot-sizing problems with concave costs. To our knowledge, this is one of the first works that simultaneously convexifies both nonconvex functions and binary variables to strengthen the relaxations of practical mixed-integer nonlinear programs.     

Temperature dependence of the magnetic hyperfine splitting of metastable iron-amalgam

The temperature dependence of the magnetic hyperfine field of a metastable iron-mercury alloy has been measured. The Mössbauer spectrum obtained at 12 K can be fitted with two magnetically split components with hyperfine fields of 39.7 and 36.0 T. With increasing temperature the hyperfine fields decrease much more rapidly than that of α-Fe. The temperature dependence is in accordance with the Bloch law up to 220 K.     

Bi-directional modulation of AMPA receptor unitary conductance by synaptic activity

Background  

Knowledge of how synapses alter their efficiency of communication is central to the understanding of learning and memory. The most extensively studied forms of synaptic plasticity are long-term potentiation (LTP) and its counterpart long-term depression (LTD) of AMPA receptor-mediated synaptic transmission. In the CA1 region of the hippocampus, it has been shown that LTP often involves a rapid increase in the unitary conductance of AMPA receptor channels. However, LTP can also occur in the absence of any alteration in AMPA receptor unitary conductance. In the present study we have used whole-cell dendritic recording, failures analysis and non-stationary fluctuation analysis to investigate the mechanism of depotentiation of LTP.     

Positron transmission and effective mass absorption coefficient in nickel

The penetration of positrons from a²²Na source into Ni foils sandwiched between different backscattering materials (Kapton, NaCl, Ni, and Mo) has been studied for the geometry commonly used in PAT experiments. Transmitted intensities and effective mass absorption coefficients have been found and the dependence on the backscattering material is discussed. General formulas for the effective mass absorption coefficients are derived. The large geometrical effects are stressed. Suggestions for source corrections are supplied.     

The Determination of Dynamic and Equlibrium Solid/Liquid Transformation Data For Sn–Pb Using DSC

Differential Scanning Calorimetric measurements were performed for accurate determination of the solidus and liquidus temperature of Sn–Pb alloys. The difference between onset and end temperature of the melting peak depended clearly on alloy composition. The results obtained were found to be in good agreement with the existing equilibrium phase diagram for the Sn–Pb system, although the data suggested a slight correction of the eutectic composition. Under cooling conditions a large variation in the onset temperature for solidification was found. A large number of heating and cooling cycles were performed in order to investigate the statistical variation of the solid nucleation process. This revised version was published online in July 2006 with corrections to the Cover Date.