Electrically-controllable RKKY interaction in semiconductor quantum wires

We demonstrate in theory that it is possible to all-electrically manipulate the RKKY interaction in a quasi-one-dimensional electron gas embedded in a semiconductor heterostructure, in the presence of Rashba and Dresselhaus spin-orbit interaction. In an undoped semiconductor quantum wire where intermediate excitations are gapped, the interaction becomes the short-ranged Bloembergen-Rowland super-exchange interaction. Owing to the interplay of different types of spin-orbit interaction, the interaction can be controlled to realize various spin models, e.g., isotropic and anisotropic Heisenberg-like models, Ising-like models with additional Dzyaloshinsky-Moriya terms, by tuning the external electric field and designing the crystallographic directions. Such controllable interaction forms a basis for quantum computing with localized spins and quantum matters in spin lattices.Comment: 5 pages, 1 figur

A 3-D vector magnetization model with interaction field

This paper presents a vector model of magnetization based on the three-dimensional (3-D) Stoner-Wohlfarth elemental operator. To account for the magnetic interactions between particles, a phenomenological mean-field approximation is employed. The paper also illustrates the numerical simulation results of the magnetization in 3-D. This model will be useful to simulate the magnetization process of complicated topology flux electromagnetic devices. © 2005 IEEE

Elliptical supra-cellular topographies regulate stem cells migratory pattern and osteogenic differentiation

In living systems, the extracellular environment is structured in a hierarchal order assembling into tissues in a myriad of shapes and complex geometries. Residing within the extracellular matrix, cells are presented and influenced by geometrical cues at several scales. While there is an emerging body of evidence that substrate with symmetric supra-cellular scale geometries (e.g. cylinders and spheres) influence the cell behavior, the effect of physiologically relevant, non-symmetric geometries with varying mean curvatures remain unexplored. In this study, we systematically explore the migratory and differentiation behavior of adipose derived stem cells (ADSCs) on arrays of elliptical cylinders (up to 80 × cell size) with varying mean curvature made from hydroxyapatite. Here, we report a new substrate-driven cell response, which we term “ridge-effect” that leads to osteogenic differentiation and nuclear deformation of cells adhered on regions of highest mean curvature at the ridge of the elliptical cylinders. This phenomenon is observed in both expansion and osteogenic medium. Live imaging combined with functional analysis shows that cells travel along-side the zero mean curvature direction on elliptical cylinders and significantly promote expression of collagen I and osteocalcin compared to a flat surface, in the absence of osteogenic supplements. Altogether, this work identifies supra-cellular scale topographies, and suggest the “ridge-effect” as a physical cue for guiding cellular mechanoresponse and promoting osteogenic differentiation. This knowledge could be utilized as an important biomaterial design parameter for the development of biomedical interfaces and bone scaffolds in tissue engineering and regenerative medicine

Measurement and modeling of core losses of soft magnetic composites under 3-D magnetic excitations in rotating motors

Soft magnetic composite (SMC) materials are especially suitable for construction of low-cost high-performance motors with three-dimensional (3-D) magnetic fields. The 3-D finite-element analysis (FEA) conducted in the design of a claw pole transverse flux motor (CPTFM) with an SMC core reveals very complicated B (flux density) loci in the core when the motor rotates. In order to understand and account for the effects of the patterns of B loci on motor parameters and performance such as core losses, a 3-D magnetic property testing system was built for magnetic measurement on a cubic SMC sample. 3-D rotational core loss models in SMC materials and rotating motors are developed. © 2005 IEEE

Improved measurement with 2-D rotating fluxes considering the effect of internal field

This paper analyzes the effect of the internal field on the measurement with two-dimensional (2-D) rotating fluxes. It is demonstrated that due to the effect of the internal field, the misalignment of H sensing coils, causing the asymmetry of H loci and the discrepancy of the rotational core losses between two opposite rotating directions cannot be corrected completely. A numerical averaging method is employed to eliminate the angular error. Finally, the B and H loci and rotational core losses of a soft magnetic composite (SMC) material were measured under circular rotating magnetic flux density vectors, and the results were corrected by the proposed averaging method. © 2005 IEEE

Solving ill-posed bilevel programs

This paper deals with ill-posed bilevel programs, i.e., problems admitting multiple lower-level solutions for some upper-level parameters. Many publications have been devoted to the standard optimistic case of this problem, where the difficulty is essentially moved from the objective function to the feasible set. This new problem is simpler but there is no guaranty to obtain local optimal solutions for the original optimistic problem by this process. Considering the intrinsic non-convexity of bilevel programs, computing local optimal solutions is the best one can hope to get in most cases. To achieve this goal, we start by establishing an equivalence between the original optimistic problem an a certain set-valued optimization problem. Next, we develop optimality conditions for the latter problem and show that they generalize all the results currently known in the literature on optimistic bilevel optimization. Our approach is then extended to multiobjective bilevel optimization, and completely new results are derived for problems with vector-valued upper- and lower-level objective functions. Numerical implementations of the results of this paper are provided on some examples, in order to demonstrate how the original optimistic problem can be solved in practice, by means of a special set-valued optimization problem

On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy

The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail

Sharp Global Bounds for the Hessian on Pseudo-Hermitian Manifolds

We find sharp bounds for the norm inequality on a Pseudo-hermitian manifold, where the L^2 norm of all second derivatives of the function involving horizontal derivatives is controlled by the L^2 norm of the sub-Laplacian. Perturbation allows us to get a-priori bounds for solutions to sub-elliptic PDE in non-divergence form with bounded measurable coefficients. The method of proof is through a Bochner technique. The Heisenberg group is seen to be en extremal manifold for our inequality in the class of manifolds whose Ricci curvature is non-negative.Comment: 13 page

Cyclic Deformation Behavior of Additive-Manufactured IN738LC Superalloys from Virgin and Reused Powders

In laser powder bed fusion (L-PBF), most powders are not melted in the chamber and collected after the printing process. Powder reuse is appreciable without sacrificing the mechanical properties of target components. To understand the influences of powder reuse on mechanical performance, a nickel-based superalloy, IN738LC, was investigated. Powder morphology, microstructure and chemical compositions of virgin and reused powders were characterized. An increase in oxygen content, generally metallic oxides, was located on the surface of powders. Monotonic tensile and cyclic fatigue were tested. Negligible deterioration in strength and tensile ductility were found, while scattered fatigue performance with regard to fatigue life was shown. Deformation and fatigue crack propagation mechanisms were discussed for describing the powder degradation effects