Magnetic Structure and Spin Waves in the Kagom\'{e} Jarosite compound KFe3(SO4)2(OH)6{\bf KFe_3(SO_4)_2(OH)_6}

We present a detailed study of the magnetic structure and spin waves in the Fe jarosite compound ${\rm KFe_3(SO_4)_2(OH)_6}$ for the most general Hamiltonian involving one- and two-spin interactions which are allowed by symmetry. We compare the calculated spin-wave spectrum with the recent neutron scattering data of Matan {\it et al.} for various model Hamiltonians which include, in addition to isotropic Heisenberg exchange interactions between nearest ($J_1$) and next-nearest ($J_2$) neighbors, single ion anisotropy and Dzyaloshinskii-Moriya (DM) interactions. We concluded that DM interactions are the dominant anisotropic interaction, which not only fits all the splittings in the spin-wave spectrum but also reproduces the small canting of the spins out of the Kagom\'e plane. A brief discussion of how representation theory restricts the allowed magnetic structure is also given.Comment: 23 pages, 17 figures, submitted to Phys. Rev. B (March 2006

A Preliminary Study on Using Multi-Nozzle Polymer Deposition System to Fabricate Composite Alginate/Carbon Nanotube Tissue Scaffolds

Three-dimensional composite alginate/single wall carbon nanotube (SWCNT) scaffolds encapsulated with endothelial cells were fabricated by a multi-nozzle biopolymer freeform deposition system. This system enables the converting of CAD designed scaffold pattern into process toolpaths and the use of computer control program to guide the nozzle deposition at spatial position for layered fabrication of 3D tissue scaffolds. The morphological, mechanical, structural and biological properties of as-fabricated scaffolds were characterized by optical microscope, SEM, Microtensile testing machine, Alamar Blue Assay, and Live-Dead Assay, respectively. The multi-nozzle deposition system demonstrated a highly efficient and effective process to build tissue scaffold or cell embedded constructs. Characterization results showed that the incorporation of SWCNT into alginate not only enhanced the mechanical strength of the scaffolds but also improved the cell affinity and the interaction with substrate. Further cell culture experimental results also showed that the incorporation of SWCNT in alginate enhanced endothelial cell proliferation compared with pure alginate scaffold.Mechanical Engineerin

Comparing the Influence of the Drop Fill and Overflow Rinsing on the Reactive Dyeing Process in a Textile Dye House

Drop/fill rinsing method was used for reducing the high water consumption of overflow rinsing process in textile dye house. However, it is not known whether this drop/fill method has adverse effects on the final fabric properties or not. In this study, the effects of using drop/fill method instead of overflow rinsing process on colorimetric and fastness properties of 100% cotton towel fabrics are investigated. It is found that, skipping to the drop fill rinsing method from the over flow rinsing process and obtained fresh water saving has no negative effect on colorimetric and color fastness values of the processed fabric samples and can be used conveniently in the industrial sized production. © Published under licence by IOP Publishing Ltd

Transition Metal-Ethylene Complexes as High-Capacity Hydrogen Storage Media

From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of 14 wt%. Dimerization, polymerizations and incorporation of the TM-ethylene complexes in nanoporous carbon materials have been also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen storage materials discovery.Comment: 5 pages, 4 figures, additional content, Phys. Rev. Lett. in pres

Unusual structural tuning of magnetism in cuprate perovskites

Understanding the structural underpinnings of magnetism is of great fundamental and practical interest. Se_{1-x}Te_{x}CuO_{3} alloys are model systems for the study of this question, as composition-induced structural changes control their magnetic interactions. Our work reveals that this structural tuning is associated with the position of the supposedly dummy atoms Se and Te relative to the super-exchange (SE) Cu--O--Cu paths, and not with the SE angles as previously thought. We use density functional theory, tight-binding, and exact diagonalization methods to unveil the cause of this surprising effect and hint at new ways of engineering magnetic interactions in solids.Comment: 4 pages, with 4 postscript figures embedded. Uses REVTEX4 and graphicx macro

A hierarchical solution approach for a multicommodity distribution problem under a special cost structure

Cataloged from PDF version of article.Motivated by the spare parts distribution system of a major automotive manufacturer in Turkey, we consider a multicommodity distribution problem from a central depot to a number of geographically dispersed demand points. The distribution of the items is carried out by a set of identical vehicles. The demand of each demand point can be satisfied by several vehicles and a single vehicle is allowed to serve multiple demand points. For a given vehicle, the cost structure is dictated by the farthest demand point from the depot among all demand points served by that vehicle. The objective is to satisfy the demand of each demand point with the minimum total distribution cost. We present a novel integer linear programming formulation of the problem as a variant of the network design problem. The resulting optimization problem becomes computationally infeasible for real-life problems due to the large number of integer variables. In an attempt to circumvent this disadvantage of using the direct formulation especially for larger problems, we propose a Hierarchical Approach that is aimed at solving the problem in two stages using partial demand aggregation followed by a disaggregation scheme. We study the properties of the solution returned by the Hierarchical Approach. We perform computational studies on a data set adapted from a major automotive manufacturer in Turkey. Our results reveal that the Hierarchical Approach significantly outperforms the direct formulation approach in terms of both the running time and the quality of the resulting solution especially on large instances. © 2012 Elsevier Ltd. All rights reserved

An Alternative Perspective on Copositive and Convex Relaxations of Nonconvex Quadratic Programs

We study convex relaxations of nonconvex quadratic programs. We identify a family of so-called feasibility preserving convex relaxations, which includes the well-known copositive and doubly nonnegative relaxations, with the property that the convex relaxation is feasible if and only if the nonconvex quadratic program is feasible. We observe that each convex relaxation in this family implicitly induces a convex underestimator of the objective function on the feasible region of the quadratic program. This alternative perspective on convex relaxations enables us to establish several useful properties of the corresponding convex underestimators. In particular, if the recession cone of the feasible region of the quadratic program does not contain any directions of negative curvature, we show that the convex underestimator arising from the copositive relaxation is precisely the convex envelope of the objective function of the quadratic program, providing another proof of Burer's well-known result on the exactness of the copositive relaxation. We also present an algorithmic recipe for constructing instances of quadratic programs with a finite optimal value but an unbounded doubly nonnegative relaxation.Comment: 26 page