Developing a framework to evaluate the existence of a complexity threshold

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering; and, (M.B.A.)--Massachusetts Institute of Technology, Sloan School of Management; in conjunction with the Leaders for Manufacturing Program at MIT, 2006.Includes bibliographical references (leaves 49-51).An automotive manufacturer facing decreasing average product volumes as a result of market fragmentation while simultaneously reducing its manufacturing plant footprint must adapt to the difficult challenge of increased product mix within its manufacturing system. The increase in complexity resulting from greater product mix is considered to be a significant driver in increasing plant investment cost and reducing plant operating effectiveness. Thus, the ability to fully understand and more effectively balance the complexity trade-offs associated with different product-to-manufacturing plant allocation scenarios is critically important, as the manufacturer formulates its strategy and analyzes the associated costs and benefits. The ultimate question to be addressed is whether there exists a "complexity threshold" in terms of the maximum number of differentiated body styles (unique vehicle models) to be produced inside a single assembly plant. This thesis analyzes the challenge of manufacturing system and plant complexity by first developing a competitive benchmark study of body-style complexity at the major North American OEMs' plants. Then, manufacturing and operations data is analyzed for evidence of a "complexity threshold" in one manufacturer's operations.(cont.) Finally, a linear-program based optimization model is developed to enable a Manufacturing Planning group to better understand the company's tolerance for plant complexity by quantifying manufacturing costs associated with various product-to-manufacturing plant allocation scenarios. This tool enables the planner to simultaneously consider thousands of different possible combinations of which products to produce in which plants, by analyzing manufacturing investment and per-vehicle operating cost estimates for each combination. The ability to impose constraints on the maximum number of body styles produced at any one plant yields insight on the value of pursuing a higher-mix (in terms of body styles) manufacturing strategy in particular plants, or across the entire plant footprint.by Matthew J. Hasik.M.B.A.S.M

Incommensurate Order with Translationally Invariant Projected Entangled-Pair States:Spiral States and Quantum Spin Liquid on the Anisotropic Triangular Lattice

Simulating strongly correlated systems with incommensurate order poses significant challenges for traditional finite-size-based approaches. Confining such a phase to a finite-size geometry can induce spurious frustration, with spin spirals in frustrated magnets being a typical example. Here, we introduce an Ansatz based on infinite projected entangled-pair states which overcomes these limitations and enables the direct search for the optimal spiral in the thermodynamic limit, with a computational cost that is independent of the spiral's wavelength. Leveraging this method, we simulate the Heisenberg model on the anisotropic triangular lattice, which interpolates between the square and isotropic triangular lattice limits. Besides accurately reproducing the magnetically ordered phases with arbitrary wavelength, the simulations reveal a quantum spin liquid phase emerging between the Néel and spin spiral phases.</p

Polyaniline/palladium nanohybrids for moisture and hydrogen detection.

Palladium nanoparticles display fascinating electronic, optical and catalytic properties, thus they can be used for various applications such as sensor fabrication. Conducting polymers such as polyaniline have also been widely used in sensor technology due to its cost effectiveness, versatility, and ease of synthesis. In this research, attention was given to unify the exceptional properties of these two materials and construct palladium nanoparticle coated polyaniline films to detect hydrogen and moisture. Electrochemical polymerization of aniline was carried out on gold sputtered epoxy resin boards. Polyaniline film was generated across a gap of 0.2 mm created by a scratch made on the gold coating prior to electrochemical polymerization. A palladium nanoparticle dispersion was prepared using sonochemical reduction method and coated on to polyaniline film using drop-drying technique. Polyaniline only films were also fabricated for comparative analysis. Sensitivity of films towards humidity and hydrogen was evaluated using impedance spectroscopy in the presence of the respective species. According to the results, polyaniline films exhibited an impedance drop in the presence of humidity and the response was significantly improved once palladium nanoparticles were incorporated. Interestingly, polyaniline only films did not respond to hydrogen. Nevertheless, palladium nanoparticle coated polyaniline films exhibited remarkable response towards hydrogen

Optimization of infinite projected entangled pair states: The role of multiplets and their breaking

The infinite projected entangled pair states (iPEPS) technique [J. Jordan et al., Phys. Rev. Lett. 101, 250602 (2008)] has been widely used in the recent years to assess the properties of two-dimensional quantum systems, working directly in the thermodynamic limit. This formalism, which is based upon a tensor-network representation of the ground-state wave function, has several appealing features, e.g., encoding the so-called area law of entanglement entropy by construction; still, the method presents critical issues when dealing with the optimization of tensors in order to find the best possible approximation to the exact ground state of a given Hamiltonian. Here, we discuss the obstacles that arise in the optimization by imaginary-time evolution within the so-called simple and full updates and connect them to the emergence of a sharp multiplet structure in the \u201cvirtual\u201d indices of tensors. In this case, a generic choice of the bond dimension D is not compatible with the multiplets and leads to a symmetry breaking (e.g., generating a finite magnetic order). In addition, varying the initial guess, different final states may be reached, with very large deviations in the magnetization value. In order to exemplify this behavior, we show the results of the S=1/2 Heisenberg model on an array of coupled ladders, for which a vanishing magnetization below the critical interladder coupling is recovered only for selected values of D, while a blind optimization with a generic D gives rise to a finite magnetization down to the limit of decoupled ladders