Folding of the triangular lattice in a discrete three-dimensional space: Crumpling transitions in the negative-bending-rigidity regime

Folding of the triangular lattice in a discrete three-dimensional space is studied numerically. Such ``discrete folding'' was introduced by Bowick and co-workers as a simplified version of the polymerized membrane in thermal equilibrium. According to their cluster-variation method (CVM) analysis, there appear various types of phases as the bending rigidity K changes in the range -infty < K < infty. In this paper, we investigate the K<0 regime, for which the CVM analysis with the single-hexagon-cluster approximation predicts two types of (crumpling) transitions of both continuous and discontinuous characters. We diagonalized the transfer matrix for the strip widths up to L=26 with the aid of the density-matrix renormalization group. Thereby, we found that discontinuous transitions occur successively at K=-0.76(1) and -0.32(1). Actually, these transitions are accompanied with distinct hysteresis effects. On the contrary, the latent-heat releases are suppressed considerably as Q=0.03(2) and 0.04(2) for respective transitions. These results indicate that the singularity of crumpling transition can turn into a weak-first-order type by appreciating the fluctuations beyond a meanfield level

Crystal growth of device quality GaAs in space

It was established that the findings on elemental semiconductors Ge and Si regarding crystal growth, segregation, chemical composition, defect interactions, and materials properties-electronic properties relationships are not necessarily applicable to GaAs (and to other semiconductor compounds). In many instances totally unexpected relationships were found to prevail. It was further established that in compound semiconductors with a volatile constituent, control of stoichiometry is far more critical than any other crystal growth parameter. It was also shown that, due to suppression of nonstoichiometric fluctuations, the advantages of space for growth of semiconductor compounds extend far beyond those observed in elemental semiconductors. A novel configuration was discovered for partial confinement of GaAs melt in space which overcomes the two major problems associated with growth of semiconductors in total confinement. They are volume expansion during solidification and control of pressure of the volatile constituent. These problems are discussed in detail

Computation of liquid-liquid equilibria and phase stabilities: implications for RH-dependent gas/particle partitioning of organic-inorganic aerosols

Semivolatile organic and inorganic aerosol species partition between the gas and aerosol particle phases to maintain thermodynamic equilibrium. Liquid-liquid phase separation into an organic-rich and an aqueous electrolyte phase can occur in the aerosol as a result of the salting-out effect. Such liquid-liquid equilibria (LLE) affect the gas/particle partitioning of the different semivolatile compounds and might significantly alter both particle mass and composition as compared to a one-phase particle. We present a new liquid-liquid equilibrium and gas/particle partitioning model, using as a basis the group-contribution model AIOMFAC (Zuend et al., 2008). This model allows the reliable computation of the liquid-liquid coexistence curve (binodal), corresponding tie-lines, the limit of stability/metastability (spinodal), and further thermodynamic properties of multicomponent systems. Calculations for ternary and multicomponent alcohol/polyol-water-salt mixtures suggest that LLE are a prevalent feature of organic-inorganic aerosol systems. A six-component polyol-water-ammonium sulphate system is used to simulate effects of relative humidity (RH) and the presence of liquid-liquid phase separation on the gas/particle partitioning. RH, salt concentration, and hydrophilicity (water-solubility) are identified as key features in defining the region of a miscibility gap and govern the extent to which compound partitioning is affected by changes in RH. The model predicts that liquid-liquid phase separation can lead to either an increase or decrease in total particulate mass, depending on the overall composition of a system and the particle water content, which is related to the hydrophilicity of the different organic and inorganic compounds. Neglecting non-ideality and liquid-liquid phase separations by assuming an ideal mixture leads to an overestimation of the total particulate mass by up to 30% for the composition and RH range considered in the six-component system simulation. For simplified partitioning parametrizations, we suggest a modified definition of the effective saturation concentration, C_j^*, by including water and other inorganics in the absorbing phase. Such a C_j^* definition reduces the RH-dependency of the gas/particle partitioning of semivolatile organics in organic-inorganic aerosols by an order of magnitude as compared to the currently accepted definition, which considers the organic species only

Synthesis And Characterization Of Transition Metal Arsenide Nanocrystals And The Metastability And Magneto-Structural Phase Transition Behavior Of Mnas Nanocrystals

This dissertation study focuses on (1) probing the magneto-structural phase transformation in nanoscale MnAs; (2) evaluation of the size-dependent phase stability of type-B MnAs (prepared by rapid injection); and (3) developing a general synthetic method for transition metal arsenide nanoparticles. Discrete MnAs nanoparticles that adopt different structures at room temperature (type-A, α-structure and type-B, β-structure) have been prepared by the solution-phase arrested precipitation method. Atomic pair distribution and Rietveld refinement were employed on synchrotron data to explore the structural transitions of the bulk and nanoparticle samples, and these results were compared to AC magnetic susceptibility measurements of the samples. The results confirm that the structural transition and the magnetic transition are highly correlated in bulk MnAs and suggest that for type-A MnAs nanoparticles, there is a similar magneto-structural phase transition occurring in the same temperature region as that for bulk MnAs. However, for type-B MnAs nanoparticles, there is no magneto-structural phase transition, consistent with that type-B MnAs nanoparticles being kinetically trapped in the β-structure. Type-B MnAs nanoparticles adopting the β-strucuture undergo a transformation from β to α upon cooling. Temperature dependent XRD studies and magnetic measurements suggest that the TP for α → β conversion is suppressed to lower temperatures relative to bulk and type-A MnAs nanoparticles and that the transformation is reversible but has an enhanced hysteresis, which results in a large coexistence temperature range for the α and β structure. The transformation temperature correlated with the compression of the lattice parameters of the type-B MnAs nanoparticles due to the decrease in the particle size or the presence of chemical doping, or both. A new general synthetic method for transition metal arsenide (Ni11As8, FeAs and CoAs) nanocrystals synthesis was developed by directly injecting the metal precursor into pre-reacted arsenic precursors. This method enables more control of the nanoparticle growth and monodispersity than is achieved by the direct conversion of metal nanoparticles

Effect of wearout processes on the critical timing parameters and reliability of CMOS bistable circuits

The objective of the research presented in this thesis was to investigate the effects of wearout processes on the performance and reliability of CMOS bistable circuits. The main wearout process affecting reliability of submicron MOS devices was identified as hot-carrier stress (and the resulting degradation in circuit performance). The effect of hot-carrier degradation on the resolving time leading to metastability of the bistable circuits also have been investigated. Hot-carrier degradation was identified as a major reliability concern for CMOS bistable circuits designed using submicron technologies. The major hot-carrier effects are the impact ionisation of hot- carriers in the channel of a MOS device and the resulting substrate current and gate current generation. The substrate current has been used as the monitor for the hot-carrier stress and have developed a substrate current model based on existing models that have been extended to incorporate additional effects for submicron devices. The optimisation of the substrate current model led to the development of degradation and life-time models. These are presented in the thesis. A number of bistable circuits designed using 0.7 micron CMOS technology design rules were selected for the substrate current model analysis. The circuits were simulated using a set of optimised SPICE model parameters and the stress factors on each device was evaluated using the substrate current model implemented as a post processor to the SPICE simulation. Model parameters for each device in the bistable were degraded according to the stress experienced and simulated again to determine the degradation in characteristic timing parameters for a predetermined stress period. A comparative study of the effect of degradation on characteristic timing parameters for a number of latch circuits was carried out. The life-times of the bistables were determined using the life-time model. The bistable circuits were found to enter a metastable state under critical timing conditions. The effect of hot-carrier stress induced degradation on the metastable state operation of the bistables were analysed. Based on the analysis of the hot-carrier degradation effects on the latch circuits, techniques are suggested to reduce hot-carrier stress and to improve circuit life-time. Modifications for improving hot- carrier reliability were incorporated into all the bistable circuits which were re-simulated to determine the improvement in life-time and reliability of the circuits under hot-carrier stress. The improved circuits were degraded based on the new stress factors and the degradation effects on the critical timing parameters evaluated and these were compared with those before the modifications. The improvements in the life-time and the reliability of the selected bistable circuits were quantified. It has been demonstrated that the hot-carrier reliability for all the selected bistable circuits can be improved by design techniques to reduce the stress on identified critically stressed devices