Improved Lower Bounds for the Critical Probability of Oriented Bond Percolation in Two Dimensions

We present a coupled decreasing sequence of random walks on Z that dominate the edge process of oriented bond percolation in two dimensions. Using the concept of random walk in a strip, we describe an algorithm that generates an increasing sequence of lower bounds that converges to the critical probability of oriented percolation p_c. From the 7th term on, these lower bounds improve upon 0.6298, the best rigorous lower bound at present, establishing 0.63328 as a rigorous lower bound for p_c. Finally, a Monte Carlo simulation technique is presented; the use thereof establishes 0.64450 as a non-rigorous five-digit-precision (lower) estimate for p_c. Mathematics Subject Classification (1991): 60K35 Supported by CNPq (grant N.301637/91-1). Supported by a grant from CNPq.     

Recent study of nanomaterials prepared by inert gas condensation using ultra high vacuum chamber

The ultra high vacuum chamber was developed in the Department of Nuclear Physics, University of Madras with the funding from DST, India. This UHV chamber is used to prepare nanocrystalline materials by inert gas condensation technique (IGCT). Nanocrystalline materials such as PbF₂, Mn²⁺-doped PbF₂, Sn-doped In₂O₃ (ITO), ZnO, Al₂O₃, Ag₂O, CdO, CuO, ZnSe:ZnO etc., were prepared by this technique and characterized. Results of some of these materials will be presented in this paper. In solid-state²⁰⁷Pb NMR on PbF₂ a separate signal due to the presence of grain boundary has been observed. The structural phase transition pressure during the phase transformation from the cubic phase to orthorhombic phase under high pressure shows an increase with the decrease in grain size. Presence of electronic centres in nanocrystalline PbF₂ is observed from Raman studies and the same has been confirmed by photoluminescence studies. Al₂O₃ was prepared and⁵⁶Fe ions were implanted. After implantation segregation of⁵⁶Fe ions was examined by SEM. The oxidation properties of ITO were studied by HRTEM. As against the expectation of oxide coating on individual nanograins of In-Sn alloy, ITO nanograins grew into faceted nanograins on heat treatment in air and O₂ atmosphere. The growth of ITO under O₂ atmosphere showed pentagon symmetry. The PMN was initially prepared by solid-state reaction. Further, this PMN relaxor material will be used to convert into nanocrystalline PMN by IGCT with sputtering and will be studied     

Effect of cooling rate on microstructure of friction-stir welded AA1100 aluminum alloy

In this work, the microstructural changes occurring during cooling of friction-stir welded aluminum alloy AA1100 were evaluated. To this end, friction-stir welding (FSW) was performed in a wide range of cooling rates of 20–62 K/s and the evolved microstructures were studied by using electron backscatter diffraction. Below 0.6 T_m (T_m being the melting point), the stir zone material was found to experience no significant changes during cooling. At higher FSW temperatures, however, notable changes occurred in the welded material, including grain growth, sharpening of texture, reduction of the fraction of high-angle boundaries and material softening.     

Pourbaix diagrams in weakly coupled systems: a case study involving acridinol and phenanthridinol pseudobases

The thermodynamics of proton‐coupled electron transfer (PCET) in weakly coupled organic pseudobases was investigated using 2,7‐dimethyl‐9‐hydroxy‐9‐phenyl‐10‐tolyl‐9,10‐dihydroacridine (AcrOH) and 6‐phenylphenanthridinol (PheOH) as model compounds. Pourbaix diagrams for two model compounds were constructed using the oxidation potentials and the pK_a values obtained, respectively, from cyclic voltammetry and photometric titrations. Our comparative study reveals the importance of having the redox active –N center closer to –OH functionality on the thermodynamics of PCET process: PheOH exhibits a wider range of pH values (pH = 2.8 to 13.3) in which both the alcohol and the corresponding alkoxy radical are expected to coexist in solution. This result indicates that a concerted mechanism is more likely to be discovered in pseudobases analogous to PheOH. The thermochemical data also indicate that the concerted PCET mechanism cannot be achieved if water is used as the proton acceptor: assuming the pK_a of hydronium ions as ?1.7, the PCET involving PheOH or AcrOH as proton/electron donors and water as the proton acceptor is expected to follow the stepwise ET/PT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Ultra-low specific on-resistance high-voltage vertical double diffusion metal–oxide–semiconductor field-effect transistor with continuous electron accumulation layer

A new ultra-low specific on-resistance(Ron,sp) vertical double diffusion metal–oxide–semiconductor field-effect transistor(VDMOS) with continuous electron accumulation(CEA) layer, denoted as CEA-VDMOS, is proposed and its new current transport mechanism is investigated. It features a trench gate directly extended to the drain, which includes two PN junctions. In on-state, the electron accumulation layers are formed along the sides of the extended gate and introduce two continuous low-resistance current paths from the source to the drain in a cell pitch. This mechanism not only dramatically reduces the Ron,sp but also makes the Ron,sp almost independent of the n-pillar doping concentration(Nn). In off-state, the depletion between the n-pillar and p-pillar within the extended trench gate increases the Nn, and further reduces the Ron,sp.Especially, the two PN junctions within the trench gate support a high gate–drain voltage in the off-state and on-state, respectively. However, the extended gate increases the gate capacitance and thus weakens the dynamic performance to some extent. Therefore, the CEA-VDMOS is more suitable for low and medium frequencies application. Simulation indicates that the CEA-VDMOS reduces the Ron,sp by 80% compared with the conventional super-junction VDMOS(CSJ-VDMOS)at the same high breakdown voltage(BV).     

Electron-impact dissociation and transient properties of a stored LiH2 - beam

The fragmentation of LiH₂ ^- anions after electron impact was investigated at the heavy-ion storage ring TSR. The main reaction channel was found to be electron detachment followed by a breakup into LiH + H. In the first ms after production of the molecular ions in a cesium sputtering ion source, additional contributions were observed in the Li + H₂ and Li^- + H₂ channels, hinting at an initial population of a short-lived state of the anion. To gain a better understanding of the mechanisms underlying the observed behavior of the system, ab initio calculations of relevant potential energy surfaces were performed at selected geometries. The experimental findings are discussed in the light of these calculations.     

Role of the d-f Coulomb interaction in intermediate valence and Kondo systems: a numerical renormalization group study

Using the numerical renormalization group method, the dependences on temperature of the magnetic susceptibility χ(T) and specific heat C(T) are obtained for the single-impurity Anderson model with inclusion of d-f the Coulomb interaction. It is shown that the exciton effects caused by this effect (charge fluctuations) can significantly change the behaviour of C(T) in comparison with the standard Anderson model at moderately low temperatures, whereas the behaviour of χ(T) remains nearly universal. The ground-state and temperature-dependent renormalizations of the effective hybridization parameter and f-level position caused by the d-f interaction are calculated, and satisfactory agreement with the Hartree-Fock approximation is derived.     

Electromagnetic proton-neutron knockout off <Superscript>16</Superscript>O : New achievements in theory

Results for the cross-sections of the exclusive ¹⁶O(e, e'pn)¹⁴N and ¹⁶O(γ, pn)¹⁴N knockout reactions are presented and discussed in different kinematics. In comparison with earlier work, a complete treatment of the center-of-mass (CM) effects in the nuclear one-body current is considered in connection with the problem of the lack of orthogonality between initial bound and final scattering states. The effects due to CM and orthogonalization are investigated in combination with different treatments of correlations in the two-nucleon overlap function and for different parametrizations of the two-body currents. The CM effects lead in super-parallel kinematics to a dramatic increase of the ¹⁶O(e, e'pn) cross-section to the 1₂ ⁺ excited state (3.95MeV) of ¹⁴N . In all the situations considered the results are very sensitive to the treatment of correlations. A crucial role is played by tensor correlations, but also the contribution of long-range correlations is important.     

Size‐Dependent Room Temperature Oxidation of Tin Particles

Using transmission electron microscopy, the size‐dependent room temperature oxidation of tin nanoparticles is studied. The oxide that forms during room temperature oxidation of Sn particles is amorphous SnO, and it retains this stoichiometry and structure over extended time periods. From the investigation of arrays of Sn nanoparticles with broad size distribution, under identical conditions, the Sn oxide thickness is evaluated as a function of size and oxidation time. The oxide thickness depends strongly on the size of the Sn nanoparticles, which is in excellent agreement with predictions for a Mott–Cabrera model corrected for a non‐uniform electric field. The results demonstrate the accelerated oxidation kinetics of nanoscale particles with high curvature, due to the amplified electric field at the interface to a continuously shrinking metal core.     

High‐mobility solution‐processed zinc oxide thin films on silicon nitride

A high effective electron mobility of 33 cm² V^–1 s^–1 was achieved in solution‐processed undoped zinc oxide (ZnO) thin films. The introduction of silicon nitride (Si₃N₄) as growth substrate resulted in a mobility improvement by a factor of 2.5 with respect to the commonly used silicon oxide (SiO₂). The solution‐processed ZnO thin films grown on Si₃N₄, prepared by low‐pressure chemical vapor deposition, revealed bigger grain sizes, lower strain and better crystalline quality in comparison to the films grown on thermal SiO₂. These results show that the nucleation and growth mechanisms of solution‐processed films are substrate dependent and affect the final film structure accordingly. The substantial difference in electron mobilities suggests that, in addition to the grain morphology and crystalline structure effects, defect chemistry is a contributing factor that also depends on the particular substrate. In this respect, interface trap densities measured in high‐κ HfO₂/ZnO MOSCAPs were about ten times lower in those fabricated on Si₃N₄ substrates. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)