On the quantum phase transition of a quantum Hall liquid

It is shown that, in the absence of disorder, a quantum Hall liquid undergoes a second-order quantum phase transition as function of the applied magnetic field. The analysis is carried out in the framework of the Chem-Simons-Ginzburg-Landau theory which is shown to exhibit a nontrivial infrared stable fixed point. The corresponding critical exponents are found to be gaussian, and thus universal and independent of the filling fraction.     

Enantioselective addition of organolithium reagents to imines mediated by C2-symmetric bis(aziridine) ligands

The C₂-symmetric bis(aziridine) ligands 1–5 have been screened in the enantioselective addition of organolithium reagents to imines. Ligand 1 (used in stoichiometric amounts) was found to be superior in terms of chemical yield and enantioselectivity, the best result being 90% yield and 89% e.e. in the addition of vinyllithium to imine 6a. Use of ligand 1 in substoichiometric amounts gave poorer yield and lower enantioselectivity. The enantioselectivity of the reaction was investigated as a function of substrate, reagent, stoichiometry and temperature, but no firm mechanistic conclusions could be drawn. Preliminary results with deuterium-labelled methyllithium indicate complexation/exchange processes involving ligand, reagent and substrate.     

Molecular simulation of gas permeability: poly(2,6-dimethyl-1,4-phenylene oxide)

Self-diffusion and solubility coefficients of four gas molecules (O₂, N₂, CH₄, and CO₂) in poly(2,6-dimethyl-1,4-phenylene oxide) have been investigated by means of molecular simulation using the COMPASS molecular mechanics force field. Diffusion coefficients were obtained from molecular dynamics (NVT ensemble) using up to 2 ns simulation times. Solubility coefficients were obtained by means of the Widom particle insertion method. Simulation values for O₂, N₂, and CH₄ agree reasonably well with published data. Agreement was less satisfactory for CO₂. Possible explanations for the CO₂ results are discussed on the basis of the partial immobilization model and considerations of simulation time and the size of the simulation box.     

Optimization of the texture formation and characterization of large size top-seeded-melt-grown YBCO pellets

The thermal texture cycle for the top seeding of Ce doped YBCO pellets has been studied and optimised. Several quenches have proved the importance of a presintering step to have a homogeneous distribution of the Y₂BaCuO₅ particles in the melt. The temperature window, in which a heterogeneous nucleated grain can grow, without being hindered by homogeneous nucleated grains, has been determined. By adjusting the process rate to the crystal growth rate in this window, large well textured single domains of YBa₂Cu₃O_7−δ have been obtained, with a high degree of reproducibility. Microstructure and texture homogeneity, as well as superconducting properties have been investigated as a function of the position in the bulk pellet. The results have shown the high quality of the textured sample, and its high homogeneity.     

快速电子辐照F-40的ESR和DSC研究

室温时空气中辐照F-40样品中残存有链侧和链端过氧化自由基,这些自由基在室温下衰变缓慢.DSC研究表明,F-40样品熔融温度T_m随辐照剂量的增加而下降,本文用聚合物片晶熔融的一般模型加以解释.     

快速电子辐照F-40残存自由基热衰减的ESR研究

空气中辐照F-40样品中残存的链侧过氧自由基没有链端过氧自由基稳定.这些自由基的热衰减呈明显二级反应特征.反应表观活化能与指前因子这两个对反应速率常数起着相反影响的动力学参数之间存在着动力学补偿效应,表明自由基热衰减是同时在不同活性中心上进行的复杂反应.     

Calculated energy barriers for the identity SN2 reaction H2O + CH3OH2+ → +H2OCH3 + OH2 in the gas phase,in water clusters,and in aqueous solution

The bimolecular nucleophilic substitution reaction H₂O + CH₃OH₂⁺ → ⁺H₂OCH₃ + OH₂ has been studied using various quantum chemical methods. Accurate barriers for the reaction in the gas phase are presented and discussed. The effect of microsolvation by water molecules in small clusters has been investigated. Extrapolation of the barrier obtained in the small clusters, using a linear relationship between the activation energy and the proton affinity of water clusters, gives a barrier for the reaction in aqueous solution which is in good agreement with that obtained in separate model calculations (polarized continuum model of a super molecule with the first solvation shell included).     

Magnetic and magneto-optical properties of MnSb films on various substrates

Magnetic and magneto-optical properties of MnSb films with different crystalline orientations on various semiconductors of GaAs(1 0 0), GaAs(1 1 1)A, B, and sapphire(0 0 0 1) have been measured by a vibrating sample magnetometer (VSM) and a home-made magneto-optical Kerr effect (MOKE) system. All these samples have their easy axes in the plane and show ferromagnetic properties. Among these samples, the film on GaAs(1 1 1)B has the lowest coercive force H_c and the largest squareness (SQ) value, whereas the film on GaAs(1 0 0) shows the largest H_c and the lowest SQ value. A large Kerr rotation angle of about 0.3° was observed at a wavelength of λ=632.8 nm for the film on sapphire in the field applied both parallel and perpendicular to the film plane. However, the MnSn films on other substrates do not have an observable Kerr rotation. The dynamic effect of the hysteresis was also measured using our MOKE system. As the frequency of applied magnetic field increases, the loop rounds off at the corners and the loop area increases.     

Symbole de Newton. Symbole de Jacobi-Carlitz

First of all we define the “Newton symbol” of two polynomials with coefficients in a commutative ring. The “Artinian symbol” of two polynomials of F₂[t] is then defined by analogy with the quadratic residue symbol. We prove that the Artinian symbol satisfies an “Euler's criterion”, and we define a “Jacobi-Carlitz symbol” in terms of the Newton symbol.