Backward extensions of subnormal operators

The concept of backward extension for subnormal weighted shifts is generalized to arbitrary subnormal operators. Several differences and similarities in these contexts are explored, with emphasis on the structure of the underlying measures.

     

SPSA vs Simplex in statistical machine translation optimization

Most statistical machine translation systems are combinations of various models and tuning scaling factors is an important step. However, this optimisation problem is hard because the objective function has many local minima and the available algorithms cannot achieve a global optimum. Consequently, optimisations starting from different initial settings can converge to fairly different solutions. We present tuning experiments with the Simultaneous Perturbation Stochastic Approximation (SPSA) algorithm, and compare them with the widely used downhill simplex method. With IWSLT 2005 Chinese-English data, both methods showed similar performance, but SPSA was more robust to the choice of initial settings. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simple modeling of dipolar coupled 7Li spins and stimulated-echo spectroscopy of single-crystalline beta-eucryptite

Stimulated-echo spectroscopy has recently been applied to study the ultra-slow dynamics of nuclear spin-3/2 probes such as 7Li and 9Be in solids. Apart from the dominant first-order quadrupolar interaction in the present article also the impact of the homonuclear dipolar interactions is considered in a simple way: the time evolution of a dipole coupled pair of spins with I = 3/2 is calculated in an approximation, which takes into account that the satellite transitions usually do not overlap. Explicit analytical expressions describing various aspects of a coupled quadrupolar pair subjected to a Jeener-Broekaert pulse sequence are derived. Extensions to larger spin systems are also briefly discussed. These results are compared with experimental data on a single-crystalline Li ion conductor.     

Anomalous behavior of superconducting-normal mesoscopic structures near T(c)

We observe a maximum in the conductance of Al/n-GaAs junctions at temperatures 20 mK lower than the superconducting transition temperature (T(c)). This is the first observation of a peak in the conductance near the superconducting transition in superconducting-normal (S/N) junctions. To accommodate this effect we calculate the full temperature dependence of the conductance of these structures, invoking quasiclassical Green's functions in the diffusive limit. In addition to the well-known low-temperature peak at temperatures on the order of the Thouless energy, we find a maximum near T(c). This peak has the same origin as the subgap conductance observed in S/N junctions at low temperatures.     

Vertical and nonvertical participation by sulfur,selenium, and tellurium

The mechanism of stabilization of positive charge on carbon by sulfur, selenium, or tellurium at the beta-position has been investigated kinetically, by measurement of rate enhancements, and structurally, by variation of the bond strength to the leaving group. Stabilization can occur either nonvertically with formation of a bridged intermediate or vertically through hyperconjugation within an open carbocation. We observed large rate enhancements (10(5) for S, 10(6) for Se) in 97% aqueous trifluoroethanol with trifluoroacetate as the leaving group. These enhancements are consistent with either mechanism. Product structures also are consistent with either mechanism. Nine crystal structures revealed that the bond to the leaving group (C-O) is lengthened by the presence of S or Se at the beta-position, in proportion to the basicity of the leaving group. This lengthening is not accompanied by angle distortions expected for the bridging mechanism. The crystallographic data support vertical (hyperconjugative) character along the reaction coordinate, more so for selenium than sulfur.     

Temperature dependence of the Raman cross section and light absorption in cubic BaTiO3

The temperature dependence of the Raman spectrum in the cubic phase of BaTiO₃ is described. The cross section decreases when the temperature increases, which is contrary to a normal second order effect behaviour. The associated measurements of the optical absorption definitely prove that this variation is intrinsic. The anomalous Raman spectrum is related to the existence in the crystal of anisotropic fluctuation domains.