Molecular mobility of substituted poly(p-phenylenes) characterized by a range of polymer relaxation techniques

The free volume and related mobility properties of substituted poly(p-phenylene) polymers are examined. The techniques used range from positron annihilation, dielectric relaxation, and dynamic mechanical spectroscopy to thermally stimulated currents. Fractional free volume is determined for the samples with different substituted side groups and related to the glass transition temperature. Bulkier groups lead to a greater fractional free volume and lower glass transition temperatures. Comparison of molecular relaxation times using the different characterization techniques demonstrates that there is strong coupling between motion of the main chain and the side groups, on which the dipoles reside. Intermolecular coupling between the main chains at the primary relaxation is shown in this work to be related to the nature of the side chains and resultant free volume, as are the temperature locations of local, secondary relaxations. A qualitative model describing the effect of regiochemistry on the motions and packing of these materials is also proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1465–1481, 1998     

Finite element approximation of a model for phase separation of a multi-component alloy with a concentration-dependent mobility matrix

We consider a model for phase separation of a multi-componentalloy with a concentration-dependent mobility matrix and logarithmicfree energy. In particular we prove that there exists a uniquesolution for sufficiently smooth initial data. Further, we provean error bound for a fully practical piecewise linear finiteelement approximation in one and two space dimensions. Finallynumerical experiments with three components in one space dimensionare presented.     

The mechanism of the norrish type II reaction of α-keto-acids and esters

Methyl-α-keto-octanoate undergoes a Type II reaction to give pent-1-ene and photophysical measurements show that this and the Type II reactions of α-keto-acids occur, contrary to previous claims, from the excited singlet state.     

Near- and far-field emission distributions of mesa stripe geometry double heterostructure lasers

Mesa stripe geometry double heterostructure lasers have been made with cavity widths down to 16 μm. The lowest threshold current values are down to 75 mA but more typical values are 110 mA, and the slope efficiencies are typically 40%. Measurements of near- and far-field distributions have shown that the problem of high order mode structure parallel to the junction is not as serious as expected although emission is normally in the lowest order only for very low drive currents. Improved mode control is demonstrated by a hybrid mesa-contact stripe geometry device which gave a pulse power output of 30 mW in the lowest order mode at a drive current of 310 mA.     

Variance or spectral density in sampled data filtering?

Most physical systems operate in continuous time. However, to interact with such systems one needs to take samples. This raises the question of the relationship between the sampled response and the response of the underlying continuous-time system. In this paper we review several aspects of the sampling process. In particular, we examine the role played by variance and spectral density in describing discrete random processes. We argue that spectral density has several advantages over variance. We illustrate the ideas by reference to the problem of state estimation using the discrete-time Kalman filter.     

Mn(I) in an extended oxide: the synthesis and characterization of La(1-x)Ca(x)MnO(2+δ) (0.6 ≤ x ≤ 1)

Reduction of La(1-x)Ca(x)MnO(3) (0.6 ≤ x ≤ 1) perovskite phases with sodium hydride yields materials of composition La(1-x)Ca(x)MnO(2+δ). The calcium-rich phases (x = 0.9, 1) adopt (La(0.9)Ca(0.1))(0.5)Mn(0.5)O disordered rocksalt structures. However local structure analysis using reverse Monte Carlo refinement of models against pair distribution functions obtained from neutron total scattering data reveals lanthanum-rich La(1-x)Ca(x)MnO(2+δ) (x = 0.6, 0.67, 0.7) phases adopt disordered structures consisting of an intergrowth of sheets of MnO(6) octahedra and sheets of MnO(4) tetrahedra. X-ray absorption data confirm the presence of Mn(I) centers in La(1-x)Ca(x)MnO(2+δ) phases with x < 1. Low-temperature neutron diffraction data reveal La(1-x)Ca(x)MnO(2+δ) (x = 0.6, 0.67, 0.7) phases become antiferromagnetically ordered at low temperature.     

New intraband magneto-optical studies on n-InSb

A highly sensitive photoconductive technique has been used to provide the first direct evidence of emission of five optical phonons during the energy relaxation process of the photoexcited electrons induced by a CO₂ laser in n-InSb. The CO₂ laser-induced magnetophotoconductivity exhibits a number of very sharp resonances whose positions depend strongly upon photon energy. All observed transitions can be described with one single set of band parameters.