Polybenzopinacols. I. Photolytic coupling of aromatic diketones

Low molecular weight polybenzopinacols were obtained by the photolytic coupling of m- and p-dibenzoylbenzene and 4,4′-dibenzoyldiphenyl ether in isopropanol, tetrahydrofuran–isopropanol, benzene–isopropanol, and benzene–ethanol solutions. The polypinacols were soluble in common organic solvents such as tetrahydrofuran, ether, and benzene. The inherent viscosities ranged from 0.06 to 0.14. Average molecular weight (M?_n) data indicated that the polymers were mostly dimers and trimers.     

Lattice dynamics and elastic constants of scandium

The lattice dynamics and the second order elastic constants of scandium are calculated with twelve second order parameters using Keating's approach. The agreement between theory and measurement is satisfactory. The results are compared with those of Wakabayashi et al. who have used the MAS model with seventeen parameters to fit their dispersion data.     

First passage time distribution in an oscillating field

Siegert's integral equation approach to calculate the first passage time distribution is generalized to the case of a one-dimensional diffusion process in an oscillating drift field. A simple algorithm to solve the integral equations is developed and numerical results are presented.     

Direct observation of the flux-line vortex glass phase in a type II superconductor

The order of the vortex state in La1.9Sr0.1CuO4 is probed using muon-spin rotation and small-angle neutron scattering. A transition from a Bragg glass to a vortex glass is observed, where the latter is composed of disordered vortex lines. In the vicinity of the transition the microscopic behavior reflects a delicate interplay of thermally induced and pinning-induced disorder.     

Nonreciprocity of counterpropagating signals in a monolithically integrated Sagnac interferometer

We demonstrate nonlinearly induced nonreciprocity of counterpropagating waves in a monolithically integrated Sagnac interferometer that employs a semiconductor optical amplifier as the nonlinear element. We show that the dependence of the linewidth enhancement factor on charge injection can influence the third-order nonlinearity in the semiconductor gain medium to a surprisingly large degree. This effect is utilized to control the phases of the counterpropagating signals in the interferometer. A theoretical model is used to explain the experimental observations. We show that these effects have significant practical implications by demonstrating an all-optical controlled-NOT gate.     

Distribution functions, loop formation probabilities, and force-extension relations in a model for short double-stranded DNA molecules

We obtain, using transfer-matrix methods, the distribution function P(R) of the end-to-end distance, the loop formation probability, and force-extension relations in a model for short double-stranded DNA molecules. Accounting for the appearance of "bubbles," localized regions of enhanced flexibility associated with the opening of a few base pairs of double-stranded DNA in thermal equilibrium, leads to dramatic changes in P(R) and unusual force-extension curves. An analytic formula for the loop formation probability in the presence of bubbles is proposed. For short heterogeneous chains, we demonstrate a strong dependence of loop formation probabilities on sequence.     

Rectification properties of carbon nanotube "Y-junctions"

Quantum conductivity of single-wall carbon nanotube Y-junctions is calculated. The current versus voltage characteristics of these junctions show asymmetry and rectification, in agreement with recent experimental results. Furthermore, rectification is found to be independent of the angle between the branches of these junctions, indicating this to be an intrinsic property of symmetric Y-junctions. The implications for the Y-junction to function as a nanoscale molecular electronic switch are investigated.     

Stability constant for the lanthanide fluoride complexes in aqueous solution at 25°C

Stability constants for the mono- and difluoride complexes of fourteen lanthanides in aqueous solution 25°C and an ionic strenght of 0.5 M have been measured using the newly developed Orion expandable and digital ion analyzer and their fluoride specific ion electrode. A simplified potentiometric procedure was employed to measure the stability constant for the monofluoride complex and this value was used to compute analytically the stability constant for the difluoride complex in separate experiments. In the case of heavy lanthanides a linear regression method was also used to compute simultaneously the stability constant for both complexes. The stability constant for the monofluoride complexes may be arranged into four groups that shows the tetrad effect with maxima at Pr, Sm, Dy and Er. There is a gradual but unsystematic increase in the stability constant for the monofluoride complex with atomic number of the lanthanides.     

Exact results on scattering by the potentialae −λr /r withλ=+0

A new, non-perturbative, coordinate space method is formulated to calculate the full and partial wave amplitudes for the potentialae ^−λr /r withλ=+0. The basic ingredients are a plausible use of the point Coulomb wave function up to moderate distances and a Wronskian identity to take care of the large distance behaviour of integrands.