fruitful discussions, Guanghan Cao and Zhicheng Wang for assisting with 3 He-SQUID measurements, and Xiaoyan Xiao for assistance with single crystal x-ray diffraction.

We report on comprehensive results identifying the ground state of a triangular-lattice structured YbZnGaO_{4} as a spin glass, including no long-range magnetic order, prominent broad excitation continua, and the absence of magnetic thermal conductivity. More crucially, from the ultralow-temperature ac susceptibility measurements, we unambiguously observe frequency-dependent peaks around 0.1 K, indicating the spin-glass ground state. We suggest this conclusion holds also for its sister compound YbMgGaO_{4}, which is confirmed by the observation of spin freezing at low temperatures. We consider disorder and frustration to be the main driving force for the spin-glass phase.

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We report on the measurements of the superconducting order parameter in the nonmagnetic borocarbides LuNi2B2C and YNi2B2C. Andreev conductance spectra are obtained from nanoscale metallic junctions on single crystal surfaces prepared along three major crystallographic orientations: These observations are robust and reproducible among all the measurements on two different sets of LuNi2B2C crystals and one set of YNi2B2C crystals. We suggest that the possible gap nodes in the [100] direction may be masked by two effects: different gap anisotropy across multiple Fermi surfaces, as reported in the recent photoemission spectroscopy, and the large tunneling cone. Our results provide a consistent picture of the superconducting gap structure in these materials, addressing the controversy particularly in the reported results of point-contact Andreev reflection spectroscopy.

Luminescent oxygen sensors are devices in which the active element involves a luminescent dye in a polymer film. Oxygen partitions into the polymer from an adjacent gas or liquid phase and quenches the dye luminescence to an extent that depends on the amount of oxygen present in the film. When the dyes are dissolved in or attached to the molecules of a pure polymer film, the quenching kinetics can be described completely in terms of parameters that can be determined independently:  the excited-state lifetime of the dye and the permeability (P O 2 ) and diffusion coefficient (D O 2 ) of oxygen in the polymer. In many sensors, nanometer-sized inorganic particles are often added to the active matrix. These particles are added either as carriers for the dye molecules or to reinforce the polymer film. The presence of these particles in the polymer complicates the quenching kinetics, because both the dyes and the oxygen molecules partition between the polymer matrix and the particle surfaces. The purpose of this article is to review the factors that operate to affect quenching kinetics in particle-filled polymer films. We provide a brief review of polymer composite systems used in sensors and a more detailed review of the factors that affect quenching kinetics in these systems. We end with a description of more sophisticated models that have been employed to analyze oxygen quenching of dyes adsorbed on the surface of inorganic particles in the hopes that similar models might be developed in the future to describe oxygen quenching in polymer composite films.

In this paper, we examine the influence of 10 nm diameter silica nanospheres on oxygen diffusion in films of two different amorphous polymers characterized by a low glass-transition temperature and a high oxygen permeability. The two polymers, poly(dimethylsiloxane) (PDMS) and poly(n-butylamino thionylphosphazene) (C 4PATP), are useful matrixes for oxygen sensors based upon luminescence quenching. In these applications, the dye platinum octaethylporphine (PtOEP), with a long-lived excited state, is incorporated into the polymer, and the presence of oxygen is registered through a quenching of the dye luminescence. For some sensor applications, these linear polymers themselves are too soft and tacky. Silica as a filler improves the mechanical properties of the matrix but perturbs the measurement of oxygen diffusion and permeation. We show that PtOEP adsorbs to the silica particles in PDMS but remains in the polymer matrix in C4PATP. The quenching kinetics of dye fluorescence is complex in PDMS because of contributions of oxygen adsorbed to the silica surface and that dissolved in the polymer matrix. In contrast, the quenching kinetics in C4PATP remains almost unaffected by the presence of silica. Time-scan experiments on PDMS films show good accord with Fick's laws of diffusion for films containing up to 30 wt % (16 vol %) silica. The diffusion constant, DO2, is reduced but only by a factor of about two. In C4PATP, the time-scan experiments give more complex results because the oxygen adsorbed to the surface of the silica particles serves as an additional reservoir for oxygen in the system. Because the PtOEP remains in the polymer matrix in C4PATP, the oxygen adsorbed to the silica does not participate in quenching until it diffuses away from the particles and into the matrix.

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