On the Anomalous Balmer Line Strengths in Globular Clusters

Spectral feature index diagrams with integrated globular clusters and simple stellar population models often show that some clusters have weak H beta, so weak that even the oldest models cannot match the observed feature depths. In this work, we rule out the possibility that abundance mixture effects are responsible for the weak indices unless such changes operate to cool the entire isochrone. We discuss this result in the context of other explanations, including horizontal branch morphology, blue straggler populations, and nebular or stellar emission fill-in, finding a preference for flaring in M giants as an explanation for the H beta anomaly.Comment: Submitted to the A

Persistant Photoconductivity of Strontium Titanate

Strontium titanate (SrTiO$_3$) is a transparent conducting oxide with a range of interesting properties, including a large, temperature-dependent dielectric constant and superconductivity at low temperatures. It has a wide indirect band gap of 3.2 eV at room temperature. Annealing in a reducing atmosphere with additional strontium oxide (SrO) powder at 1200^\\mathrm{o}C results in the creation of native defects. These annealed samples show persistent photoconductivity (PPC) at room temperature, when exposed to light of energy 2.9 eV or greater. The three or more order of magnitude change in resistance persists long after the light is turned off. This effect is attributed to an electron being excited from an acceptor defect, with a large barrier for recapture, to the conduction band.This work investigates many of the changes that occur and factors that affect PPC. The right amount of SrO powder is crucial to the formation of PPC. The presence of some oxygen vacancies is also necessary for PPC; however, too many will mute the dramatic change in resistivity. Peaks at 430 nm and 520 nm appear in the visible region of the spectrum. The peak at 430 nm is due to iron, while the peak at 520 nm has not been identified. The infrared region of the spectrum also shows changes. First, the intensity of the transmitted signal drops significantly after light exposure, due to free carrier absorption.Additionally, a hydrogen line at 3500 cm$^{-1}$ and satellites are often observed in as-received samples. The satellites disappear during annealing and return during PPC. The hydrogen lines have the same thermal kinetics as the 520 nm peak. Hydrogen lines at 3355 and 3384 cm$^{-1}$, if present, will prevent PPC. An exposed chip can be erased (i.e. returned to its pre-light exposed state) by using a heat treatment.Erasing and polishing an annealed chip prior to light exposure can result in weakly $p$-type behavior with high mobility holes ( $>$ 100 cm$^2$/Vs). This is an order of magnitude higher than those commonly measured for electrons (5-10 cm$^2$/Vs). The average hole densities were in the 10$^9$-10$^{10}$ cm$^{-3}$ range, consistent with a deep acceptor

P-type conductivity in annealed strontium titanate

Hall-effect measurements indicate p-type conductivity in bulk, single-crystal strontium titanate (SrTiO3, or STO) samples that were annealed at 1200°C. Room-temperature mobilities above 100 cm2/V s were measured, an order of magnitude higher than those for electrons (5-10 cm2/V s). Average hole densities were in the 109-1010 cm−3 range, consistent with a deep acceptor