William Savin

High-energy mass spectrometry of 10Be using a tandem van de graaff accelerator

Abstract A method of measurement of specific 10Be contents of naturally occurring materials is described. The method is based on mass spectrometry of 10Be/9Be ratios, using the Rutgers-Bell tandem van de Graaff accelerator. The detection limit is currently approximately 107 atoms 10Be, and the precision, when not limited by counting statistics, is 13%. The linear proportionality of observed 10Be/9Be ratio to actual ratio in sample has been verified within uncertainty over more than two orders of magnitude in ratio. Results are presented for the application of the method to determination of 10Be contents of six samples from various locations in the Antarctic H-chondrite ALHA 78084. No significant variations among the samples due to differences in shielding from cosmic rays were observed.

Infrared quenching of photoinduced persistent conductivity in YBa2Cu3O6+x

Experimental evidence for infrared quenched photoinduced persistent conductivity in oxygen deficient YBa2Cu3O6+x films is presented. Results are interpreted in terms of both a charge transfer model, in which photogenerated electrons are trapped in oxygen vacancy defects, and a photoassisted oxygen ordering model. Infrared illumination of the photoinduced state results in detrapping of electrons, a recombination of photogenerated holes/electrons, and a partial quenching of the photoinduced state. Experimental results suggest the possibility of optically controlled superconducting devices in which the superconducting state is generated and quenched with different colors of light.

Local structural and electronic changes accompanying photodoping in YBA2Cu3O6+x

Abstract Polarized X-ray absorption measurements reveal that photodoping of oxygen deficient YBa2Cu3O6+x (YBCO) produces a local axial distortion of the CuOx chains. The broad normal state axial Cu(1)O(4) bond distribution becomes two well-defined peaks in the photodoped state and suggest local ordering. XANES measurements reveal a transfer of hole density into the CuO2 planes. These results support a common structure-related transport enhancement mechanism in chemically doped and photodoped high temperature superconductor systems.

Defect mechanism of photoinduced superconductivity in YYBa 2 Cu 3 O x

Persistent photoconductivity and photoinduced superconductivity have been previously observed in various stoichiometries of YBa2Cu3Ox. The physical mechanism which is responsible for these effects is still under debate. Through an investigation of laser ablated YBa2Cu3Ox thin film compositions, photoluminescence spectra and infrared spectroscopy, the possibility that the mechanism may be defect related has been supported. A correlation between photoluminescence spectra and wavelength dependence support an oxygen defect model of photoinduced persistent conductivity and superconductivity in which the oxygen vacancies act as weakly luminescent F-centers and F+-centers under illumination. Upon infrared illumination the trapped electrons are photoexcited resulting in a partial quenching of the persistent photoconductivity state.