New directions in InP solar cell research

Recent research efforts representing new directions in InP solar cell research are reviewed. These include heteroepitaxial growth on silicon and gallium arsenide substrates, V-grooved cells, large area high efficiency cells, and surface passivation. Improvements in heteroepitaxial cell efficiency are described together with processing of 19.1 percent, 4 sq cm cells. Recommendations are made for improvements in processing leading to increased efficiencies

Monitoring the Very-Long-Term Variability of X-ray Sources in the Giant Elliptical Galaxy M87

We report on our search for very-long-term variability (weeks to years) in X-ray binaries (XRBs) in the giant elliptical galaxy M87. We have used archival Chandra imaging observations to characterise the long-term variability of 8 of the brightest members of the XRB population in M87. The peak brightness of some of the sources exceeded the ultra luminous X-ray source (ULX) threshold luminosity of ~ 10^{39} erg/s, and one source could exhibit dips or eclipses. We show that for one source, if it has similar modulation amplitude as in SS433, then period recoverability analysis on the current data would detect periodic modulations, but only for a narrow range of periods less than 120 days. We conclude that a dedicated monitoring campaign, with appropriately defined sampling, is essential if we are to investigate properly the nature of the long-term modulations such as those seen in Galactic sources.Comment: 19 pages, 19 figures, 1 table, Accepted for publication in MNRAS. Updated to correct typos in previous versio

A double junction model of irradiated silicon pixel sensors for LHC

In this paper we discuss the measurement of charge collection in irradiated silicon pixel sensors and the comparison with a detailed simulation. The simulation implements a model of radiation damage by including two defect levels with opposite charge states and trapping of charge carriers. The modeling proves that a doubly peaked electric field generated by the two defect levels is necessary to describe the data and excludes a description based on acceptor defects uniformly distributed across the sensor bulk. In addition, the dependence of trap concentrations upon fluence is established by comparing the measured and simulated profiles at several fluences and bias voltages.Comment: Talk presented at the 10th European Symposium on Semiconductor Detectors, June 12-16 2005, Wildbad Kreuth, Germany. 9 pages, 4 figure

Magnetic Moment Formation in Graphene Detected by Scattering of Pure Spin Currents

Hydrogen adatoms are shown to generate magnetic moments inside single layer graphene. Spin transport measurements on graphene spin valves exhibit a dip in the non-local spin signal as a function of applied magnetic field, which is due to scattering (relaxation) of pure spin currents by exchange coupling to the magnetic moments. Furthermore, Hanle spin precession measurements indicate the presence of an exchange field generated by the magnetic moments. The entire experiment including spin transport is performed in an ultrahigh vacuum chamber, and the characteristic signatures of magnetic moment formation appear only after hydrogen adatoms are introduced. Lattice vacancies also demonstrate similar behavior indicating that the magnetic moment formation originates from pz-orbital defects.Comment: accepted to Phys. Rev. Let

UHF and VHF radar observations of thunderstorms

A study of thunderstorms was made in the Summer of 1985 with the 430-MHz and 50-MHz radars at the Arecibo Observatory in Puerto Rico. Both radars use the 300-meter dish, which gives a beam width of less than 2 degrees even at these long wavelengths. Though the radars are steerable, only vertical beams were used in this experiment. The height resolution was 300 and 150 meters for the UHF and VHF, respectively. Lightning echoes, as well as returns from precipitation and clear-air turbulence were detected with both wavelengths. Large increases in the returned power were found to be coincident with increasing downward vertical velocities at UHF, whereas at VHF the total power returned was relatively constant during the life of a storm. This was attributed to the fact that the VHF is more sensitive to scattering from the turbulence-induced inhomogeneities in the refractive index and less sensitive to scatter from precipitation particles. On occasion, the shape of the Doppler spectra was observed to change with the occurrence of a lightning discharge in the pulse volume. Though the total power and mean reflectivity weighted Doppler velocity changed little during these events, the power is Doppler frequency bins near that corresponding to the updraft did increase substantially within a fraction of a second after a discharge was detected in the beam. This suggests some interaction between precipitation and lightning

Oscillatory Spin Polarization and Magneto-Optic Kerr Effect in Fe3O4 Thin Films on GaAs(001)

The spin dependent properties of epitaxial Fe3O4 thin films on GaAs(001) are studied by the ferromagnetic proximity polarization (FPP) effect and magneto-optic Kerr effect (MOKE). Both FPP and MOKE show oscillations with respect to Fe3O4 film thickness, and the oscillations are large enough to induce repeated sign reversals. We attribute the oscillatory behavior to spin-polarized quantum well states forming in the Fe3O4 film. Quantum confinement of the t2g states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations.Comment: to appear in Phys. Rev. Let

Tracheal suctioning improves gas exchange but not hemodynamics in asphyxiated lambs with meconium aspiration.

BackgroundCurrent neonatal resuscitation guidelines recommend tracheal suctioning of nonvigorous neonates born through meconium-stained amniotic fluid.MethodsWe evaluated the effect of tracheal suctioning at birth in 29 lambs with asphyxia induced by cord occlusion and meconium aspiration during gasping.ResultsTracheal suctioning at birth (n = 15) decreased amount of meconium in distal airways (53 ± 29 particles/mm(2) lung area) compared to no suction (499 ± 109 particles/mm(2); n = 14; P < 0.001). Three lambs in the suction group had cardiac arrest during suctioning, requiring chest compressions and epinephrine. Onset of ventilation was delayed in the suction group (146 ± 11 vs. 47 ± 3 s in no-suction group; P = 0.005). There was no difference in pulmonary blood flow, carotid blood flow, and pulmonary or systemic blood pressure between the two groups. Left atrial pressure was significantly higher in the suction group. Tracheal suctioning resulted in higher Pao2/FiO2 levels (122 ± 21 vs. 78 ± 10 mm Hg) and ventilator efficiency index (0.3 ± 0.05 vs.0.16 ± 0.03). Two lambs in the no-suction group required inhaled nitric oxide. Lung 3-nitrotyrosine levels were higher in the suction group (0.65 ± 0.03 ng/µg protein) compared with the no-suction group (0.47 ± 0.06).ConclusionTracheal suctioning improves oxygenation and ventilation. Suctioning does not improve pulmonary/systemic hemodynamics or oxidative stress in an ovine model of acute meconium aspiration with asphyxia

Measurements with the Chandra X-Ray Observatory's flight contamination monitor

NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor (FCM), a system of 16 radioactive calibration sources mounted to the inside of the Observatory's forward contamination cover. The purpose of the FCM is to verify the ground-to-orbit transfer of the Chandra flux scale, through comparison of data acquired during the ground calibration with those obtained in orbit, immediately prior to opening the Observatory's sun-shade door. Here we report results of these measurements, which place limits on the change in mirror--detector system response and, hence, on any accumulation of molecular contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7

Effect of dislocations on properties of heteroepitaxial InP solar cells

The apparently unrelated phenomena of temperature dependency, carrier removal and photoluminescence are shown to be affected by the high dislocation densities present in heteroepitaxial InP solar cells. Using homoepitaxial InP cells as a baseline, it is found that the relatively high dislocation densities present in heteroepitaxial InP/GaAs cells lead to increased volumes of dVoc/dt and carrier removal rate and substantial decreases in photoluminescence spectral intensities. With respect to dVoc/dt, the observed effect is attributed to the tendency of dislocations to reduce Voc. Although the basic cause for the observed increase in carrier removal rate is unclear, it is speculated that the decreased photoluminescence intensity is attributable to defect levels introduced by dislocations in the heteroepitaxial cells