Suchergebnisse

Doping mechanisms of Mn in GaAs nanowires (NWs) that have been grown self-catalytically at 600 C by molecular beam epitaxy (MBE) are investigated using advanced electron microscopy techniques and atom probe tomography. Mn is found to be incorporated primarily in the form of non-magnetic tetragonal Ga0.82Mn0.18 nanocrystals in Ga catalyst droplets at the ends of the NWs, while trace amounts of Mn (2264 at. ppm) are also distributed randomly in the NW bodies without forming clusters or precipitates. The nanocrystals are likely to form after switching off the reaction in the MBE chamber, since they are partially embedded in neck regions of the NWs. The Ga0.82Mn0.18 nanocrystals and the low Mn concentration in the NW bodies are insufficient to induce a ferromagnetic phase transition, suggesting that it is difficult to have high Mn contents in GaAs even in 1-D NW growth via the vapor-liquid-solid process ; European Union PITN-GA- 2008-215368, POIG.01.01.02- 00-108/09, REGPOT-CT-2011-285895- Al-NANOFUNC ; Polish National Science Centre 2014/13/B/ST3/04489

Magnetic fields are proposed to have played a critical role in some of the most enigmatic processes of planetary formation by mediating the rapid accretion of disk material onto the central star and the formation of the first solids. However, there have been no experimental constraints on the intensity of these fields. Here we show that dusty olivine-bearing chondrules from the Semarkona meteorite were magnetized in a nebular field of 54 ± 21 μT. This intensity supports chondrule formation by nebular shocks or planetesimal collisions rather than by electric currents, the x-wind, or other mechanisms near the sun. This implies that background magnetic fields in the terrestrial planet-forming region were likely 5-54 μT, which is sufficient to account for measured rates of mass and angular momentum transport in protoplanetary disks. ; Thomas F. Peterson ; United States. National Aeronautics and Space Administration (NASA Origins Program)) ; Jet Propulsion Laboratory (U.S.) (U.S. Rosetta Project) ; National Science Foundation (U.S.) (Graduate Research Fellowship Program) ; United States. National Aeronautics and Space Administration (NASA Solar System Exploration and Research Virtual Institute)) ; European Union (European Research Council, Seventh Framework Programme) ; Leverhulme Trust ; United States. National Aeronautics and Space Administration (Hubble Fellowship) ; United States. Defense Advanced Research Projects Agency (DARPA QuASAR program) ; National Science Foundation (U.S.)