21,328 research outputs found

    Origins of ferromagnetism in transition-metal doped Si

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    We present results of the magnetic, structural and chemical characterizations of Mn<sup>+</sup>-implanted Si displaying <i>n</i>-type semiconducting behavior and ferromagnetic ordering with Curie temperature,T<sub>C</sub> well above room temperature. The temperature-dependent magnetization measured by superconducting quantum device interference (SQUID) from 5 K to 800 K was characterized by three different critical temperatures (T*<sub>C</sub>~45 K, T<sub>C1</sub>~630-650 K and T<sub>C2</sub>~805-825 K). Their origins were investigated using dynamic secondary mass ion spectroscopy (SIMS) and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy (EELS), Z-contrast STEM (scanning TEM) imaging and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn<sup>+</sup> ions, as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T*<sub>C</sub> is attributed to the Mn<sub>4</sub>Si<sub>7</sub> precipitates identified by electron diffraction. Possible origins of and are also discussed. Our findings raise questions regarding the origin of the high ferromagnetism reported in many material systems without a careful chemical analysis

    Partonic effects on anisotropic flows at RHIC

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    We report recent results from a multiphase transport (AMPT) model on the azimuthal anisotropies of particle momentum distributions in heavy ion collisions at the Relativistic Heavy Ion Collider. These include higher-order anisotropic flows and their scaling, the rapidity dependence of anisotropic flows, and the elliptic flow of charm quarks.Comment: 7 pages, 5 figures, talk given at "Hot Quarks 2004", July 18-24, 2004, Taos Valley, NM, US

    Kinetic Roughening in Deposition with Suppressed Screening

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    Models of irreversible surface deposition of k-mers on a linear lattice, with screening suppressed by disallowing overhangs blocking large gaps, are studied by extensive Monte Carlo simulations of the temporal and size dependence of the growing interface width. Despite earlier finding that for such models the deposit density tends to increase away from the substrate, our numerical results place them clearly within the standard KPZ universality class.Comment: nine pages, plain TeX (4 figures not included

    Mechanisms that influence the formation of high-ozone regions in the boundary layer downwind of the Asian continent in winter and spring

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    The seasonal variation of ozone (O3) in the boundary layer (BL) over the western Pacific is investigated using a chemistry-transport model. The model results for January and April-May 2002 were evaluated by comparison with PEACE aircraft observations. In January, strong northwesterlies efficiently transported NOx from the continent, leading to an O3 increase of approximately 5-10 ppbv over a distance of about 3000 km. In April, southwesterlies dominated due to anticyclone development over the western Pacific. Along this flow, O3 continued to be produced by NO x emitted from East Asia. This resulted in the formation of a high-O3 (> 50 ppbv) region extending along the coastal areas of East Asia. This seasonal change in O3 was driven in part by a change in the net O3 production rate due to increases in solar UV and H 2O. Its exact response depended on the NOx values in the BL. The net O3 production rate increased between winter and spring over the Asian continent and decreased over the remote western Pacific. Model simulations show that about 25% of the total O3 (of 10-20 ppbv) increase over the coastal region of Northeast Asia was due to local production from NOx emissions from China, and the rest was due to changes in background levels as well as emissions from Korea, Japan, and east Siberia. Uplift of BL air over Asia, horizontal transport in the free troposphere, and subsidence were the principal mechanisms of transporting Asian O3 to the central and eastern North Pacific Copyright 2008 by the American Geophysical Union
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