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| RIKEN Press Release | February 27, 2008 |
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X-ray images suggest new explanation for semiconductor charge bondingCheckerboard charge-density pattern indicates what happens when semiconductor elements switch charge states.x | |
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It has long been thought that there is a clear distinction between charged and uncharged states in doped semiconductor materials, but a team of researchers including scientists from RIKEN recently reported new evidence of charge-state hybridization in the form of a checkerboard molecular pattern that occurs in at least one class of materials. Moreover, the evidence suggests that this hybridization does not follow the conventional theoretical model for molecular bonding. Writing in a journal of the American Physical Society, the team said they used X-ray microscopy to investigate the charge states of transition-metal impurities in doped manganite thin film, an oxide-based semiconductor material. Such 'on' and 'off' charge states are key to the functioning of microchip devices in use today. The photon-level study is part of continuing research into the development of 'oxide electronics,' which hold promise in many useful technological applications such as ferroelectrics, catalysts, sensors and superconductors. For the research, a single crystal of ferromagnetic manganite, which was grown in a furnace at a feeding speed of 7-9 mm per hour, was crushed into fine powder. The powder was precipitated into a thin film layer that the experimenters subjected to slight electrical currents at different temperatures until they were able to bring about the charge transition. Radiographic images were snapped to record the behavior of the oxide material during the tests. What they saw was that instead of distinct blocks of charged elements, adjacent molecules bonded into a zigzag pattern suggestive of the ordered rows of a checkerboard, with paired bonds that were both long (a weaker charge) and short (a stronger charge), or 'hybridized.' This checkered pattern of hybridized charge densities, the researchers wrote, seemed 'at a glance' to follow the theoretical model of how molecules bond together. They acknowledged that it was similar to earlier research findings by themselves and others on the charged states of ferromagnetic semiconductor materials. However, the researchers ascribed the zigzag pattern they saw to the disproportionate bonding of electrons in the hybridized state, and not to the tighter-patterned 'orbital ordering' called for by the standard theoretical model. This new finding helps explain another phenomenon they also noted in their tests. The RIKEN investigators determined that the electrostatic potential of the charges were effectively enhanced by the disproportionate pairing of the weak and strong molecular bonds. Spread over the thin film, this presented a 'contour map' of the charge densities that further evidenced the checkerboard pattern, they said, adding that the contour map is a 'powerful tool to visualize the charge characteristics of transition metal compounds.' This continuing research adds to the growing body of knowledge on oxide-type semiconductors and is among the first to describe and obtain images of charge state transitions as they occur in this class of materials. The work by the team of specialists from RIKEN, the University of Tsukuba, the University of Tokyo, Shimane University and Tokyo University of Science is described in Physical Review B, an online research documentation service of the American Physical Society.
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