Giant spin-splitting revealed on SrTiO 3 surface
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ant spin-splitting revealed on SrTiO3 surface
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onventional electronics is based on semiconductor materials such as silicon, germanium, or gallium arsenide, which are increasingly reaching their performance limits. One key approach to make these electronic devices faster and more efficient is spintronics, which requires new materials. Scientists have had their sights on oxides such as strontium titanate (SrTiO3) as an alternative to the wellestablished semiconductors. A thin conductive layer forms on the pure surface of SrTiO3—a two-dimensional electron gas (2DEG), where electrons can virtually move freely, like gas particles. Milan Radović and his colleague Nicholas Plumb at Paul Scherrer Institute have now measured
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MRS BULLETIN
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VOLUME 39 • DECEMBER 2014
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to avoid obstructing the light), and 3 mol/l KCl aqueous electrolyte. The device can be bleached to transparency by connecting the electrodes and reducing the Fe(III), which occurs over several tens of seconds and shows a maximum optical transparency change of 52.2% for red light (670 nm wavelength).When the electrodes are disconnected, the device spontaneously returns to opaque as the iron is oxidized by oxygen dissolved in the electrolyte. This occurs much more slowly; transparency is reduced by 38.5% after two hours. By applying a 2 V external bias, the researchers were able to significantly accelerate this transition, demonstrating 10-s cycling through +/–10% changes in transparency. In addition to the electrochromic behavior, the researchers note that the device also functions as a “self-recoverable” battery, with an open-circuit voltage of 1.26 V. During the electrochromic bleaching process the battery is discharging, with a measured discharge capacity of 63.6 mAh/g at –2 V for 30 s. The battery then spontaneously recovers during the electrochromic recovery process; after 24 hours the battery discharges at
61.9% of the original capacity. The spontaneous cycling behavior of the device— considered as either an electrochromic window or a battery—is accompanied by the formation of Al(OH)3 precipitate in the electrolyte, gradually consuming the Al electrode. However, the researchers note that the rate at which this occurs is negligible, and is unlikely to limit the device performance. Sebastien Lounis of Lawrence Berkeley National Laboratory agrees that the need for an external bias is a significant problem for the market deployment of smart windows: “With current electrochromic technology, you’re looking at involving both carpenters and electricians for installation, which creates a major headache for the builder and drives up costs.” A bias-free electrochromic window could therefore help accelerate the technology into much wider use. Prussian Blue has captivated artists and scientists since it was discovered over 300 years ago, and it is now used in everything from art to medicine to machining. But given these results, it may still have surprises in store. Colin McCormick
the properties of the electrons in this 2DEG, providing the clearest desc
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