SAN FRANCISCO, Oct. 3 (Xinhua) -- Researchers with Stanford University have created a two-dimensional superlattice, sent electrons through the sheet, and achieved what theory suggested would be needed to conduct terahertz signals. The new semiconductor material, made by sandwiching a sheet of atomically thin graphene in between two sheets of electrically insulating boron nitride, may allow for electromagnetic oscillations at terahertz frequency range theorized by the late Stanford professor and Nobel laureate Felix Bloch. While large portions of the electromagnetic spectrum have been harnessed for diverse technologies, from X-rays to radios, a chunk of that spectrum, known as the terahertz gap, has remained largely out of reach. It is located between radio waves and infrared radiation, two parts of the spectrum now in use in everyday technologies including cell phones, TV remotes and toasters. #graphene Bloch suggested that a specially structured material that allowed electrons to oscillate in a particular way might be able to conduct terahertz signals, and researchers have long thought that materials with repeating spatial patterns on the nanoscale might allow for Bloch's oscillations. Such a material requires that electrons travel long distances without deflection, where even the smallest imperfection in the medium through which the electrons flow can put them off their original path. In the Stanford study, published in the recent issue of the journal Science, as they are protected from air and contaminants by boron nitride above and below in the 2-D superlattice, electrons in the graphene flow along smooth paths without deflection, exactly as Bloch's theory suggested would be needed to conduct terahertz signals. The researchers were able to send electrons through the graphene sheet, collect them on the other side and use them to thus infer the activity of the electrons along the way, David Goldhaber-Gordon, a physics professor and co-author of the study, was quoted as saying in a new release. And the electrons can be confined to narrower bands of energy. Combined with very long times between deflections, it should lead the electrons to oscillate in place and emit radiation in the terahertz frequency range, a foundational success on the path toward creating controlled emission and sensing of terahertz frequencies.
http://news.xinhuanet.com/english/2016-10/04/c_135731119.htm
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