烃类也超导

Not Just for Fuel Anymore: Hydrocarbons Can Superconduct, Too
Superconductivity is one of those nearly magical properties that seem to defy all intuition for how the physical world ought to work. In a superconductor, electric currents flow without resistance—an electron passes unimpeded through the material like a torpedo through some frictionless ocean. After discovering the phenomenon in 1911 Dutch physicist Heike Kamerlingh Onnes showed that an electric current in a closed superconducting loop of mercury would keep flowing long after the driving potential was removed; he demonstrated his discovery by carrying such a persistent current from the Netherlands to England.

Since then physicists have discovered superconductors based on other metals and even ceramics. The latest entry is one rooted in a hydrocarbon, which superconducts at a relatively high temperature compared with elemental metals.

The finding comes from a team of Japanese researchers, who report in the March 4 issue of Nature that the hydrocarbon molecule picene, ordinarily a semiconductor, becomes a potential superconductor when interspersed with an alkali metal such as potassium or rubidium. (Scientific American is part of Nature Publishing Group.) And when picene (C22H14) is doped with potassium, it superconducts at a relatively toasty 18 kelvins (–255 degrees C). Although the transition temperature is far below that of superconductors based on the ceramiclike copper oxides, where electricity can flow without resistance at around 160 kelvins, or –113 degrees C, the discovery could lead to the development of new, high-temperature superconductors rooted in organic chemistry.

Yoshihiro Kubozono, a professor of solid-state chemistry and physics at Okayama University in Japan and a study co-author, says that picene is the first example of a superconducting hydrocarbon. (Researchers have had similar success coaxing strictly carbon-based compounds to superconduct, for instance by doping the fullerene C60 with potassium.) Hydrocarbons are better known for their combustibility than their electric properties: Gasoline is a blend of hydrocarbons, including the familiar octane (C8H18); the natural gas that heats about half of U.S. homes is mostly methane (CH4).

Picene occurs naturally in coal tar and is found in residues from petroleum refining, but Kubozono and his colleagues synthesized the compound for the new study. The picene molecule is flat, and it forms crystals comprising stacked layers. Cooking the compound for days with an alkali metal such as potassium introduced metallic atoms between the hydrocarbon layers, which enhanced its conductivity in the planes parallel to the picene slabs.

Kubozono says that he and his colleagues are now doping picene with other metals—in the new study, the group reports trying sodium and cesium, in addition to potassium and rubidium—to further boost the material's superconducting temperature. They are also experimenting with other hydrocarbons to see which ones can superconduct, Kubozono says.

Brian Maple, a physicist at the University of California, San Diego, says the research is an encouraging step in expanding the realm of superconductivity, particularly in the high-temperature regime. "This is just another example of how prevalent superconductivity is amongst a broad range of materials if you can make them into metals," he says. "Globally, I think this just shows that one should be very optimistic about finding superconductors with higher transition temperatures by looking in directions that heretofore one would have thought wouldn't be very promising."
烃类也超导
超导是很神奇的一种性质，这种性质与人们对物理世界的一些直观印象相悖。超导材料没有电阻：在超导材料中，电子如同鱼雷，在丝毫没有阻力的海里自由穿行。1911年荷兰物理学家海克昂尼斯（Dutch physicist Heike Kamerlingh Onnes）发现在水银构成的超导回路里，电流能在外界电势撤去后保持很长一段时间。这一装置被从荷兰一直搬到了英国，而电流依然存在。他用这证明了他的发现。
从此以后物理学家陆续发现了基于其他金属甚至是陶瓷的超导材料。最近超导家族又添新成员。这是一种基于烃类的超导材料，其临界温度比纯金属材料要相对高一些。
这项发现来自于日本研究人员。他们在3月4日《自然（Nature）》杂志上撰文称，一种叫苉的烃类原本是半导体，但在其中扩散过碱金属（如钾、铷）后就会变成超导体。扩散钾以后，这种材料的临界温度是相对“暖和”的18K（-255℃）。尽管这种材料的转变温度相对于陶瓷一般的、基于氧化铜的超导材料的160K（-113℃）很低，但是这项发现的意义在于它将超导研究的战火烧到了有机化学的地界。
日本冈山大学研究固态化学和物理学的丸独子教授（Yoshihiro Kubozono）参与了此项研究，他说苉是世界上{dy}种烃类超导体。（科学家此前在诸如向足球烯C60中扩散钾的实验中获得的超导体，严格来说只能算碳基化合物。）人们更了解烃类的可燃性：汽油是包括我们熟悉的辛烷在内的烃类混合物、美国大一半家庭使用的天然气的主要成分是甲烷。但人们很少知道烃类的电学性质。
（待续）