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超导百年

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The world"s first "quantum"" computer -- a machine that harnessesthe magic of quantum phenomena to perform memory and processingtasks incredibly faster than today"s silicon-based computer chips --was recently sold by D-Wave Systems of Canada to Lockheed-Martin.And, while some question whether the machine is truly a quantumcomputer, its designers have published articles in peer-reviewed3 journalsdemonstrating that the basic elements of this novel computer are indeedsuperconducting quantum bits.

This spring marked the ! 00th anniversary of the discovery ofsuperconductivity -- the ability of materials to carry electrical currentwith no loss. Currents set up in superconducting wires can exist for yearswithout any measurable decay.

Because of this property, superconductors have unique features thatcan be exploited in many ways. They can carry enormous amounts of,

世界上第一台“量子”计算机——利用奇异的量子现象执行数据记忆和处理任务,比今天使用硅芯片的计算机快得多——最近由加拿大D—wave系统公司出售给洛克希德一马丁公司。就这台机器是否真是一台量子计算机的问题,它的设计者们已经在经同行评议的杂志上发表文章,证明这台新奇的计算机的基本要素确实是超导量子位。

今年春天是超导电性——材料无损失传送电流的能力——发现100周年的日子。在超导导线中产生的电流能持续若干年而没有任何可测量的衰减。

由于这个特性,超导体具有独特的、能够多方面加以利用的特征。它们current, making them ideal for urban power grids. And,when wound into coils, they can produce extremelystrong magnetic fields.

Such superconducting magnetshave been applied in a variety oftechnologies. The best-knownexamples are the magnets that drivethe magnetic resonance imaging(MRI) machines found in mosthospitals. Perhaps the most exotic7 arethe huge magnets used to accelerateparticles in the Large HadronCollider, which seeks to discover thefundamental principles of matter.

Despite their great promise,however, superconductors havelimits the primary one being thatmost superconduct at very low temperatures -- indeed,near absolute zero (-273 ℃). Such temperatures canbe achieved only through liquid-helium8 cooling.Thus, Swiss researchers caused excitement in 1986by announcing the discovery of superconductivity inan oxide of copper at twice the temperature of theprevious record holder. 能传输巨大的电流,这使其成为城市电网的理想之选。而且,当它们被绕成线圈时能产生极强的磁场。

这样的超导磁体已在各种技术中得到应用。最著名的例子是在多数医院都可见到的驱动磁共振成像(Mill)设备的超导体。也许最新的应用是在试图发现物质基本原理的大型强子对撞机(LHC)上用以加速粒子的巨大磁体。

尽管有巨大的应用前景,然而超导体有诸多局限,主要是大多数超导现象在极低的温度下发生——实际上接近绝对零度(一273~C)。只有通过液氦冷却才能达到这样的温度。因此,瑞士研究人员在1986年宣称发现铜的氧化物的超导电性,其超导温度两倍于前记录保持者,此举引发了轰动。

Shortly thereafter, researchers in the United States founda related material that superconducts above the temperatureat which air liquefies. As Time magazine proclaimed in May1987, with the discovery of these so-called "cuprates," thesuperconducting revolution had begun.

Alas, the revolution soon *bogged down. Cuprates arenotoriously difficult materials to work with, because they are verybrittle. This is exacerbated14 by their strong anisotropy -- thematerials have a quasi-two-dimensional structure consisting ofa weakly coupled stack of conducting sheets. As such, they are

a challenge for industry, though applications are beginning toappear.

Since the cupratesfirst appeared, a variety ofother "high temperature"superconductors have beendiscovered -- one is a simplecompound of magnesium andboron, and another involvesa mixture of iron and arsenic.Although none of themsuperconduct at temperaturesas high as liquid air, they may ultimately be better materialswith which to work. Given the vast number of combinations ofelements that can form compounds, there is a good chance thatbetter superconductors await our discovery.

In the coming years, superconductors are expected to playa growing role in technology. Already, "second generation"cuprate wires are being used to make high-capacity cables forelectric-power transmission, and lighter-weight generators forwind turbines. Stronger superconducting magnets are leadingto the development of MRIs with more sophisticated diagnosticcapabilities. Superconductors are being used for levitatedtrains in high-speed rail transport, and as microwave filtersfor improved signal bandwidth in cellular base stations. Thediscovery of a new superconductor with enhanced propertiescould lead to even greater technological innovation.

This brings us to the intellectual challenge ofsuperconductors. It took 46 years from the discovery ofsuperconductivity to the 1957 Bardeen, Cooper, and Schrieffer(BCS) theory of how the phenomenon occurs. Along the way, anumber of famous physicists tried and failed to get the answer --Albert Einstein, *Werner Heisenberg26, and *Richard Feynmanbeing notable examples.

Discovering the solution required the development of advancedtheoretical techniques. What had been difficult to figure out was how to

此后不久,美国研究人员发现了一种相关的材料,其超导温度高于空气液化的温度。随着这些所谓的“铜氧化物”的发现,《时代》周刊1987年5月宣告,超导革命开始了。

遗憾的是,这场革命不久就陷入困境。都知道铜氧化物材料极其难用,因为它们非常脆。另外它们有很强的各向异性——这种材料具有导电层间存在弱连接的准二维结构——用起来就更困难。严格说来,它们对于生产制造是个挑战,尽管已开始首次应用。

自铜氧化物首次出现以来,其他各种各样的“高温”超导体已经被发现——一种是镁和硼的简单化合物,另一种涉及铁和砷的混合物。尽管它们的超导温度都没有液态空气高,但它们最终可能会是更好的实用材料。鉴于有大量的能够形成化合物的元素组合,我们有充分的机会去发现更好的超导体。

在未来的岁月里,超导体预计会在技术中发挥越来越大的作用。“第二代”铜氧化物导线已经被用来制造高容量输电电缆和轻量的风力涡轮发电机。更强的超导磁体正在使磁共振成像向具有更精密的诊断能力上发展。超导体正被用于高速铁路运输的悬浮列车,用作微波滤波器来改善蜂窝基站信号带宽。一种新型的性能更强的超导体的发现会使技术创新更胜一筹。

超导体对我们的智力提出了挑战。从超导电性的发现,到1957年阐释超导现象发生的“巴丁一库珀一施里弗(BCS)理论”的形成,用了46年时间:在这条路上,一些著名的物理学家试图解答但失败了——阿尔伯特·爱因斯坦、沃纳·海森伯和理查德·费因曼就是著名的例证。

要找到问题的答案需要发展先进get electrons to superconduct. The basic discovery of BCS was that ifthe electrons pair up, those couples could indeed superconduct.

Fortunately, the mechanism for such coupling was known.Although electrons are negatively charged, and therefore repel oneanother, the positive ions that theyleave behind when they flow througha metal can mediate29 an effectiveattraction between two electrons underrestrictive conditions (for example, themetal must be very cold).

The suspicion, though, is

that this is not the case in thenew superconductors. Cupratessuperconduct at much highertemperatures, but, more importantly,they possess some exotic properties:they are formed by doping electricalcarriers into a host material thatis a magnetic insulator -- the lastplace one would look for a conventional superconductor. And,unlike BCS theory, in which the pairs are isotropic -- withidentical properties in all directions in space -- the pairs incuprates are strongly anisotropic, resembling a cloverleaf.

How can one pair electrons without ions holdingthem together, thereby enabling higher-temperaturesuperconductors? While ideas about this abound, newtheoretical breakthroughs most likely will be needed todevelop the machinery required to solve such electron-electrontheories, perhaps even involving black holes. Whatever thetheory turns out to be, it is certain to revolutionize physics. 的理论技巧。一直难以解决的是,如何使电子具有超导性。BCS理论的基本发现是,如果电子配成对,这些电子对确实能超导。

幸运的是,人们了解了电子成对的机制。虽然电子带负电,相互之间排斥,但当电子在金属中流动时,它们留下的正离子在特定的条件下(例如,金属必须非常冷)能够促成两个电子间的有效吸引。

然而,问题是,在新的超导体中情况不是这样。铜氧化物显示超导性时的温度要高得多,而更重要的是,它们具有一些奇异的特性:它们是通过在磁性绝缘体的基质材料中掺杂电载流子形成的——人们没想到会在基质材料里发现一种常规超导体。而且,与BCS理论不同(BCS理论中电子对是各向同性的——在空间所有方向具有相同的性质),铜氧化物中的电子对有强烈的各向异性,类似一个苜蓿叶。

没有离子作用的一对电子怎样聚在一起,从而使高温超导体成为可能?尽管这方面的思路很多,但新的理论突破很可能将必须发展能解决这样的电子一电子之间作用的理论体系,也许甚至涉及到黑洞。无论这种理论结果如何,都必定彻底改变物理学。

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