The device has three electrodes: a niobium injector electrode; a common electrode that consists of a layer of niobium and a layer of aluminium; and a niobium detector electrode. The electrodes are separated by insulating barriers, or junctions, through which current can pass by quantum tunnelling. At the operating temperature of 4.2 K, all the niobium layers are superconducting. In the absence of any injected current, the aluminium layer is also superconducting, as a result of the proximity effect. In a superconduting metal the electrons form pairs, and an amount of energy equal to the superconducting energy gap is needed to split a pair.
The operation of the device can be understood in terms of electrons that are injected into the common electrode. When the injection current is small, the aluminium layer remains superconducting, with an energy gap that is lower than that of niobium. The electrons become trapped in the aluminium layer, but each one is capable of splitting at most one pair of electrons, so relatively few electrons reach the detector electrode. Thus, the current gain is low.
But this changes when the injected current is increased. The aluminium layer becomes normal, so its energy gap vanishes and the injected electrons increase the electron temperature enough for a large number of electrons to tunnel through the detector junction. Indeed, the Naples Oxford team observed current gains of more than 50 and signal power gains as high as 1000. Such devices could be exploited in a wide range of cryogenic particle and radiation detectors in both astrophysics and particle physics.
Comprehension check
Answer the following questions:
1. What materials are transistors made of?
2. What are the strong points of the superconducting materials?
3. What are the main parts of a superconducting device?
4. Where could such devices be exploited?
Discussion
Comment on the text using the following prompts: first of all…, secondly... , as I see it... , personally I feel... , I’m not sure about... , I’m convinced that... , I’d say..., I can’t side with the author....
Give your own project of building a superconducting device and its exploitation.
UNIT 5
NOTHING GOES FASTER THAN LIGHT
Vocabbox
noun collocations
§ laser pulse
§ caesium gas
§ infinite energy
§ relativistic notion
§ simultaneity
§ disarray
§ space-time event
verb collocations
§ force somebody to re-examine
§ precede effect
§ travel at a far greater velocity
§ forbid by relativity
Pre-reading task
1. Nothing goes faster than light. Do you agree?
2. Have you ever heard about the experiments called “motion of effects”?
Reading
Read the text. Find some information concerning “c” velocities.
Nothing can travel faster than light. Despite a recent raft of reports in the media, this statement is as true now as it ever was. Nonetheless, experiments over the past 20 years have been forcing us to re-examine what we mean by the word "nothing". In the latest experiment, a group of researchers at the NEC Research Institute in Princeton, US, observed the peak of a laser pulse leave a small cell filled with caesium gas before it had even entered the cell. Apparently, the peak of this pulse is simply not the kind of "thing" to which Einstein's famous law applies.
At almost 300 000 km s-¹, the cosmic speed limit, c, is one of the most widely known constants in physics. A massive object needs infinite energy to reach c, while massless particles like photons always carry their energy at precisely the speed of light. More importantly, the relativistic notion of simultaneity makes it clear that no information can travel faster than light without throwing all our concepts of cause and effect into disarray. Relativity teaches us that if two space-time events are separated so that they cannot be connected by any signal travelling at c or less, then different observers will disagree as to which of the two events came first. Since most physicists still believe that cause needs to precede effect, we conclude that no information can be transmitted faster than the speed of light.
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