The thinnest superconductor yet is a layer of copper oxide material less than a nanometre thick. The feat suggests a new possible route to faster electronic components.
Making superconductors super-skinny raises the prospect of being able to switch them on and off using electric fields, says at Brookhaven National Laboratories in Upton, New York. That could allow them to be used in electronics, not just for carrying current from place to place.
鈥淪tatic electric fields cannot penetrate more than 1聽nanometre into good conductors,鈥� explains Bozovic, whose team carried out the new study. So a very thin superconductor indeed is needed to use electric fields in this way.
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Rough experiments
Physicists have long disagreed whether it is even possible for superconductivity to exist in such tight spaces, he says. Many have attempted to build very thin films from copper oxide-based materials called to find an answer. But the roughness of the resulting films hampers their potential superconductive properties.
鈥淥ur approach is different,鈥� says Bozovic. His team created thicker films, but built them one atomic layer at a time. They grew six layers of insulating lanthanum copper oxide, and above them six layers of metallic lanthanum-strontium copper oxide. The way electrons leak between the two copper oxides spontaneously creates a superconducting layer somewhere within the stack, able to operate at the relatively high temperature of 32聽kelvin (-241聽掳C) 鈥� most superconductors work at even lower temperatures.
Superconductor in a haystack
The team made many versions of the superconducting layer cake with superconductor-suppressing zinc in different layers. They were able to control the growth of each layer so finely that they could pinpoint the layer that was hiding the superconductivity.
Bozovic鈥檚 team found that when they tainted the second lanthanum copper oxide layer in the stack, the critical temperature at which superconductivity could occur dropped from 32聽kelvin to 18聽kelvin 鈥� no such temperature drop was seen if any other layer was doped with zinc.
That proves that all of the 鈥渉igh-temperature鈥� superconductivity traffic in the film takes place in a single copper oxide layer, says Bozovic. Many thought high-temperature superconductivity would be unstable in a single copper oxide plane, 鈥渘ow we know that鈥檚 not the case鈥�.
The thin superconductor relies on other layers in the structure to feed it electrons. But Bozovic says that in principle the same effect is possible in a single layer of the material if electric fields are used. That could produce high-temperature superconductivity in a single copper-oxide layer just 0.66聽nanometres thick.
Tour de force
, a physicist at Ohio State University in Columbus who was not involved in the study, says that the work is a 鈥渢our de force demonstration鈥� of precision atom handling and growth techniques.
Lemberger says it will be interesting to see if the new techniques can be applied to other materials. 鈥淚f these growth techniques and device architectures could be ported to cuprates with higher [operating superconductivity] temperatures, then the impact could be tremendous.鈥�
Bozovic says his team will next try to fabricate a superconducting field-effect transistor using their cuprate material. Although others have built such devices, they worked only at temperatures close to absolute zero.
He will also explore whether similar effects can be recorded in materials that have not previously been shown to be superconducting: 鈥渢hat would really stir the research community鈥�.
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