For years, physicists have assumed that Cooper pairs, the pairs of electrons that allow superconductors to conduct current without resistance, are two-trick ponies. The pairs either slide freely and create a superconductive state or create an insulating state by jamming in a material that can not move at all.
In a recent publication in Science a research team has shown that Cooper pairs can also conduct current with some resistance, as is the case with normal metals. The results describe a completely new state of aggregation, which, according to the researchers, requires a new theoretical explanation.
"There was evidence that this metallic state would occur in thin film superconductors when cooled to superconducting temperatures, but whether or not they were Cooper pairs was an open question," said Jim Valles, a professor of Physics at Brown University and the corresponding author of the study. "We have developed a technique that allows us to test this question, and we have shown that Cooper pairs are indeed responsible for charge transport in this metallic state." Interestingly, no-one knows, on a fundamental level, exactly how Doing it To understand exactly what's going on, further theoretical and experimental work is needed. "
Cooper couples were named after Leon Cooper, a physics professor with Brown, who received the Nobel Prize in 1
In 2007, Valles, in collaboration with Brown engineer and physics professor Jimmy Xu, showed that Cooper pairs can also produce insulating states as well as superconductivity. In very thin materials, the pairs conspire instead of moving together to stay in place, stranded on tiny islands within one material and unable to jump to the next island.
For this new study, Valles, Xu and colleagues in China searched for Cooper pairs in the non-superconducting metallic state using a technique similar to that revealed by Cooper pair isolators. The technique involves patterning a thin film superconductor – in this case a high temperature superconductor yttrium barium copper oxide (YBCO) – with tiny holes. When the material is traversed by a current and exposed to a magnetic field, charge carriers in the material orbit the holes like water orbiting a drain.
"We can measure the frequency at which these charges revolve," said Valles. "In this case, we found that the frequency agrees with not just one, but two electrons simultaneously, so we can conclude that the charge carriers in this state are copper pairs rather than single electrons."
The idea that boson-like Cooper pairs are responsible for this metallic state surprises the researchers, because there are elements of quantum theory that suggest that this should not be possible. Understanding what is going on in that state could lead to exciting new physics, but more research is needed.
Fortunately, the fact that this phenomenon was discovered in a high-temperature superconductor makes future research more practical. YBCO begins to become superconducting at about -181 degrees Celsius, and the metal phase begins at temperatures just above it. This is pretty cold, but it is much warmer than other superconductors that are active just above absolute zero. This higher temperature makes it easier to use spectroscopy and other techniques to better understand what is going on in this metallic phase.
Researchers believe it may be possible to use this boson metal state for new types of electronic devices.
"What's so special about the bosons is that they are in a rippled state rather than electrons, so we're talking about having a phase and producing interference in the same way as light," said Valles. "So there could be new modalities for moving charge in devices by playing with the interference between bosons."
However, researchers are happy to have discovered a new state of aggregation.
"Science is based on discoveries," Xu said, "and it's great to have discovered something completely new."
Repellent charges prevent Cooper pairs from isolating "island hopping"
Chao Yang et al., Interbosonic Metal State in Superconductor Insulator Transition, Science (2019). DOI: 10.1126 / science.aax5798
Research reveals new state of aggregation: a Cooper metal pair (2019, 15 November)
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