Physicists have assumed for years that two-trick ponies were the Cooper pairs, the electron duos that allow superconductors to conduct electricity without resistance. The pairs either glide freely, create a superconducting state, or create an isolating state by jamming up inside a material, which is impossible to move.
But a team of researchers has shown in a new paper published in Science that Cooper pairs can also conduct electricity with some resistance, as is the case with standard metals.
Small holes punched into a superconductive material of high temperature revealed that Cooper pairs, electron duos that allow superconductivity, can also conduct electricity in the same way as metals do.
[Credit: Valles lab / Brown University]
[Credit: Valles lab / Brown University]
"There was evidence that this metallic state would arise in thin film superconductors as they were cooled down to their superconducting temperature, but whether or not that state involved Cooper pairs was an open question. We've developed a technique that enables us to test that question and we showed that, indeed, Cooper pairs are responsible for transporting charge in this metallic state. What's interesting is that no one is quite sure at a fundamental level how they do that, so this finding will require some more theoretical and experimental work to understand exactly what's happening." - said Jim Valles, a physics professor at Brown University and the corresponding author of the study.
Cooper pairs are named for Leon Cooper, a Brown physics professor who received the 1972 Nobel Prize to explain their role in allowing superconductivity. Resistance is produced by rattling electrons in a material's atomic lattice as they pass about. Yet they undergo a fascinating transition as electrons come together to become Cooper pairs.
Electrons on their own are fermions, particles that obey the rule of Pauli exclusion, meaning that each electron appears to preserve its own quantity. Nevertheless, cooper pairs behave as bosons that can happily share the same state. That bosonic behaviour allows Cooper pairs to coordinate their movements in a way that reduces resistance to zero with other sets of Cooper pairs.
In 2007, Valles, working with Brown engineering and physics professor Jimmy Xu, showed that both isolating states and superconductivity could also be produced by Cooper pairs. The couples conspire to stay in place in very small fabrics, instead of travelling in tandem, trapped in a substance on tiny islands and unable to move to the next island.
Valles, Xu and Chinese colleagues searched for Cooper pairs in the non-superconducting metallic system for this new study using a similar technique to the one that identified Cooper pair insulators. The technique involves patterning a superconductor of thin-film — in this case, a superconductor of high-temperature Yttrium Barium Copper Oxide (YBCO) — with clusters of small holes. If the material has a current running through it and is exposed to a magnetic field, the material's charging carriers will orbit the holes like water running around a drain.
"We can measure the frequency of the circle of these charges. In this case, we found that the frequency is consistent with two electrons going around at one time instead of just one. So we can conclude that in this state the charging carriers are Cooper pairs and not single electrons." - said Valles.
The suggestion that boson-like Cooper pairs are responsible for this metallic state is somewhat shocking, the researchers say, as there are aspects of quantum theory indicating that this should not be plausible. And discovering what's going on in this system might lead to some exciting new theories, but it will take further work.
Luckily, the researchers say, it will make future research more practical to detect this phenomenon in a high-temperature superconductor. YBCO begins to superconduct at about -181 degrees Celsius, and the metal phase begins just above that at temperatures. That's pretty cold, but it's a lot warmer than other superconductors, operating just above absolute zero. The higher temperature makes spectroscopy and other methods easier to use in order to better understand what is going on in this metallic process.
Down the road, researchers say, it may be feasible for new types of electronic devices to exploit this bosonic metal state.
"The thing about the bosons is that they tend to be more wavelike than electrons, so we're talking about having a phase and creating interference in much the same way that light does. So there may be new ways to move charge in devices by playing with interference between bosons."
- said Valles.
But for now, the researchers are pleased to have discovered a new state of affairs.
"Science is built on discoveries, and it's great to find something totally new." - said Xu.
Reference:
Chao Yang et al, Intermediate bosonic metallic state in the superconductor-insulator transition, Science (2019). DOI: 10.1126/science.aax5798
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