In a discovery that reveals a whole new state of matter, research published in the journal Science shows that Cooper pairs, electron duos that facilitate superconductivity, can also conduct current like normal metals. [1
But in a new publication published today (14 November 2019) in Science a research team has shown that Cooper couples, like normal metals, can also conduct electricity with some resistance. 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 their superconducting temperature. But whether or not it was a Cooper pair was an open question, "said Jim Valles, professor of physics at Brown University and author of the study. "We have developed a technique to test this question, and we have shown that Cooper pairs are indeed responsible for charge transport in this metallic state. Interestingly, no one on a fundamental level knows exactly how they do it. Therefore, this result requires some theoretical and experimental work to understand exactly what happens. "
Cooper pairs are named after physics professor Leon Cooper at Brown, who received the Nobel Prize in 1972 for describing their role in facilitating superconductivity. Resistance arises when electrons move in the atomic lattice of a material. But when electrons fuse into Cooper pairs, they undergo a remarkable transformation. Electrons in themselves are fermions, particles that obey the Pauli principle of exclusion, which means that every electron tends to maintain its own quantum state. Cooper couples, however, behave like bosons happily sharing the same state. This boson behavior allows Cooper pairs to coordinate their movements with other Cooper pair sets so that resistance is reduced to zero.
In 2007, Valles, in collaboration with Brown engineer and physics professor Jimmy Xu, showed that Cooper pairs can produce both insulating states and superconductivity. In very thin materials, rather than moving together, the couples conspire 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 with which these charges revolve," said Valles. "In that case, we've found that the frequency agrees with two electrons going around rather than just one. So we can conclude that the charge carriers in this state are Cooper pairs rather than single electrons.
The notion that boson-like Cooper pairs are responsible for this metallic state surprises the researchers, because there are such elements of quantum theory that suggest 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 just above the absolute zero active. 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 might be possible to use this boson metal state for new types of electronic devices.
"The special thing about the bosons is that they are more in a wave-like state than electrons. We therefore say that they have a phase and generate interference in the same way as light, "said Valles. "There may be new modalities for moving charge in devices by playing with the interference between bosons."
However, researchers are pleased 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.
Reference: "Interbosonic Metal State in the Superconductor-Insulator Transition" by Chao Yang, Yi Liu, Yang Wang, Liu Feng, Qianmei He, Jian Sun, Yue Tang, Chunchun Wu, Jie Xiong, Wanli Zhang, Xi Lin, Hong Yao, Haiwen Liu, Gustavo Fernandes, Jimmy Xu, James M. Valles Jr., Jian Wang and Yanrong Li, 14 November 2019, Science
DOI: 10.1126 / science.aax5798