In a finding that reveals a whole new state of matter, research published in the journal Science shows that Cooper pairs, electron beams that allow for superconductivity, can conduct electricity just like normal metals do.
For years, physicists have speculated that Cooper's pairs, electronic duels that allow superconductors to conduct electricity without resistance, were double-decker ponies. Couples either slip freely, creating a superconducting state, or they create an insulating state by slamming into the material, unable to move at all.
But in a new paper published today (14.11.2019) at Science, a team of researchers have shown that Cooper pairs can also conduct electricity with a certain amount of resistance, just as regular metals do. The findings describe a completely new state of matter, the researchers say, which will require a new theoretical explanation.
"There was evidence that this metal state would appear in superconductors with thin films as they cool to their superconducting temperature, but whether those states were included in Cooper's pairs was an open question," said Jim Wallace, a professor of physics at Brown. University and corresponding author of the study. "We have developed a technique that allows us to test this question and have shown that, indeed, Cooper's pairs are responsible for transporting cargo in this metallic state. What is interesting is that no one is fundamentally sure how they did it, so this discovery will require even more theoretical and experimental work to understand exactly what is going on. "
Cooper's couples are named for Leon Cooper, a professor of physics at Brown who won the Nobel Prize in 1972 for describing their role in enabling superconductivity. Resistance is created when electrons are scattered in the atomic grid of the material as they move. But when electrons come together to become Cooper pairs, they undergo a remarkable transformation. The electrons themselves are fermions, particles that adhere to the Pauli exclusion principle, meaning that each electron tends to maintain its quantum state. Pile pairs, however, act as bosons, which can happily share the same state. This bosonic behavior allows Cooper pairs to coordinate their movements with Cooper's other groups of couples in a way that reduces resistance to zero.
In 2007, Wallace, working with Brown Engineering and Professor of Physics Jimmy Xu, demonstrated that Cooper pairs can also produce insulating states as well as superconductivity. In very thin material, instead of moving to a concert, couples want to stay in place, trapped in small islands within the material and cannot jump to the next island.
For this new study, Wallace, Xu, and colleagues in China looked for Cooper pairs in a non-superconducting metal state using a technique similar to that discovered by Dome insulators. The technique involves modeling a thin film superconductor – in this case a high-temperature yttrium barium copper oxide (YBCO) superconductor – with a series of small holes. When the material has electricity through it and is exposed to a magnetic field, the charge carriers in the material will orbit the holes like circulating water.
"We can measure the frequency with which these charges are circulated," Wallace said. "In this case, we found that the frequency is consistent with two electrons going around at the same time, instead of just one. So we can conclude that the charge carriers in this state are Cooper pairs, not single electrons. "
The idea that boson pairs, like Cooper is responsible for this metal state, is a surprise, the researchers say, because there are elements of quantum theory that suggest this should not be possible. So understanding just what is going on in this state can lead to some exciting new physics, but more research will be needed.
Fortunately, researchers say, the fact that this phenomenon has been detected in a high-temperature superconductor will make future research more practical. YBCO begins with superconductivity at about -181 degrees Celsius, and the metal phase begins at temperatures well above that. It's pretty cold, but much hotter than other superconductors that are active on top absolute zero. This higher temperature facilitates the use of spectroscopy and other techniques in order to better understand what is happening in this metal phase.
Along the way, researchers say, it may be possible to use this state of boson metal for new types of electronic devices.
"The thing about bosons is that they tend to be more wavy than electrons, so we're talking about them having a phase and creating interference in the same way that light does," Wallace said. "So, there may be new charging modes in devices that play with mixing between bosons."
But for now, researchers are happy to discover a new state of matter.
"Science is based on discoveries," Xu said, "and it's nice to discover something completely new."
Reference: "Intermediate boson metal state in the superconductor-insulator transition" by Zhao Yang, Shi Liu, Yang Wang, Liu Feng, Qianmei He, Jian Sun, Yue Tang, Chunchun Wu, Jie Xiong, Wanli Zhang, Xi Lin, Hong Ya , Haven Liu, Gustavo Hernandez, Jimmy Xu, James M. Wallace Runner, Yiannian Wang and Shanrong Lee, 14.11.2019, Science.
DOI: 10.1126 / science.aax5798