In a discovery that reveals a whole new state of matter, a study published in the journal Science shows that Cooper's pairs, electronic duos that enable superconductivity, can also conduct electricity just like normal metals.
For years, physicists have assumed that Cooper pairs, electronic duos that allow superconductors to conduct current without resistance, were two-trick ponies. The vapors either slide freely, creating a superconducting state, or create an insulating state, blocking in the material, unable to move at all.
But in a new article published today (November 14, 2019) W. Science, a team of scientists has shown that Cooper vapors can conduct electricity with some resistance, just like ordinary metals do. The findings describe a completely new state of matter, according to scientists, which will require a new theoretical explanation.
"There is evidence that this metallic state would arise in thin-film superconductors when they were cooled to superconducting temperature, but the question of whether this state applies to Cooper pairs was an open question," said Jim Valles, professor of physics at Brown University and the author study. "We've developed a technique that allows us to test this question and showed that Cooper pairs are responsible for transporting cargo in this metallic state. It is interesting that no one is sure at the basic level of how they do it, so this discovery will require a little more theoretical and experimental work to understand exactly what is going on. "
Cooper pairs were named after Leon Cooper, a professor of physics at Brown who won the Nobel Prize in 1972 for describing their role in enabling superconductivity. Resistance arises when electrons rattle in the atomic network during movement. But when the electrons combine to form Cooper pairs, they undergo an extraordinary transformation. The electrons themselves are fermions, particles that follow the principle of Pauli exclusion, which means that each electron tends to maintain its own quantum state. Cooper couples, however, act like bosons that happily share the same state. This bosonic behavior allows Cooper pairs to coordinate their movements with other sets of Cooper pairs in a way that reduces drag to zero.
In 2007, Valles, working with Brown's professor of engineering and physics, Jimmy Xu, showed that Cooper pairs could also produce isolation and superconductivity. In very thin materials, instead of moving in harmony, the pairs conspire to stay in place, get stuck on small islands in the material and cannot jump to the next island.
In this new study, Valles, Xu, and colleagues from China searched for Cooper vapors in a non-superconducting metallic state, using a technique similar to that which disclosed Cooper's vapor isolators. This technique involves the design of a thin-film superconductor – in this case, the high-temperature barium yttrium-copper superconductor (YBCO) – through a series of small holes. When the material flows through it and is exposed to a magnetic field, the charge carriers in the material will circulate around the holes like water circulating in the gutter.
"We can measure the frequency with which these charges circulate," said Valles. "In this case, we've found that the frequency is consistent with two electrons circling instead of one. We can therefore conclude that the charge carriers in this state are Cooper pairs, not individual electrons. "
Researchers say the idea that boson-like pairs are responsible for this metallic state, because there are elements of quantum theory that suggest that this should not be possible. Understanding what happens in this state can lead to exciting new physics, but more research will be needed.
Fortunately, scientists say that detecting this phenomenon in high-temperature superconductor will make future research more practical. YBCO begins superconducting at a temperature of about -181 degrees Celsius, and the metallic phase begins at slightly higher temperatures. It's quite cold, but it's much warmer than other superconductors that are active just above absolute zero. This higher temperature facilitates the use of spectroscopy and other techniques to better understand what is happening in this metallic phase.
Scientists say that this barefooted metal can be used later on in the road for new types of electronic devices.
"The problem with bosons is that they are more wavelike than electrons, so we are talking about being in phase and causing interference in a similar way to light," said Valles. "So there may be new ways of transferring cargo in devices by playing with interference between bosons."
But for now, researchers are happy that they have discovered a new state of matter.
"Science is based on discoveries," said Xu, "and it's great to discover something completely new."
Reference: "Indirect bosonic metallic state in superconductor-isolator transition" 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, November 14, 2019, Science.
DOI: 10.1126 / science.aax5798