An international team of physicists and materials scientists from the United States and China has discovered a new state of matter they’ve named ‘Cooper pair metal.’
Cooper pairs are pairs of coupled electrons that enable superconductors to conduct electricity without resistance.
They are named for Leon Cooper, an American physicist who won the Nobel Prize in 1972 for describing their role in enabling superconductivity.
Resistance is created when electrons rattle around in the atomic lattice of a material as they move. But when electrons join together to become Cooper pairs, they undergo a remarkable transformation.
Electrons by themselves are fermions, particles that obey the Pauli exclusion principle, which means each electron tends to keep its own quantum state. Cooper pairs, however, act like bosons, which can happily share the same state.
That bosonic behavior allows Cooper pairs to coordinate their movements with other sets of Cooper pairs in a way that reduces resistance to zero.
In 2007, Brown University’s Professor Jim Valles and colleagues showed that Cooper pairs could also produce insulating states as well as superconductivity.
In very thin materials, rather than moving in concert, the pairs conspire to stay in place, stranded on tiny islands within a material and unable to jump to the next island.
For the new study, the researchers looked for Cooper pairs in the non-superconducting metallic state using a technique similar to the one that revealed Cooper pair insulators.
The technique involves patterning a thin-film superconductor — in this case a high-temperature superconductor yttrium barium copper oxide — with arrays of tiny holes.
When the material has a current running through it and is exposed to a magnetic field, charge carriers in the material will orbit the holes like water circling a drain.
The scientists measured the frequency at which these charges circle and found it is consistent with there being two electrons going around at a time instead of just one.
So they concluded that the charge carriers in this state are Cooper pairs and not single electrons.
“There had been evidence that this metallic state would arise in thin film superconductors as they were cooled down toward their superconducting temperature, but whether or not that state involved Cooper pairs was an open question,” Professor Valles said.
“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.”
The research is described in a paper in the journal Science.
Chao Yang et al. Intermediate bosonic metallic state in the superconductor-insulator transition. Science, published online November 14, 2019; doi: 10.1126/science.aax5798