In a finding that reveals
an entirely new state of matter, research published in the journal Science
shows that Cooper pairs, electron duos that enable superconductivity, can also
conduct electricity like normal metals do.
PROVIDENCE, R.I. [Brown
University] — November 14, 2019 -- For years, physicists have assumed that Cooper pairs, the
electron duos that enable superconductors to conduct electricity without
resistance, were two-trick ponies. The pairs either glide freely, creating a
superconducting state, or create an insulating state by jamming up within a
material, unable to move at all.
But in a new paper published in Science, a team of researchers has shown
that Cooper pairs can also conduct electricity with some amount of resistance,
like regular metals do. The findings describe an entirely new state of matter,
the researchers say, that will require a new theoretical explanation.
“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,” said Jim Valles, a professor of
physics at Brown University and the study’s corresponding author. “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.”
Cooper pairs are named for Leon Cooper,
a physics professor at Brown 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 the reduces resistance to
zero.
In 2007, Valles, working with Brown
engineering and physics professor Jimmy Xu, 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 this new study, Valles, Xu and
colleagues in China 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
(YBCO) — 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.
“We can measure the frequency at which
these charges circle,” Valles said. “In this case, we found that the frequency
is consistent with there being two electrons going around at a time instead of
just one. So we can conclude that the charge carriers in this state are Cooper
pairs and not single electrons.”
The idea that boson-like Cooper pairs
are responsible for this metallic state is something of a surprise, the
researchers say, because there are elements of quantum theory that suggest this
shouldn’t be possible. So understanding just what is happening in this state
could lead to some exciting new physics, but more research will be required.
Luckily, the researchers say, the fact
that this phenomenon was detected in a high-temperature superconductor will
make future research more practical. YBCO starts superconducting at around -181
degrees Celsius, and the metallic phase starts at temperatures just above that.
That’s pretty cold, but it’s much warmer than other superconductors, which are
active at just above absolute zero. That higher temperature makes it easier to
use spectroscopy and other techniques aimed at better understand what’s
happening in this metallic phase.
Down the road, the researchers say, it
might be possible to harness this bosonic metal state for new types of
electronic devices.
“The thing about the bosons is that they
tend to be in more of a wavelike state than electrons, so we talk about them
having a phase and creating interference in much the same way light does,”
Valles said. “So there might be new modalities for moving charge around in
devices by playing with interference between bosons.”
But for now, the researchers are happy
to have discovered a new state of matter.
“Science is built on discoveries,” Xu
said, “and it’s great to have discovered something completely new.”
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