A mysterious superconductor’s wave could reveal the physics behind the materials

Physicists have finally captured a
superconductor’s wave.

The first direct evidence of a phase of matter known as a pair-density wave helps reveal the physics that underlies mysterious high-temperature superconductors, which conduct electricity without resistance at surprisingly high temperatures. The wave was detected using a scanning tunneling microscope, researchers report April 1 in Nature.

Physicists had suspected that
pair-density waves existed in these materials, and previous experiments had
hinted at their existence. But without direct proof, scientists couldn’t
advance their understanding of the materials. “Investigating and proving [that]
this phase not only might exist, but actually does exist, is very important,”
says theoretical physicist Eun-Ah Kim of Cornell University, who wasn’t
involved in the work.

High-temperature superconductors wowed
physicists when the materials came on the scene in the 1980s. Known as cuprates because they contain copper, the materials conduct electricity without resistance
at temperatures much higher than most other superconductors, some around 100
kelvins (about –173° Celsius) or higher (SN:
12/8/17).

Although still chilly, such temperatures
are much easier to attain than the nearly absolute zero temperatures required
for many superconductors. The materials’ discovery led to high hopes that a
room-temperature superconductor could soon be found, possibly leading to new
technologies such as vastly more energy-efficient electric grids, magnetically
levitated trains and powerful supercomputers.

But, decades later, a room-temperature
superconductor has yet to appear on the scene. What’s more, scientists still
don’t fully understand the physics that makes these materials so special. In
particular, “we want to understand the microscopic mechanism of how superconductivity
occurs in those materials,” says physicist Kazuhiro Fujita of Brookhaven National
Laboratory in Upton, N.Y. Now, scientists are drawing a bit closer to a
solution.

In superconductors, electrons buddy up into
duos called Cooper pairs,
a partnership that allows them to slip smoothly through the material without
resistance (SN: 5/13/15). In these
materials, scientists observe a gap in the energies of electrons, rather than a
continuous spectrum.

Physicists predicted that, in
high-temperature superconductors, the gap in the electrons’ energies would periodically
vary across the surface of the material in a strange kind of wave. That effect might
be linked to another unusual state that exists in the same materials at higher
temperatures, called the pseudogap phase. That state inhabits a strange
purgatory: It’s neither superconductor nor insulator, and it conducts
electricity but not all that well.

Fujita and colleagues detected the wave by
skimming across the surface of a superconducting compound — a bismuth-based copper
oxide — with a scanning tunneling microscope. The microscope has an extremely
thin tip that detects electrons that pass across the space between the superconductor
and tip via a quantum process known as tunneling. In this case, the researchers
also affixed a tiny piece of superconductor to the microscope’s tip, to search
for electrons tunneling from one bit of the superconductor to another. The
energy gap, the team reported, periodically varied across the surface of the
material in a wave, as predicted.

“This is actually a direct measurement
of the pair-density wave component,” says theoretical physicist Eduardo Fradkin
of the University of Illinois at Urbana-Champaign. “It’s a really exciting
experiment.”

That pseudogap phase may be important in
the quest to increase the temperature range of high-temperature superconductors.
The new result could help scientists understand that phase better by illuminating
how these materials behave as they warm up.