|
High-temperature superconductivity doesn’t happen all at once. It starts in
isolated nanoscale patches that gradually expand until they take over.
That discovery, from atomic-level observations at Cornell
University and the University of Tokyo,
offers a new insight into the puzzling “pseudogap” state observed in
high-temperature superconductors; it may be another step toward creating new
materials that superconduct at temperatures high enough to revolutionize
electrical engineering.
Using extremely precise scanning tunneling microscopes (STM) that can
observe the states of electrons around atoms, an international research team
led by J.C. Séamus Davis, the J.G. White Distinguished Professor in the
Physical Sciences, and by Hidenori Takagi, professor of physics at the
University of Tokyo, has for the first time observed how a high-temperature
superconductor evolves as its chemical composition is modified. They found that
as more “dopant” atoms are added, small, scattered superconducting areas,
some just a few atoms across, appear. These grow until they touch and
eventually fill the entire space, whereupon the entire material becomes a
superconductor.
“Some theorists have imagined that this is what happens,” Davis said, “but
there has been no evidence until now.” The research was reported in an
online edition of Nature Physics.
Superconductivity, in which an electric current flows with zero resistance,
was first discovered in metals cooled very close to absolute zero (-273 C). New
materials called cuprates—copper oxides “doped” with other atoms—superconduct
as “high” as -123 C.
Observations of high-temperature superconductors with the STM and other
instruments show an “energy gap” where electronic states are missing.
Theory says that electrons have left to join into “Cooper pairs” that
can carry an electric current without interference. A puzzler for physicists is
that sometimes this energy gap appears but the material still does not superconduct—a
so-called “pseudogap” phase. The pseudogap appears at higher
temperatures than any superconductivity, offering the promise of someday
developing materials that would superconduct at or near room temperature.
The researchers use STMs to scan a surface in steps smaller than an atom,
measuring what electron energy levels are occupied and what electrons are
conspicuous by their absence. They examined a series of samples of a material
known as sodium-doped calcium cuprate, prepared with gradually increasing
sodium content. As more sodium is added to the mix it displaces calcium atoms,
changing the crystal structure and the arrangement of electrons in ways not
completely understood. This particular cuprate was chosen because its simple
chemistry allows fine tuning, Davis
said. The phenomena observed had not been seen before because most cuprates
make abrupt transitions from insulator to pseudogap to superconductor, he
explained.
At a moderate level of doping, the STM finds small, scattered areas with the
pseudogap signature. These areas also show a “broken symmetry” where
the arrangement of electrons between copper and oxygen atoms differs between
“north and south” and “east and west” in the square crystal
lattice. Davis and colleagues had found this broken symmetry in earlier
observations of the same superconductors.
As doping increases, these areas become larger until finally they touch, and
the entire sample becomes a superconductor. It’s presumed that the scattered
pseudogap regions occur in the vicinity of dopant atoms, but those atoms were
not observed in the current study, Davis
said.
Previously, the researchers noted, it was thought that the pseudogap phase
in cuprates might be in competition with superconductivity, something that had
to be gotten out of the way before superconductivity could happen. This work,
they said, suggests that it is beneficial—a necessary step in the evolution of
a superconductor.
