Using precision techniques for making superconducting thin films layer-by-layer,
physicists at the U.S. Department
of Energy's (DOE) Brookhaven National Laboratory have identified a single
layer responsible for one such material's ability to become superconducting,
i.e., carry electrical current with no energy loss. The technique, described
in the October 30, 2009, issue of Science, could be used to engineer ultrathin
films with "tunable" superconductivity for higher-efficiency electronic
devices.
This graphic shows the inside of the molecular beam epitaxy chamber where thin films are built layer by layer, showing an artists rendition of the film synthesis process.
"We wanted to answer a fundamental question about such films," said
Brookhaven physicist and the group leader Ivan Bozovic. "Namely: How thin
can the film be and still retain high-temperature superconductivity?"
The thinner the material (and the higher its transition temperature to a superconductor),
the greater its potential for applications where the superconductivity can be
controlled by an external electric field. "This type of control is difficult
to achieve with thicker films, because an electric field does not penetrate
into metals more than a nanometer or so," Bozovic explained.
To explore the limits of thinness, Bozovic's group synthesized a series of
films based on the high-temperature superconducting cuprates (copper-oxides)
— materials that carry current with no energy loss when cooled below a
certain transition temperature (Tc). Since zinc is known to suppress the superconductivity
in these materials, the scientists systematically substituted a small amount
of zinc into each of the copper-oxide layers. Any layer where the zinc's presence
had a suppressing effect would be clearly identified as essential to superconductivity
in the film.
"Our measurements showed that the zinc doping had essentially no effect,
except when placed in a single, well-defined layer. When the zinc was in that
layer, the superconductivity was dramatically suppressed," Bozovic said.
The material studied by Bozovic's team was unusual in that it consists of layers
of two materials, one metallic and one insulating, that are not superconductors
on their own, but rather exhibit superconductivity at the interface between
them [see http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=822].
The layer identified as essential to the superconductivity by the zinc-substitution
experiment represents the second copper-oxide layer away from the interface.
The scientists found that the presence of zinc had no effect on the transition
temperature at which superconductivity sets in, about 32 kelvin (-241 Celsius),
except when placed in that particular layer. In the latter case, the scientists
observed a dramatic drop in the transition temperature to 18 kelvin (-255 Celsius).
The reduction in transition temperature provides a clear indication that that
particular layer is the "hot" one responsible for the relatively high
temperature at which superconductivity normally sets in for this material.
"We now have a clean experimental proof that high-temperature superconductivity
can exist, undiminished, in a single copper-oxide layer," Bozovic said.
"This piece of information gives important input to our theoretical understanding
of this phenomenon."
Bozovic explained that, in the material he studied, the electrons required
for superconductivity actually come from the metallic material below the interface.
They leak into the insulating material above the interface and achieve the critical
level in that second copper-oxide layer.
But in principle, he says, there are other ways to achieve the same concentration
of electrons in that single layer, for example, by doping achieved by applying
electric fields. That would result in high-temperature superconductivity in
a single copper-oxide layer measuring just 0.66 nanometers.
From a practical viewpoint, this discovery opens a path toward the fabrication
of electronic devices with modulated, or tunable, superconducting properties
which can be controlled by electric or magnetic fields.
"Electronic devices already consume a large fraction of our electricity
usage — and this is growing fast." Bozovic continued. "Clearly,
we will need less-power hungry electronics in the future." Superconductors,
which operate without energy loss — particularly those that operate at
warmer, more-practical temperatures — may be one way to go.
Bozovic's layer-by-layer synthesis method and ability to strategically alter
individual layers' composition might also be used to explore and possibly control
other electronic phenomena and properties that emerge at the interfaces between
layered materials.