Scanning transmission electron micrscopy image of superlattice consisting of an
alternating sequence of 5 atomic unit cells of neodymium nickelate (blue) and 5
atomic unit cells of samarium nickelate (yellow). Courtesy: Bernard Mundet /
EPFL.
Today's
silicon-based electronics consume a substantial and ever-increasing share of
the world's energy. A number of researchers are exploring the properties of
materials that are more complex than silicon but that show promise for the
electronic devices of tomorrow—and that are less electricity-hungry. In keeping
with this approach, scientists from the University of Geneva (UNIGE) have been
working in collaboration with the Swiss Federal Institute of Technology in
Lausanne (EPFL), the University of Zurich, the Flatiron Institute of New York
and the University of Liège. The scientists have discovered a hitherto-unknown
physical phenomenon in an artificial material made up of very thin layers of
nickelates. This could be exploited to accurately control some of the
material's electronic properties, such as the sudden transition from a
conductive to an insulating state. It could also be used to develop new, more
energy-efficient devices. You can read about this technological advance in the
journal Nature Materials.
"Nickelates
are known for a special characteristic: they suddenly switch from an insulating
state to that of an electrical conductor when their temperature rises above a
certain threshold," begins Jean-Marc Triscone, a professor in the
Department of Quantum Matter Physics in UNIGE's Faculty of Science. "This
transition temperature varies according to the composition of the
material."
Nickelates
are formed from a nickel oxide with the addition of an atom belonging to
so-called "rare earth" elements (i.e. a set of 17 elements from the
Periodic Table). When this rare earth is samarium (Sm), for example, the
metal-insulator jump takes place at around 130°C, while if it is neodymium (Nd),
the threshold drops to -73°C. This difference is explained by the fact that
when Sm is replaced by Nd, the compound's crystal structure is deformed—and it
is this deformation that controls the value of the transition temperature.
In their
attempt to learn more about these materials, the Geneva-based scientists
studied samples made up of repeated layers of samarium nickelate deposited on
layers of neodymium nickelate—a kind of "super sandwich" where all
the atoms are perfectly arranged.
Behaving
like a single material
Claribel
Domínguez, a researcher in the Department of Quantum Matter Physics and the
article's first author, explains: "When the layers are quite thick, they
behave independently, with each one keeping its own transition temperature.
Oddly enough, when we refined the layers until each one was no larger than
eight atoms, the entire sample began behaving like a single material, with only
one large jump in conductivity at an intermediate transition temperature."
A very
detailed analysis performed by electron microscope at EPFL—backed up by
sophisticated theoretical developments undertaken by American and Belgian
colleagues—showed that the propagation of the deformations in the crystal
structure at the interfaces between the materials only takes place in two or
three atomic layers. Accordingly, it is not this distortion that explains the
observed phenomenon. In reality, it is as though the furthest layers somehow
know that they are very close to the interface but without being physically
deformed.
It's not
magic
"There's
nothing magical about it," says Jennifer Fowlie, a researcher in the
Department of Quantum Matter Physics and co-author of the article. "Our
study shows that maintaining an interface between a conductive region and an
insulating region, as is the case in our samples, is very expensive in terms of
energy. So, when the two layers are thin enough, they are able to adopt much
less energy-intensive behavior, which consists of becoming a single material,
either totally metallic or totally insulating, and with a common transition
temperature. And all this happens without the crystal structure being changed.
This effect, or coupling, is unprecedented."
This
discovery was made possible thanks to the support provided by the Swiss
National Science Foundation and the Q-MAC ERC Synergy Grant (Frontiers in
Quantum Materials' Control). It provides a new way of controlling the
properties of artificial electronic structures, which, in this instance, is the
jump in conductivity obtained by the Geneva researchers in their composite
nickelate, which represents an important step forward for developing new
electronic devices. Nickelates could be used in applications such as
piezoelectric transistors (reacting to pressure).
More
generally, the Geneva work fits into a strategy for producing artificial
materials "by design," i.e. with properties that meet a specific
need. This path, which is being followed by many researchers around the world,
holds promise for future energy-efficient electronics.