click to enlarge An artist’s rendering of a transition metal oxide superlattice, with an actual transmission electron microscopy image superimposed on the left panel. The red is manganese, yellow is lanthanum and blue is strontium. The top is a Fermi surface map which illustrates how electrons move in the material. Image: Kyle Shen

Materials called transition metal oxides have physicists intrigued by their
potentially useful properties—from magnetoresistance to superconductivity.

By combining two sophisticated experimental tools—oxide molecular beam
expitaxy and angle-resolved photoemission spectroscopy—researchers have gained
the first insights into quantum interactions in transition metal oxide
superlattices, which are artificial stacked layers of alternating materials,
each just a few atoms thick.

Even slight modifications to the stacking sequence can switch the entire
superlattice from a conductive to insulating state, due to the enhancement of
quantum interactions between the electrons. The findings were published online
in Nature Materials.

“We are interested in superlattices of transition metal oxides because
they can exhibit all sorts of exotic electronic and magnetic properties that do
not exist in the bulk of these materials,” says Kyle Shen, assistant
professor of physics and paper’s senior author. “They might be useful
someday, but from a scientific standpoint, they are just really fascinating
because the electrons can conspire to give rise to very unexpected emergent
phenomena.”

For some transition metal oxide superlattices, it has been shown that adding
just one extra layer of atoms to the stacked layers switches them from
conductor to insulator. Shen and his colleagues wanted to understand why this
occurs.

To do this, the team tapped the expertise of co-author Darrell Schlom, the
Herbert Fisk Johnson Professor of Industrial Chemistry in the Department of
Materials Science and Engineering, who with postdoctoral scholar Carolina
Adamo, created specifically designed stacks of two oxides, lanthanum manganese
oxide and strontium manganese oxide, each just a few atomic layers thick and
with atomic precision. To make the superlattices, they used molecular beam
epitaxy, which is like spray painting with the elements of the periodic table.

The team then used a unique piece of instrumentation designed and built by
Shen and Schlom’s groups at Cornell. It allowed them to study the superlattices
after synthesis by angle-resolved photoemission spectroscopy without exposing
the surfaces to air, which would contaminate the sample and obscure the
sensitive experiments. Eric Monkman, a graduate student in Shen’s group, and
colleagues then measured and analyzed how the electrons move through different
kinds of superlattices.

click to enlarge The molecular beam epitaxy-angle resolved photoemission spectroscopy instrument in Duffield Hall, with graduate student Eric Monkman. Image: Kyle Shen

It turned out that the distances between the interfaces of the lanthanum and
strontium oxides were the key: Pushing the interfaces farther apart made the electrons
more confined to each individual interface, resulting in an enhancement of the
quantum interactions, which drive the entire superlattice into an insulating
state.

By pushing the interfaces closer together, the electrons could start to move
between interfaces, resulting in a metallic state. The researchers were able to
reach these conclusions through the use of photoemission spectroscopy, which
maps the motion of electrons in solids at the atomic scale.

Advanced transmission electron microscopy imaging led by David A. Muller,
Cornell professor of applied and engineering physics and co-director of the
Kavli Institute at Cornell for Nanoscale Science, and graduate student Julia
Mundy, confirmed that the interfaces between the lanthanum and strontium were
indeed sharp, which helped confirm the quantum interactions.

Source: Cornell University