"Polar metals should not be possible."Chang-Beom Eom, University of Wisconsin-Madison
Most materials are only capable of being one thing at a time, but a team of engineers and physicists at the University of Wisconsin-Madison have created an entirely new material that simultaneously possesses completely contradictory properties. The compound, which the researchers described in a paper in Nature, is a polar metal.
"Polar metals should not be possible," says Chang-Beom Eom, professor of materials science and engineering, who led the research.
Undeterred by known physical laws, Eom and his colleagues created a compound that is a scientific oxymoron. Through a new synthesis approach supported by computational modeling, the group made a crystal with multiple personalities: part polar, part metallic. Metals conduct electricity because electrons flow freely throughout them; polar materials, by contrast, impede the free flow of electrons and work as electrical insulators.
In order to produce a material with both insulating and conducting properties, Eom’s team separated the polar and metallic parts of a perovskite crystal. This caused some of the electrons in the crystal to act like they were in a metal, moving within the material to conduct electricity, while other electrons contributed to the polar properties.
Because the natural molecular structure of the material is symmetrical, however, even after separating the two components, the material as a whole would not display polar properties. The reason for this was that the equal and opposite arrangements of electrons canceled each other out. To overcome this obstacle, the researchers synthesized a version of the material with slightly off-kilter atoms, which threw off the internal symmetry enough to make the material polar.
"The initial calculations that the theory suggested did not show the polar nature so we experimentally tested the materials, then went back and improved the models," explains Eom. "We looped between theory and experiments, but most importantly, we actually created the material, demonstrated its polar and metallic properties, and developed an understanding of how this is happening."
Eom and colleagues made the polar metal by painstakingly growing thin films of perovskite crystal one atom at a time. Crucially, they grew the substance on top of a supporting lattice with a slightly offset molecular organization. Tightly clamping the growing film to this support skewed the internal arrangement of their material, stabilizing its internal geometry in the asymmetrical orientation necessary to maintain the polar properties.
Synthesizing and characterizing this first-of-its-kind material required patience and precision. As the material slowly grew one layer at a time, the researchers counted every atom deposited on the surface. They then used multiple optical, electronic and structural measurements to determine its properties.
This work is an attempt to accelerate the discovery of multifunctional materials with unusual coexisting properties, paving the way for devices with the ability to perform simultaneous electrical, magnetic and optical functions. "This has been a complex effort, and theoretical and experimental contributions from all collaboration members has been essential. The project would not succeed otherwise," Eom asserts.
This story is adapted from material from the University of Wisconsin-Madison, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.