A New Material Combines Contradictory Properties

For the most part, it is generally accepted that materials tend to be capable of being only one thing at any given time. However, a team of engineers and physicists at the University of Wisconsin-Madison have created an entirely new material with multiple, contradictory properties.

The atomic scale model of this polar material. Neodymium atoms are pink and the nickel atoms are yellow.

Chang-Beom Eom, the Theodore H. Geballe Professor and Harvey D. Spangler Distinguished Professor of materials science and engineering at UW-Madison led the research team to develop a crystal with two unique qualities: part polar, part metallic.

“Polar metals should not be possible,” says Eom.

Metals conduct electricity because electrons flow freely throughout them. Polar materials, by contrast, impede the free flow of electrons and work as electrical insulators. Eom’s team worked to develop a material that offered both insulating and conducting abilities.

The team first separated the polar and metallic parts of the crystal. Some electrons gave rise to the metallic nature, moving within the material to conduct electricity. Other electrons contributed to the polar properties.

The natural molecular structure of the material is symmetrical, even after separating the two components. Therefore, the material as a whole would not act polar. Equal and opposite arrangements of electrons canceled each other out. To overcome this obstacle, the researchers synthesized the substance with slightly off-kilter atoms, which threw off the internal symmetry enough to make the material polar.

Tae Heon Kim, the lead author on the paper, peers into the synthesis chamber that made the polar metal, while Chang-Beom Eom (right) and Mark Rzchowski observe. Photo credit: Sam Million-Weaver

“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,” says 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.”

In order to overcome this challenge, the team grew thin films of material one atom at a time. 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 polarity.

The researchers counted every atom deposited on the surface, as the substance slowly grew one layer at a time. They then used multiple complex optical and electronic and structural measurements to determine its properties.

The discovery of multifunctional materials with unusual coexisting properties will pave the way to develop devices with the ability to perform simultaneous electrical, magnetic and optical functions.

www.engr.wisc.edu