New class of polar metals provide polar distortion with low electrical resistivity

Polar metals have free carriers and show polar structural distortion at the same time. Oxide-based polar metals are among the most studied, but their poor electrical conductivity limits their usage in practical applications.

Du et al. identified a new class of polar metals, called hexagonal Heusler compounds, which proved to be much more conductive than their oxide counterparts. This conductivity could make hexagonal Heusler compounds useful in a type of non-volatile memory based on ferroelectric capacitors, which consist of a ferroelectric film sandwiched between two metal electrodes. Using polar metals for the electrodes instead of normal metals might allow researchers to make thinner ferroelectric capacitors, therefore allowing more efficient, higher density storage.

The authors grew films of two hexagonal Heusler compounds, LaAuGe and LaPtSb, using molecular beam epitaxy, and studied their properties. They characterized the films’ polar structure with scanning transmission electron microscopy and X-ray diffraction. Magnetotransport measurements revealed both Heusler compounds to be more conductive than oxide-based polar metals. Combining their measurements with density functional theory modeling, the authors determined that atomic packing is the mechanism that causes polar distortion in this class of polar metals.

While previously studied polar metals have not been conductive enough to serve as electrodes in ferroelectric capacitors, “hexagonal Heusler compounds are an ideal candidate material for making that full ferroelectric capacitor stack with a higher conductivity metal,” said author Jason Kawasaki. The authors plan to fabricate a ferroelectric capacitor stack using a metallic hexagonal Heuser compound as the polar electrode, and an insulating hexagonal Heusler as the ferroelectric material.

Source: “High electrical conductivity in the epitaxial polar metals LaAuGe and LaPtSb,” by Dongxue Du, Amber Lim, Chenyu Zhang, Patrick J. Strohbeen, Estiaque H. Shourov, Fanny Rodolakis, Jessica L. McChesney, Paul Voyles, Daniel C. Fredrickson, and Jason K. Kawasaki, APL Materials (2019). The article can be accessed at https://doi.org/10.1063/1.5132339 .