High electrical conductivity in the epitaxial polar metals LaAuGe and LaPtSb

TL;DR

This paper reports the successful epitaxial growth of polar metals LaAuGe and LaPtSb with high electrical conductivity, revealing their structural, electronic, and potential for multiferroic heterostructures, advancing polar metal research.

Contribution

First demonstration of epitaxial growth of highly conductive polar metals LaAuGe and LaPtSb with detailed structural and electronic characterization.

Findings

LaAuGe and LaPtSb exhibit low resistivity and high carrier density.

Structural analysis confirms polar buckled planes in these materials.

Band structure aligns with DFT calculations, supporting their metallic nature.

Abstract

Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning transmission electron microscopy (STEM) reveals a polar structure with unidirectionally buckled $BC$ (PtSb, AuGe) planes. Magnetotransport measurements demonstrate good metallic behavior with low residual resistivity ($\rho_{LaAuGe}=59.05$ $\mu\Omega\cdot$cm and $\rho_{LaAPtSb}=27.81$ $\mu\Omega\cdot$cm at 2K) and high carrier density ($n_h\sim 10^{21}$ cm$^{-3}$). Photoemission spectroscopy measurements confirm the band metallicity and are in quantitative agreement with density functional theory (DFT) calculations. Through DFT-Chemical Pressure and Crystal Orbital Hamilton Population analyses, the atomic packing factor is found to support the polar buckling of the structure, though the degree of direct interlayer $B-C$ bonding is limited by repulsion at the $A-C$ contacts. When combined with insulating hexagonal Heuslers, these materials provide a new platform for fully epitaxial, multiferroic heterostructures.