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 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 reveals a polar structure with unidirectionally buckled BC (PtSb and AuGe) planes. Magnetotransport measurements demonstrate good metallic behavior with low residual resistivity (ρLaAuGe = 59.05 μΩ cm and ρLaAPtSb = 27.81 μΩ cm at 2 K) and high carrier density (nh ∼ 1021 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 although the degree of direct interlayer B–C bonding is limited by repulsion at the A–C contacts. When combined with predicted ferroelectric hexagonal Heuslers, these materials provide a new platform for fully epitaxial, multiferroic heterostructures.
Highlights
INTRODUCTIONPolar metals were once assumed to be rare because of free carrier screening, there exist a number of known polar metals, including bulk LiOsO3,3 Ca3Ru2O7,4 PtTi1−xNbxO3,5 epitaxially stabilized LaNiO3/LaAlO3,1 and trilayer WTe2.6 Polar metals show a number of promising properties and applications, for example, nonlinear optics, nonreciprocal charge transport, and potential use as electrode materials to suppress the critical thickness limit in ferroelectric capacitors. A significant challenge, is that most of the well-studied oxide polar metals are poor electrical conductors with residual resistivity greater than 500 μΩ cm. Several transition metal dichalcogenides (TMDs) have recently been demonstrated as polar metals with a residual resistivity of approximately 100 μΩ cm; the weak out-of-plane van der Waals interactions make
We demonstrate the molecular beam epitaxial 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
Through density functional theory (DFT)-chemical pressure and crystal orbital Hamilton population analyses, the atomic packing factor is found to support the polar buckling of the structure the degree of direct interlayer B–C bonding is limited by repulsion at the A–C contacts
Summary
Polar metals were once assumed to be rare because of free carrier screening, there exist a number of known polar metals, including bulk LiOsO3,3 Ca3Ru2O7,4 PtTi1−xNbxO3,5 epitaxially stabilized LaNiO3/LaAlO3,1 and trilayer WTe2.6 Polar metals show a number of promising properties and applications, for example, nonlinear optics, nonreciprocal charge transport, and potential use as electrode materials to suppress the critical thickness limit in ferroelectric capacitors. A significant challenge, is that most of the well-studied oxide polar metals are poor electrical conductors with residual resistivity greater than 500 μΩ cm. Several transition metal dichalcogenides (TMDs) have recently been demonstrated as polar metals with a residual resistivity of approximately 100 μΩ cm; the weak out-of-plane van der Waals interactions make. A significant challenge, is that most of the well-studied oxide polar metals are poor electrical conductors with residual resistivity greater than 500 μΩ cm.. It is important to identify polar metals that are both good conductors and adopt crystal structures amenable to integration with common semiconductor platforms.. 2θ − ω scans for LaPtSb (blue trace) and LaAuGe (red trace) thin films grown on Al2O3 (0001) with substrate reflections labeled by asterisks. (g) In-plane rotation φ scan of the LaPtSb film 101 ̄2 and Al2O3 101 ̄4 reflections. We demonstrate the epitaxial growth of single crystalline LaAuGe and LaPtSb films on (0001) oriented Al2O3 substrates. Magnetotransport measurements reveal that the residual resistivity is an order of magnitude lower than most oxide based polar metals. Our results provide a new epitaxial platform for multifunctional polar materials and devices for engineering the subtle interplay between polarization, band topology, charge, and spin
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