Electronically enhanced layer buckling and Au-Au dimerization in epitaxial LaAuSb films

Abstract

We report the molecular beam epitaxial growth, structure, and electronic measurements of singlecrystalline LaAuSb films on Al2O3 (0001) substrates. LaAuSb belongs to a broad family of hexagonal ABC intermetallics in which the magnitude and sign of layer buckling have strong effects on properties, e.g., predicted hyperferroelecticity, polar metallicity, and Weyl and Dirac states. Scanning transmission electron microscopy reveals highly buckled planes of Au-Sb atoms, with strong interlayer Au-Au interactions and a doubling of the unit cell. This buckling is four times larger than the buckling observed in other ABCs with similar composition, e.g. LaAuGe and LaPtSb. Photoemission spectroscopy measurements and comparison with theory suggest an electronic driving force for the Au-Au dimerization, since LaAuSb, with a 19-electron count, has one more valence electron per formula unit than most stable ABCs. Our results suggest that the electron count, in addition to conventional parameters such as epitaxial strain and chemical pressure, provides a powerful means for tuning the layer buckling in ferroic ABCs.

Heusler interfaces—Opportunities beyond spintronics?

“…Other structural variants include Jahn-Teller driven cubic to tetragonal distortions, variations in the atomic site ordering (e.g. "inverse Heusler," D0 3 , and B2 cubic variants), and polar vs antipolar layer buckling patterns in the hexagonal variants [67,69,70].…”

“…A significant challenge is that only a small subset of hexagonal Heuslers is natively insulating -an assumed requirement for switching via applied electric fields [105]. Epitaxial strain, quantum confinement, and Peierls-like distortions [67,70,106] may provide routes for tuning the buckling and opening a gap in polar compounds that are natively metallic.…”

“…Significant advances have been made on the epitaxial growth and control of Heusler interfaces over the past 20 years, primarily driven by applications in spintronics. These include the development of Heusler molecular beam epitaxy (MBE) [48,[157][158][159][160][161], the identification of semi adsorption-controlled growth windows [50,70,119,161,162], the use of epitaxial diffusion barriers and low temperature seed layers [41,44,156,163], the use of chemical templating layers [160,164], and the development simple theoretical frameworks based on electron counting [57,165,166] for predicting stability and structural distortions at surfaces and interfaces [50,166].…”

“…[241,242]). For the MBE-grown samples, TiCoSb was grown on lattice matched InAlAs/InP(001) [49] and on MgO(001) [236], LuPtSb on InAlSb/GaSb(001) [119], and the hexagonal compounds LaAuSb, LaAuGe, and LaPtSb on Al2O3(0001) [70]. Most sputtered films were grown on MgO(001).…”

Semi-adsorption-controlled growth window for half Heusler FeVSb epitaxial films

“…For example, while the intrinsic carrier concentration of silicon is n i ∼ 10 10 cm −3 at room temperature, typical experimental carrier concentrations for semiconducting half Heuslers are typically ∼ 10 19 to 10 21 cm −3 due to defects and nonstoichiometry, which are difficult to control to better than 1% [21]. In select cases it has been shown that several Sb-containing Heuslersincluding CoTiSb [22,23], NiMnSb [24,25], LuPtSb [26], LaPtSb [27], and LaAuSb [28] -can be grown with an excess Sb flux, in which the ratio of Sb to (X + Y ) is self-limiting. Since the stoichiometry of one out of three elements is self-limiting, this can be called semi adsorption control.…”