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

Patrick J. Strohbeen, Dongxue Du, Chenyu Zhang, Estiaque H. Shourov, Fanny Rodolakis, Jessica L. McChesney, Paul M. Voyles, and Jason K. Kawasaki

Phys. Rev. Materials 3, 024201 – Published 6 February 2019

Abstract

We report the molecular beam epitaxial growth, structure, and electronic measurements of single-crystalline LaAuSb films on ${Al}_{2} O_{3}$ (0001) substrates. LaAuSb belongs to a broad family of hexagonal $A B C$ 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 $A B C s$ 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 $A B C s$ . 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 $A B C s$ .

Received 10 December 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.3.024201

Physics Subject Headings (PhySH)

Electric polarization

Heusler alloy

Molecular beam epitaxy Photoemission spectroscopy Transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Patrick J. Strohbeen¹, Dongxue Du¹, Chenyu Zhang¹, Estiaque H. Shourov¹, Fanny Rodolakis², Jessica L. McChesney², Paul M. Voyles¹, and Jason K. Kawasaki^1,*

¹Materials Science and Engineering, University of Wisconsin Madison, Madison, Wisconsin 53706, USA
²Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA

^*jkawasaki@wisc.edu

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Vol. 3, Iss. 2 — February 2019

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Images

Figure 1
Structural distortions in hexagonal $A B C$ compounds [12]. (a) The centrosymmetric ZrBeSi-type structure, consisting of graphitelike $B C$ planes. (b) The polar LiGaGe-type structure, defined by unidirectional buckling $d$ of the $B C$ planes. In both ZrBeSi- and LiGaGe-type structures, the stacking of the $B C$ planes follows a $B − C − B − C$ sequence. (c) The centrosymmetric YPtAs-type structure consisting of an alternating “up-down” buckling of the $B C$ planes and a doubled unit cell along the $c$ axis. The $B C$ planes follow a $B − B − C − C$ stacking sequence. (d) In-plane crystal structures of the $A B C$ compounds and comparison to the sapphire substrate.
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Figure 2
RHEED patterns and intensity oscillations. (a), (b) RHEED patterns of the ${Al}_{2} O_{3}$ substrate along the $[12 \bar{3} 0]$ and $10 \bar{1} 0]$ azimuths, respectively. (c), (d) The corresponding RHEED patterns for the LaAuSb film. (e) RHEED intensity oscillations of the specular (red) and +1 (green) spots observed during growth. The corresponding integration boxes are shown in panel (c).
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Figure 3
X-ray diffraction of LaAuSb films on ${Al}_{2} O_{3}$ (0001). (a) $θ - 2 θ$ scan of LaAuSb film (19 valence electrons) and comparison to LaAuGe (18 valence electrons) [7]. The LaAuSb shows half-order superstructure reflections, corresponding to a doubled unit cell along the $c$ axis. Reflections from the substrate are marked by asterisks. (b) Higher-resolution $θ - 2 θ$ scan around the 0002 reflection of LaAuSb, showing sharp Kiessig fringes. (c) Pole figure of the LaAuSb $10 \bar{1} 4$ reflection and sapphire $10 \bar{1} 2$ reflection, showing the epitaxial relationship between the film and the substrate. (d) Rocking curve scans of the substrate and film showcasing the high crystallinity of this film.
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Figure 4
(a) HAADF-STEM image of the LaAuSb film showing the long-range ordering of the Au-Au dimerized structure, oriented along a $[12 \bar{3} 0]$ zone axis. The [0001] growth direction points upwards. (b) Zoomed-in region of the image in panel (a) to show finer details of the atomic structure. The cartoon ball-and-stick model in panel (b) is added as a guide to the eye where the gray, gold, and purple balls represent La, Au, and Sb atoms, respectively.
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Figure 5
(a) Resistivity (zero-field), carrier densities, and mobilities versus temperature. The densities and mobilities were extracted from a two-band fit to the $ρ_{x y} (B$ ) data. RRR is the residual resistivity ratio ( $ρ_{300 K} / ρ_{2 K}$ ). (b) Top panel: Longitudinal magnetoresistance $[ρ_{x x} (B) - ρ_{x x} (0)] / [ρ_{x x} (0)]$ . Bottom panel: Transverse (Hall) resistivity $ρ_{x y}$ versus magnetic field, at select temperatures.
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Figure 6
(a) Angle-integrated shallow core levels measured at a photon energy of 1486 eV. (b) Valence band measurement at a photon energy of 300 eV and comparison to the DFT (GGA + SO) density of the dimerized and undimerized structures.
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