Relativistic Effects in LaBi$_2$ Thin Films

Chemical substitution in crystalline quantum materials is a powerful way to explore the consequences of strong spin-orbit coupling on their structural and electronic properties. In this work, we present an investigation of thin films of the La$\textit{Pn}_2$ ($\textit{Pn}$~=Sb, Bi) class of layered square-net intermetallics. We report the growth of LaBi$_2$ with a pristine layer-by-layer growth mode, classifying it as a good metal displaying superconductivity at $\sim$0.55K. Compared to LaSb$_2$, we attribute the enhanced metallic behavior and improved growth dynamics of LaBi$_2$ to significant relativistic corrections to its electronic band structure and the resulting impact on both surface energy and intrinsic phonon scattering.

💡 Research Summary

In this work the authors investigate the structural, electronic, and superconducting properties of LaBi₂ thin films grown by molecular‑beam epitaxy (MBE), focusing on how the substitution of antimony (Sb) by the heavier bismuth (Bi) atom enhances relativistic spin‑orbit coupling (SOC) and thereby modifies material behavior. The study begins with a clear motivation: SOC, which scales with atomic number, can open gaps at symmetry‑protected band crossings, generate topological states, and influence superconducting pairing symmetry (e.g., Ising pairing). By replacing Sb with Bi, the authors aim to amplify SOC without altering the overall crystal chemistry of the LaPn₂ (Pn = Sb, Bi) family of layered square‑net intermetallics.

Growth experiments were performed on MgO (001) substrates under ultra‑high vacuum (10⁻¹⁰ mbar). A two‑step temperature protocol was devised: an initial high‑temperature “buffer” stage (≈ 450 °C) to promote a flat, layer‑by‑layer template, followed by an extended low‑temperature stage (≈ 330 °C) that allows Bi incorporation while suppressing its volatile desorption. Real‑time reflection high‑energy electron diffraction (RHEED) shows streaky, oscillatory patterns during the buffer stage, indicating smooth epitaxy, whereas the low‑temperature stage maintains the pattern without degradation, leading to phase‑pure LaBi₂ films. In contrast, single‑step growth at either temperature results in mixed phases (LaBi, LaBi₃, etc.) or decomposition into LaBi (111) grains.

X‑ray diffraction (θ‑2θ scans) of the optimized films reveals sharp (001) Bragg peaks with pronounced Laue fringes, confirming high crystallinity and a c‑axis lattice constant of ~17.5 Å. Rocking‑curve analysis yields a full width at half maximum (FWHM) of ~0.045°, and φ‑scans demonstrate a unique epitaxial relationship: