Relativistic Effects in LaBi$_2$ Thin Films

TL;DR

The study addresses how strong spin-orbit coupling (SOC) shapes growth, structure, and electronic transport in LaBi$_2$ thin films within the LaPn$_2$ square-net family. It combines layer-by-layer MBE growth on MgO(001) with extensive structural, transport, and first-principles analyses (SOC-inclusive DFT, Wannier90/EPW, Boltzmann transport) to link band-structure modifications to reduced phonon scattering and improved growth. The key findings are that LaBi$_2$ stabilizes in a monoclinic Yb-monoclinic stacking, supports intrinsic superconductivity at $T_c \approx 0.55$ K, and exhibits higher metallicity than LaSb$_2$ due to SOC-induced band shifts and lower surface energy. This work shows SOC as a tunable parameter for thin-film growth and electronic properties in pnictide intermetallics, with implications for surface engineering and superconductivity in related materials.

Abstract

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$0.55~K. Compared to LaSb$_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.

Figures (15)

• Figure 1: (a) Yb-monoclinic structure observed in thin films of the light rare-earth dipnictides. Substitution of the Sb anion with Bi is expected to enhance SOC due to the larger atomic mass. (b) Degree of spacer-layer Pn dimerization in several reported compounds wang:1967, including the results presented in this work.
• Figure 2: (a) Schematic view of growth approaches attempted for LaBi2 synthesis, showing substrate temperature over time with the provided elemental fluxes represented by colored bars at the top. (b) Intensity of specular RHEED diffraction over the course of growth for each approach. Inset: RHEED patterns at selected stages of growth. (c) Snippet of the X-ray diffraction (XRD) $\theta/2\theta$ diffraction pattern resulting from each growth approach.
• Figure 3: (a) $\theta/2\theta$ diffraction pattern from a 75 nm film produced by the optimized two-step growth approach. (b) Rocking curve of the (006) peak. (c) Azimuthal $\varphi$-scan of an asymmetric diffraction peak showing a [100]$_{\text{LaBi}_2}\parallel$ [110]$_{\text{MgO}}$ register with the MgO substrate. (d) Reciprocal space map along the $h=1$ rod, showing finite splitting along $Q_z$ consistent with the Yb-mono structure.
• Figure 4: (a) Temperature-dependent sheet resistivity $\rho_\mathrm{xx}$ of a 75 nm film down to 2 K. (b) Hall resistivity $\rho_\mathrm{yx}$ and (c) symmetrized magnetoresistance ratio (MRR) under an out-of-plane field at a series of temperatures. (d) Superconducting transitions below 1 K plotted as the temperature-dependent ratio of resistance to normal-state resistance $R_{0}$, for a series of perpendicular magnetic fields. (e) Upper critical field $H_{c2}^{\perp}$ (defined at $R=0.5R_0$) versus temperature. Fit to the single-gap Ginzberg-Landau formula shown as dashed line.
• Figure 5: Calculated surface energy $\gamma$ as a function of the number of quintuple layers for LaSb2 (blue) and LaBi2 (green). The filled (open) circles represent the results obtained with (without) spin-orbit coupling (SOC).