Relativistic Effects Unlock Superconductivity in New Lanthanum-Bismuth Material

Scientists have long understood that manipulating crystalline structures through chemical substitution offers a route to exploring strong spin-orbit coupling and its effects on material properties. Reiley Dorrian (California Institute of Technology), Sungmin Song (Pohang University of Science and Technology), and Jinwoong Kim (Korea Institute for Advanced Study) et al. now present a detailed investigation into thin films of LaBi, a layered square-net intermetallic compound. Their research demonstrates the successful growth of pristine, layer-by-layer LaBi films exhibiting superconductivity at 0.55 K, and crucially, attributes the material’s enhanced metallic behaviour and improved growth characteristics to significant relativistic corrections within its electronic band structure. This finding is particularly important as it highlights the substantial influence of relativistic effects on both surface energy and intrinsic phonon scattering, offering new insights for the design of future materials with tailored electronic properties.

Work focusing on thin films of the LaPn2 (where Pn represents antimony or bismuth) family of intermetallics reveals that substituting antimony with bismuth dramatically alters the material’s properties.

Specifically, LaBi2 exhibits a pristine layer-by-layer growth mode and displays superconductivity at approximately 0.55 K. This represents a significant improvement over LaSb2, with researchers attributing the enhanced performance to relativistic corrections within the electronic band structure. The study details the growth of LaBi2 thin films using molecular beam epitaxy on magnesium oxide substrates, employing a large bismuth-to-lanthanum beam-flux ratio of 10 to 15 during the process.
Films were grown at a rate of approximately 0.034 Angstroms per second, and finalized with a germanium capping layer to mitigate air sensitivity. Detailed structural characterisation via X-ray diffraction confirmed the high quality of the grown films, while electrical measurements revealed superior metallic behaviour in LaBi2 compared to its antimony counterpart.

This difference is not attributed to changes in hopping amplitudes, but rather to a reduction in phonon scattering at elevated temperatures. Theoretical calculations utilising density functional theory further illuminate the underlying mechanisms. These calculations demonstrate that the increased spin-orbit coupling induced by bismuth shifts electronic states, suppressing intrinsic phonon scattering.

This suppression contributes to the observed enhancement in metallic conductivity and the emergence of superconductivity. The research highlights the power of anion-tunable spin-orbit coupling as a means to engineer the electronic properties of layered square-net intermetallics, opening avenues for the development of novel quantum materials with tailored functionalities. The findings suggest a route towards materials with improved resilience to magnetic fields and potentially unconventional pairing symmetries within the superconducting state.

LaBi2 thin film deposition and characterisation via molecular beam epitaxy

Molecular beam epitaxy was employed to synthesize LaBi₂ thin films on MgO substrates, achieving a base pressure of 10⁻¹⁰ mbar within the system. Prior to growth, the MgO substrates underwent laser annealing, a procedure detailed in previous studies, to enhance surface quality. La and Bi fluxes were supplied using conventional effusion cells, maintaining a substantial Bi to La beam-flux ratio of 10 to 15, while substrate heating was achieved with a standard SiC heating coil.

A La flux of approximately 7.5 × 10⁻⁹ mbar regulated a growth rate of 0.034 Å/s. To mitigate oxidation, all films received an in-situ deposition of a 5nm amorphous Ge capping layer at room temperature, followed by storage within a nitrogen-atmosphere glovebox. Despite these precautions, film degradation was noted within a few hours of exposure to air, as documented in the Supplemental Material.

Structural characterisation was performed using X-ray diffraction with a Smart-Lab Rigaku instrument. Electrical measurements, conducted above 2 K, utilised a Quantum Design Dynacool PPMS system equipped with a 14 T superconducting magnet. Films were configured into approximately 1-2mm square pieces and wire-bonded in a four-point van der Pauw geometry using Al wire.

Magnetoresistance and Hall resistivity were determined by symmetrizing the ρxx channel or anti-symmetrizing the Ryx channel, respectively, with a magnetic field applied perpendicularly to the film. Superconducting transitions below 1 K were measured using a Leiden dilution refrigerator with a two-axis vector magnet capable of 9 and 3 T.

Samples were shaped into approximately 1 × 2mm rectangles and bonded using a linear four-point configuration. Density functional theory calculations were performed using the Quantum ESPRESSO package, employing the PBEsol functional to describe exchange-correlation interactions. Norm-conserving pseudopotentials were used to treat ion-electron interactions, and electronic wavefunctions were expanded using a plane-wave basis set with a kinetic energy cutoff of 100 Ry. The Methfessel-Paxton smearing technique, with a broadening width of 0.01 Ry, was implemented to account for partial occupancies of electronic states.

Relativistic effects and structural properties of superconducting LaBi2 thin films

LaBi₂ thin films exhibit superconductivity at 0.55 K, demonstrating a pristine layer-by-layer growth mode. Compared to LaSb₂, the enhanced metallic behaviour and improved growth dynamics of LaBi₂ are attributed to significant relativistic corrections to its electronic band structure. These corrections impact both surface energy and intrinsic phonon scattering within the material.

Structural characterisation via X-ray diffraction confirms the layered structure, despite previous conflicting reports regarding the crystal class of LaBi₂. The degree of spacer-layer dimerisation in LaBi₂ was assessed, revealing values consistent with other compounds in the LnPn₂ family, where Ln represents a lanthanide element and Pn is either Sb or Bi.

Measurements indicate a Bi/La beam-flux ratio of 10 to 15 was used during molecular beam epitaxy, with a La flux of 7.5 × 10⁻⁹ mbar establishing a growth rate of approximately 0.034 Å/s. Despite precautions including a 5nm amorphous Ge capping layer, film degradation was observed within a few hours of air exposure.

Electrical measurements reveal superior metallic behaviour in LaBi₂ compared to LaSb₂, not due to changes in hopping amplitudes but rather due to suppressed phonon scattering at elevated temperatures. Calculations utilising a k-grid of 8 × 6 × 6 for bulk calculations and 8 × 8 × 1 for slab models, alongside a kinetic energy cutoff of 100 Ry, demonstrate the influence of spin-orbit induced shifting of electronic states.

Integration for conductivity and scattering rates employed an energy window of ±0.5 eV around the Fermi level, ensuring coverage of active transport states. These findings suggest a pathway towards anion-tunable manipulation of spin-orbit coupling and its impact on material properties.

Relativistic effects drive enhanced superconductivity and layer-by-layer growth in lanthanum dibismuth

Researchers have successfully grown thin films of lanthanum dibismuth (LaBi) exhibiting layer-by-layer growth and superconductivity at 0.55 Kelvin. This achievement builds upon prior work with lanthanum disbismuth (LaSb), demonstrating improved metallic behaviour and growth characteristics in LaBi. The enhanced properties are attributed to significant relativistic corrections within the electronic band structure of LaBi, influencing both surface energy and phonon scattering.

These relativistic effects fundamentally alter the Fermi surface, shifting bands relative to the Fermi energy and resulting in weaker phonon scattering and a lower surface energy. Consequently, LaBi displays more efficient layer-by-layer growth and superior electrical conductivity compared to LaSb. Structural analysis revealed a monoclinic structure, differing from previous bulk studies which may have misindexed the compound.

This work clarifies the subtle but important role of spin-orbit coupling in determining both the growth dynamics and metallic properties of these layered intermetallic films. Acknowledging limitations, the authors note discrepancies between their structural analysis and prior bulk studies, suggesting a need for re-evaluation of existing data.

Future research may focus on further exploring the impact of spin-orbit coupling on similar materials and investigating the potential for enhancing superconducting properties through compositional tuning or strain engineering. These findings establish a foundation for understanding the interplay between relativistic effects and material properties in layered intermetallics, potentially guiding the development of novel electronic materials.