Multigap superconductivity in the filled-skutterudite compound LaRu4As12 probed by muon spin rotation

Muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and inelastic x-ray scattering were used to investigate the superconducting properties of the filled-skutterudite compound ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$. A two-gap isotropic ($s+s$)-wave model can explain the temperature dependence of the superfluid density. Zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements confirm that the time-reversal symmetry does not break upon entering the superconducting state. The measurements of lattice dynamics at 2, 20, and 300 K revealed temperature dependences of the phonon modes that do not strictly follow a hardening of phonon frequencies upon cooling as expected within the quasiharmonic picture. The 20 K data rather mark a turning point for the majority of the phonon frequencies. Indeed, a hardening is observed approaching 20 K from above, while for a few branches a weak softening is visible upon further cooling to 2 K. The observed dispersion relations of phonon modes throughout the Brillouin zone matches with the density functional theory prediction quite closely. Our results point out that cubic ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$ is a good reference material for studying multiband superconductivity, including those with lower crystallographic symmetries such as iron arsenide--based superconductors.