Strong coupling nature of kagome superconductivity in LaRu$_3$Si$_2$

TL;DR

This study combines muon spin rotation experiments and first-principles calculations to reveal that LaRu$_3$Si$_2$ exhibits strong coupling, unconventional kagome superconductivity with robust properties under pressure and features associated with flat bands and van Hove points.

Contribution

It provides the first detailed investigation into the strong coupling and unconventional nature of kagome superconductivity in LaRu$_3$Si$_2$, highlighting the roles of flat bands and electron correlations.

Findings

Superconductivity in LaRu$_3$Si$_2$ is isotropic and type-II.

The superconducting gap ratio indicates strong coupling.

Electron-phonon coupling alone cannot explain the high $T_c$.

Abstract

We report muon spin rotation (${\mu}$SR) experiments together with first-principles calculations on microscopic properties of superconductivity in the kagome superconductor LaRu$_3$Si$_2$ with $T_{\rm c}$ ${\simeq}$ 7K. We find that the calculated normal state band structure features a kagome flat band and Dirac as well as van Hove points formed by the Ru-$dz^2$ orbitals near the Fermi level. Below $T_{\rm c}$, ${\mu}$SR reveals isotropic type-II superconductivity, which is robust against hydrostatic pressure up to 2 GPa. Intriguingly, the ratio 2$\Delta/k_{\rm B}T_{\rm c}$ ${\simeq}$ 4.3 (where ${\Delta}$ is the superconducting energy gap) is in the strong coupling limit, and $T_{\rm c}$/$\lambda_{eff}^{-2}$ (where ${\lambda}$ is the penetration depth) is comparable to that of high-temperature unconventional superconductors. We also find that electron-phonon coupling alone can only reproduce small fraction of $T_{\rm c}$ from calculations, which suggests other factors in enhancing $T_{\rm c}$ such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and high density of states from narrow kagome bands. Our experiments and calculations taken together point to strong coupling and the unconventional nature of kagome superconductivity in LaRu$_3$Si$_2$.