HfRuP and ZrRuAs have garnered significant interest in recent years as potential topological superconductors. However, the mechanism underlying their superconducting pairing, particularly whether it is mediated by electron-phonon coupling (EPC), remains unresolved. Here, we conducted polarized and temperature-dependent Raman scattering experiments to comparatively investigate their EPC. Six Raman-active phonon modes were assigned for each compound based on polarized Raman spectra and first-principles calculations. Temperature-dependent Raman scattering measurements revealed a large and continuous softening of some low-energy phonon modes for both crystals in the measurement temperature range from $1.7\mathrm{to}300\mathrm{K}$. In HfRuP, the $E^{″}$ $\left(91.5{\mathrm{cm}}^{-1}\right)$ and $E^{\prime }$ $\left(102.9{\mathrm{cm}}^{-1}\right)$ modes exhibit phonon softening and linewidth broadening upon cooling. In ZrRuAs, the $E^{″}$ $\left(88.8{\mathrm{cm}}^{-1}\right)$ mode shows similar behavior. Furthermore, the $E^{\prime }$ mode in HfRuP $\left(357.2{\mathrm{cm}}^{-1}\right)$ and the $A_1^{\prime }$ mode in ZrRuAs $\left(171.1{\mathrm{cm}}^{-1}\right)$ exhibit conventional frequency hardening but anomalous linewidth broadening upon cooling. Our first-principles calculations attribute these anomalies to a resonancelike enhancement of the EPC between the Fermi-surface electrons and the Ru-dominated $E^{″}$ phonon mode. The electronic band structures also demonstrate that the enhancement stems from the exact matching in energy scale between the Ru-dominated low-frequency phonon modes and the distribution of their electronic densities of states around $E_F$. These results provide direct evidence that EPC drives the superconducting pairing in both compounds, highlighting the pivotal role of Ru atoms in these ternary transition metal pnictide superconductors.

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• Phonons
• Superconductivity
• Topological superconductors
• BCS theory
• Density functional calculations
• Raman spectroscopy