Superconductivity enhanced by abundant low-energy phonons in (Sr$_{1-x}$Ca$_x$)$_3$Rh$_4$Sn$_{13}$

TL;DR

This study shows that structural quantum criticality in (Sr$_{1-x}$Ca$_x$)$_3$Rh$_4$Sn$_{13}$ enhances low-energy phonons, leading to stronger superconductivity and altered electron-phonon scattering behavior.

Contribution

It reveals how structural quantum criticality increases low-energy phonons and enhances strong-coupling superconductivity in (Sr$_{1-x}$Ca$_x$)$_3$Rh$_4$Sn$_{13}$.

Findings

Lattice specific heat increases near the SQCP.

Superconducting gap correlates with low-temperature lattice specific heat.

Electrical resistivity shows T^2 dependence despite strong electron-phonon interaction.

Abstract

The effects of structural quantum criticality on the strong-coupling superconductivity of (Sr$_{1-x}$Ca$_x$)$_3$Rh$_4$Sn$_{13}$ have been investigated via electrical resistivity and specific heat measurements. We demonstrate that the lattice specific heat at low temperatures considerably increases toward the structural quantum critical point (SQCP), $x_{\rm c}\approx0.9$. The superconducting gap increases with $x$ in the exact same fashion as the low-temperature lattice specific heat, clearly indicating that the abundant low-energy phonons cause strong-coupling superconductivity. Despite the electron-phonon interaction, which is much more enhanced than the electron correlation, the low-temperature electrical resistivity near the SQCP varies as $\propto T^2$. This finding suggests that structural quantum fluctuation affects the power law arising from the electron-phonon scattering.