# Soft phonon modes in the vicinity of the structural quantum critical point

## TL;DR

This study investigates how soft phonon modes emerge near the structural quantum critical point in quasi-skutterudite superconductors, revealing phonon softening and increased electron-phonon coupling close to structural instability.

## Contribution

It constructs a phase diagram and analyzes phonon density of states to demonstrate phonon softening near the quantum critical point in these materials.

## Key findings

- Phonon softening occurs near the quantum critical point.
- Enhanced electron-phonon coupling is observed near structural instability.
- The results support phonon softening as a key feature of quantum criticality in these compounds.

## Abstract

The quasi-skutterudite superconductors $A_3T_4$Sn$_{13}$ ($A$=Sr, Ca; $T$=Ir, Rh, Co) are highly tunable featuring a structural quantum critical point. We construct a temperature-lattice constant phase diagram for these isovalent compounds, establishing Ca$_{3}$Rh$_4$Sn$_{13}$ and Ca$_{3}$Co$_4$Sn$_{13}$ as members close to and far away from the structural quantum critical point, respectively. Deconvolution of the lattice specific heat and the electrical resistivity provide an approximate phonon density of states $F(\omega)$ and the electron-phonon transport coupling function $\alpha_{tr}^2F(\omega)$ for Ca$_{3}$Rh$_4$Sn$_{13}$ and Ca$_{3}$Co$_4$Sn$_{13}$, enabling us to investigate the influence of the structural quantum critical point. Our results support the scenario of phonon softening close to the structural quantum critical point, and explain the enhancement of the coupling strength on approaching structural instability.