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

Abstract

The quasiskutterudite superconductors ${A}_{3}{T}_{4}{\mathrm{Sn}}_{13}$ ($A=\text{Sr}$, Ca; $T=\text{Ir}$, Rh, Co) are highly tunable, featuring a structural quantum critical point. We construct a temperature-lattice constant phase diagram for these isovalent compounds, establishing ${\mathrm{Ca}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ and ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ 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 $F(\ensuremath{\omega})$ and the electron-phonon transport coupling function ${\ensuremath{\alpha}}_{\text{tr}}^{2}F(\ensuremath{\omega})$ for ${\mathrm{Ca}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ and ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$, 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.