Structural phase transition, precursory electronic anomaly and strong-coupling superconductivity in quasi-skutterudite (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ir$_{4}$Sn$_{13}$ and Ca$_{3}$Rh$_{4}$Sn$_{13}$

TL;DR

This study investigates the interplay between structural phase transitions and superconductivity in quasi-skutterudite compounds using NMR, revealing precursory electronic anomalies and strong-coupling superconductivity, with implications for understanding similar phenomena in other materials.

Contribution

The paper uncovers precursory electronic anomalies before structural transitions and demonstrates strong-coupling superconductivity in specific quasi-skutterudite compounds, advancing knowledge of their electronic behavior.

Findings

Precursor electronic anomalies observed above structural transition temperature.

Strong-coupling superconductivity indicated by large energy gap without coherence peak.

Distinct differences in NMR responses between compounds with and without structural transitions.

Abstract

The interplay between superconductivity and structural phase transition has attracted enormous interests in recent years. For example, in Fe-pnictide high temperature superconductors, quantum fluctuations in association with structural phase transition have been proposed to lead to many novel physical properties and even the superconductivity itself. Here we report a finding that the quasi-skutterudite superconductors (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ir$_{4}$Sn$_{13}$ ($x$ = 0, 0.5, 1) and Ca$_{3}$Rh$_{4}$Sn$_{13}$ show some unusual properties similar to the Fe-pnictides, through $^{119}$Sn nuclear magnetic resonance (NMR) measurements. In (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ir$_{4}$Sn$_{13}$, the NMR linewidth increases below a temperature $T^*$ that is higher than the structural phase transition temperature $T_{\rm s}$. The spin-lattice relaxation rate ($1/T_1$) divided by temperature ($T$), $1/T_1T$, and the Knight shift $K$ increase with decreasing $T$ down to $T^*$, but start to decrease below $T^*$ and followed by more distinct changes at $T_{\rm s}$. In contrast, none of the anomalies was observed in Ca$_{3}$Rh$_{4}$Sn$_{13}$ that does not undergo a structural phase transition. The precursory phenomenon above structural phase transition resembles that occurs in Fe-pnictides. In the superconducting state of Ca$_{3}$Ir$_{4}$Sn$_{13}$, $1/T_{1}$ decays as ${\rm exp}(-\Delta/k_{\rm B}T)$ with a large gap $\Delta = 2.21 k_{\rm B}T_{\rm c}$, yet without a Hebel-Slichter coherence peak, which indicate strong-coupling superconductivity. Our results provide new insight into the relationship between superconductivity and the electronic-structure change associated with structural phase transition.