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The interplay between superconductivity and structural phase transition hasattracted enormous interests in recent years. For example, in Fe-pnictide hightemperature superconductors, quantum fluctuations in association withstructural phase transition have been proposed to lead to many novel physicalproperties and even the superconductivity itself. Here we report a finding thatthe quasi-skutterudite superconductors(Sr$_{1-x}$Ca$_{x}$)$_{3}$Ir$_{4}$Sn$_{13}$ ($x$ = 0, 0.5, 1) andCa$_{3}$Rh$_{4}$Sn$_{13}$ show some unusual properties similar to theFe-pnictides, through $^{119}$Sn nuclear magnetic resonance (NMR) measurements.In (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ir$_{4}$Sn$_{13}$, the NMR linewidth increasesbelow a temperature $T^*$ that is higher than the structural phase transitiontemperature $T_{\rm s}$. The spin-lattice relaxation rate ($1/T_1$) divided bytemperature ($T$), $1/T_1T$, and the Knight shift $K$ increase with decreasing$T$ down to $T^*$, but start to decrease below $T^*$ and followed by moredistinct changes at $T_{\rm s}$. In contrast, none of the anomalies wasobserved in Ca$_{3}$Rh$_{4}$Sn$_{13}$ that does not undergo a structural phasetransition. The precursory phenomenon above structural phase transitionresembles that occurs in Fe-pnictides. In the superconducting state ofCa$_{3}$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 aHebel-Slichter coherence peak, which indicate strong-couplingsuperconductivity. Our results provide new insight into the relationshipbetween superconductivity and the electronic-structure change associated withstructural phase transition.

chinese physics b/chinese physics b

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$_{13}$