Structural phase transition, precursory electronic anomaly, and strong-coupling superconductivity in quasi-skutterudite (Sr_1-xCa_x)₃Ir₄Sn₁₃ and Ca₃Rh₄Sn₁₃

The interplay between superconductivity and structural phase transition has attracted enormous interest 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-xCa_x)₃Ir₄Sn₁₃ (x=0, 0.5, 1) and Ca₃Rh₄Sn₁₃ show some unusual properties similar to the Fe-pnictides, through ¹¹⁹Sn nuclear magnetic resonance (NMR) measurements. In (Sr_1-xCa_x)₃Ir₄Sn₁₃, the NMR linewidth increases below a temperature T^* that is higher than the structural phase transition temperature T_s. The spin-lattice relaxation rate (1/T₁) divided by temperature (T), 1/T₁T 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_s. In contrast, none of the anomalies is observed in Ca₃Rh₄Sn₁₃ that does not undergo a structural phase transition. The precursory phenomenon above the structural phase transition resembles that occurring in Fe-pnictides. In the superconducting state of Ca₃Ir₄Sn₁₃, 1/T₁ decays as exp(-△/k_BT) with a large gap △=2.21 k_BT_c, yet without a Hebel-Slichter coherence peak, which indicates strong-coupling superconductivity. Our results provide new insight into the relationship between superconductivity and the electronic-structure change associated with structural phase transition.