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${}_{13}$

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 $T^{\ast }$ that is higher than the structural phase transition temperature $T_{\mathrm{s}}$. The spin-lattice relaxation rate ($1/T_1$) divided by temperature ($T$), $1/T_{1}T$, and the Knight shift $K$ increase with decreasing $T$ down to $T^{\ast }$, but start to decrease below $T^{\ast }$ and followed by more distinct changes at $T_{\mathrm{s}}$. 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 $\exp (-\Delta /k_{\mathrm{B}}T)$ with a large gap $\Delta =2.21k_{\mathrm{B}}{T}_{\mathrm{c}}$, 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.

arXiv commentChin. Phys. B (in press)

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• Order-Disorder Transitions in (Ca${}_x$Sr${}_{1-x}$)${}_3$Rh${}_4$Sn${}_{13}$
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• Possible quantum fluctuations in the vicinity of the quantum critical point of $(\mathbf{Sr},\mathbf{Ca}{)}_3\mathbf{I}{\mathbf{r}}_4\mathbf{S}{\mathbf{n}}_{13}$ revealed by high-energy X-ray diffraction study
• Fully-gapped superconductivity with rotational symmetry breaking in pressurized kagome metal CsV${}_3$Sb${}_5$
• Robust superconductivity and the suppression of charge-density wave in ${\text{Ca}}_3({\text{Ir}}_{1-x}{\text{Rh}}_x{)}_4{\text{Sn}}_{13}$ single crystals at ambient pressure