Ambient Pressure Structural Quantum Critical Point in the Phase Diagram   of (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$
  The quasi-skutterudite superconductor Sr$_3$Rh$_4$Sn$_{13}$ features a pronounced anomaly in electrical resistivity at $T^*\sim$138 K. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centred around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of $T^*$ as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., $x_c=0.9$). This establishes (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures. 
S. K. Goh, D. A. Tompsett, P. J. Saines, H. C. Chang, T. Matsumoto, M.   Imai, K. Yoshimura, and F. M. Grosche
cond-mat.supr-con cond-mat.str-el