Approaching a quantum critical point (QCP) has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce due to the lack of suitable systems with a continuous structural transition that can be conveniently tuned to 0~K. Using inelastic X-ray scattering, we examine the phonon spectrum of the nonmagnetic quasi-skutterudite (CaxSr1−x)3Rh4Sn13, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration x. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at x=0.85, the soft mode energy squared at the M point extrapolates to zero at (−5.7±7.7)~K, providing the first compelling microscopic evidence of a structural QCP in (CaxSr1−x)3Rh4Sn13. The enhanced phonon density-of-states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP.
@article{arxiv.1810.02248,
title = {Evidence of a structural quantum critical point in (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ from a lattice dynamics study},
author = {Y. W. Cheung and Y. J. Hu and M. Imai and Y. Tanioku and H. Kanagawa and J. Murakawa and K. Moriyama and W. Zhang and K. T. Lai and K. Yoshimura and F. M. Grosche and K. Kaneko and S. Tsutsui and Swee K. Goh},
journal= {arXiv preprint arXiv:1810.02248},
year = {2018}
}