Evidence of a structural quantum critical point in (Ca_xSr_1-x)₃Rh₄Sn₁₃ from a lattice dynamics study

The nonmagnetic quasiskutterudite (Ca_xSr_1-x)₃Rh₄Sn₁₃ represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration x [1-3]. We performed inelastic x-ray scattering to probe the phonon spectrum of (Ca_xSr_1-x)₃Rh₄Sn₁₃ for various calcium contents. We detected a complete phonon softening at the M point when approaching the structural transition temperature from above. Intriguingly, at x = 0.85, the energy squared of the soft mode at M extrapolates to zero at (-5.7 ± 7.7) K, providing the first compelling microscopic evidence of a structural quantum critical point (QCP) in (Ca_xSr_1-x)₃Rh₄Sn₁₃. The observed phonon softening around the M point provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP [4].

[1] S. K. Goh et al., PRL 114, 097002 (2015)
[2] W. C. Yu et al., PRL 115, 207003 (2015)
[3] Y. J. Hu et al., PRB 95, 155142 (2017)
[4] Y. W. Cheung et al., PRB 98, 161103 (R) (2018)