Proximity of SrTiO₃ to a quantum critical point [1] and evolution of underlying modes when tuned with even with modest pressure has become a promising new branch of study of quantum critical phenomena. The critical point here is associated with a soft optical phonon mode responsible for the ferroelectricity instability through a continuous displacive transition. Our recent dielectric measurements under pressure [2] exposed formation of a ‘quantum optical-acoustic state’ around 2K. Further to this, we performed triple-axis inelastic neutron scattering experiments on single-crystal STO down to 37 mK to directly observe the optical and acoustic (and hybridised) modes at and around q=0 in the multiple directions of the reciprocal space. In addition, we explored how the pressure affects the phonon mode in STO up to 5.0 kbar around 2K. Our observations shed light on the coupling of the soft optical mode with the acoustic phonons, and its response to external pressure [3]. We believe this could help us understand the importance of anharmonic lattice dynamics and unusual thermal transport in STO.  

[1] S. E. Rowley, et al. Nat. Phys., 10(5), 367–372 (2014)

[2] M. J. Coak et al., PNAS, 117, 12707 (2020)

[3] M. J. Coak, et al. Phys. Rev. B, 100 214111 (2019)