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Description
The proximity of SrTiO$_3$ to a ferroelectric quantum critical point (FE QCP) has become a promising new branch of the study of quantum critical phenomena. New forms of quantum order have been reported in SrTiO$_3$, different from the quantum paraelectric state via dielectric measurements. The critical point here is associated with a soft optical phonon mode responsible for the ferroelectric instability.
We report our recently performed triple-axis inelastic neutron scattering experiments on single-crystal SrTiO$_3$ at the temperature and pressure region of interest. These were the first direct measurements deep into the enigmatic “quantum polar-acoustic state” in the vicinity of the FE QCP. Measurements are taken at and around $q$ = 0 in multiple directions in reciprocal space to explore the transverse acoustic and soft optical phonon modes and their hybridization. In addition, we explore how pressure affects the underlying phonon modes in SrTiO$_3$. We also present first-principles calculations in the low-temperature tetragonal phase, in comparison with the experimental observations. Our explorations directly address the coupling of the soft optical mode with the acoustic phonons, and its response to external pressure. We believe this could help us understand the importance of anharmonic lattice dynamics and quantum fluctuations in SrTiO$_3$.
Acknowledgement The experiments were carried out with the support of the Institut Laue-Langevin, Grenoble, France through the approval of beamtime for proposals 7-02-212, 7-02-213, 7-02-219 and 7-02-223 on IN8 and the support of Paul Scherrer Institute for proposal 20230501 on EIGER. The calculations were performed with the support of IDRIS high-performance computer centre for the calculation time under the GENCI project n_A0150801842. The authors would like to thank G. G. Guzmán-Verri, P. B. Littlewood, E. Artacho, J. Lashley, M.B. Lepetit, and T. Weber for their generous help and discussions.
