Phonon Anharmonicity in Tunable Quantum Paraelectrics*

The quantum paraelectric behavior and strongly anharmonic lattice dynamics in SrTiO3 and KTaO3 have attracted interest for decades. By tunning temperature, doping, and external field, quantum paraelectric materials can cross multiple phase transitions and approach a ferroelectric quantum critical point (QCP). Besides the ferroelectric soft mode and phase transition, recent research also revealed unusual thermal transport properties such as thermal Hall effects in SrTiO3 and KTaO3, as well as superconductivity, which motivate detailed studies of lattice dynamics. In particular, understanding phonon anharmonicity near a QCP is crucial to rationalizing the properties of quantum paraelectrics. We used inelastic neutron scattering and first-principle simulations to probe the temperature, electric field, and doping effects on phonons in SrTiO3 and KTaO3. Our experiments reveal striking intensity changes, as well as phonon damping and shifts, reflecting strong acoustic-optic phonon coupling which are reproduced by our calculations. These results provide direct insights into the behaviors of phonon eigenvectors and ionic interactions.