Phonon anharmonicity in quantum paraelectrics beyond density-functional theory

The ABO₃ family of perovskite oxides exhibits a wide range of physical properties and technological applications, with structural instabilities associated with anharmonic potential energy surfaces playing a key role. In the case of SrTiO₃ and KTaO₃, the strongly anharmonic lattice dynamics lead to a quantum paraelectric behavior at low temperatures, whereby the ferroelectric instability is suppressed due to anharmonic quantum fluctuations. A detailed quantitative understanding of the phonon anharmonicity underpins the study of emergent properties in this class of materials, but poses significant challenges.

In this talk, I will discuss an approach for calculating the temperature-dependent anharmonic properties of these materials with beyond density-functional theory accuracy. Specifically, we employ machine-learned potentials in combination with the stochastic self-consistent harmonic approximation. I will show that this method seamlessly allows to fully capture strong anharmonicities while retaining first-principles accuracy, and furthermore opens up the possibility to perform many-body calculations beyond DFT via Δ-machine learning. I will show that the paraelectric phase in KTaO₃ and SrTiO₃ is stabilized by anharmonic quantum fluctuations down to 0 K. In the case of SrTiO₃, we further characterize the cubic to tetragonal transition by the softening of the antiferrodistortive instability. I will discuss how a quantitative description of the quantum paraelectric behavior requires a higher-level treatment of electronic correlation effects via the random phase approximation.