Origin of the AFD mode gap in tetragonal SrTiO3

Determine whether the elevation of the anti-ferrodistortive (AFD) Eg mode in tetragonal SrTiO3 to a positive frequency with an approximately 2 meV gap at the Γ point (for lattice constants a = 3.88 Å and c/a = 1.0 at low temperature) is caused by nuclear tunneling between degenerate wells on the Born–Oppenheimer potential energy surface or by anharmonic phonon interactions.

Background

In tetragonal SrTiO3, the anti-ferrodistortive (AFD) Eg mode is soft in harmonic phonon calculations. Using path-integral molecular dynamics (PIMD), the authors observe that this soft AFD mode is lifted to a positive frequency with a ~2 meV gap at the Γ point. This raises a key interpretive uncertainty about the physical mechanism responsible for the lift in frequency.

The ambiguity stems from competing explanations: nuclear tunneling (quantum fluctuations due to motion between degenerate wells on the potential energy surface) versus anharmonic phonon interactions. Conventional approaches like SSCHA may capture anharmonicity but are argued to be inadequate for tunneling, which complicates attribution of the observed gap and motivates a careful separation of these effects.