Modeling Polar Order in Compressively Strained Strontium Titanate

The perovskite oxide SrTiO₃ (STO) is both a dilute superconductor and an incipient ferroelectric whereby polar order can be induced through strain, chemical substitution, or isotope exchange. Recently, it has been shown that superconductivity is substantially enhanced in films that are deep within the polar phase, suggesting that inversion symmetry breaking may play an important role in the superconducting pairing mechanism. Carefully characterizing the nature of the polar order in STO is crucial for understanding how it may facilitate electron pairing in this material. In this work, we use density functional theory to compute the ground state structure and phonon spectrum of compressively strained STO thin films and develop a statistical model to simulate the structural phase transitions and polar order. By using computational methods that explicitly incorporate the effects of temperature and disorder on large supercells, we find that, contrary to common belief, the polar phase transition has significant order-disorder character. We place our results in the greater context of proposed superconducting pairing mechanisms in STO.