Modeling polar order in compressively strained SrTiO$_{3}$

TL;DR

This paper develops a simplified free energy model to study polar order in compressively strained SrTiO3, revealing complex transition behavior and coupling of order parameters relevant to its superconductivity.

Contribution

The authors introduce a minimal free energy model capturing key physics of polar order in strained SrTiO3 with near DFT accuracy, enabling large-scale simulations.

Findings

Polar transition is neither purely displacive nor order-disorder.

Model accurately predicts energies of large disordered systems.

Coupling between ferroelectric and antiferrodistortive phases is analyzed.

Abstract

Strontium titanate is an incipient ferroelectric in which superconductivity emerges at exceptionally low doping levels. Remarkably, stabilizing the polar phase through strain or chemical substitution has been shown to significantly enhance the superconducting critical temperature, and the polar instability plays a pivotal role in the majority of proposed superconducting pairing mechanisms. A rigorous understanding of ferroelectricity is therefore essential to elucidate the electron pairing mechanism in this material. To investigate the nature of the polar order in strontium titanate, we develop a simplified free energy model that only includes the degrees of freedom necessary to capture the relevant physics in a biaxially compressively strained system. Our model is able to calculate the energies of large, disordered systems with near DFT-level accuracy. We simulate the ferroelectric and antiferrodistortive phase transitions using the Monte Carlo method and discuss the coupling between various order parameters. Finally, we assess the character of the polar transition, which we find to be neither strictly displacive nor order-disorder.