Effects of doping on polar order in SrTiO$_{3}$ from first-principles modeling

TL;DR

This study models how doping influences polar order and lattice dynamics in SrTiO₃, revealing insights into phase transitions, electronic structure, and potential links to superconductivity.

Contribution

We develop a simplified free energy model and perform Monte Carlo and Langevin simulations to analyze doping effects on polar and antiferrodistortive phases in SrTiO₃.

Findings

Doping shifts polar and antiferrodistortive transition temperatures.

Doped SrTiO₃ exhibits polar nanodomains and altered lattice dynamics.

Electronic structure and Rashba parameters are affected by doping.

Abstract

SrTiO$_{3}$ is an incipient ferroelectric and an exceptionally dilute superconductor with a dome-like dependence on carrier concentration. Stabilization of a polar phase through chemical substitution or strain significantly enhances the superconducting critical temperature, suggesting a possible connection between the polar instability and unconventional Cooper pairing. To investigate the effects of doping on the polar order in SrTiO$_{3}$, we develop a simplified free energy model which includes only the degrees of freedom necessary to capture the relevant physics of a doped, biaxially compressively strained system. We simulate the polar and antiferrodistortive thermal phase transitions using Monte Carlo methods for different doping levels and comment on the doping dependence of the transition temperatures and the formation of polar nanodomains. In addition, the temperature-dependent phonon spectral function is calculated using Langevin simulations to investigate the lattice dynamics of the doped system. We also examine the effects of doping on the electronic structure within the polar phase, including the density of states and band splitting. Finally, we compute the polarization dependence of the Rashba parameter and the doping dependence of the Midgal ratio, and place our results in the broader context of proposed pairing mechanisms.