Effects of quantum confinement on excited state properties of SrTiO$_3$ from ab initio many-body perturbation theory

TL;DR

This study uses advanced ab initio methods to analyze how quantum confinement affects the excited state properties of SrTiO$_3$ and related layered oxides, revealing significant excitonic effects and localization phenomena.

Contribution

First ab initio calculations of excited state properties across the Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ series, demonstrating the impact of quantum confinement on excitonic behavior.

Findings

Optical gaps match experimental data.

Exciton binding energies increase with quantum confinement.

Localized excitons in Sr$_2$TiO$_4$ are confined to 2D TiO$_2$ layers.

Abstract

The Ruddlesden-Popper (RP) homologous series Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO$_3$. We use ab initio many-body perturbation theory within the $GW$ approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectrum of Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ for $n=1-5$ and $\infty$. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small $n$ and reach a value of 330~meV for $n=1$, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr$_2$TiO$_4$ ($n=1$) localizes in the 2D plane defined by the TiO$_2$ layer, and explain the origin of its localization.