Structural and electronic properties of monodomain ultrathin PbTiO3/SrTiO3/PbTiO3/SrRuO3 heterostructures: A first-principles approach

Reza Mahjoub,Valanoor Nagarajan,Javier Junquera

doi:10.1063/5.0031505

Abstract

First-principles calculations within the local density approximation were carried out to explain the ground state and electronic properties of a vacuum/PbTiO3/SrTiO3/PbTiO3/SrRuO3 multilayer in a monodomain phase. Open-circuit boundary conditions were assumed, considering the electric displacement field, D, as the fundamental electrical variable. The direction and the magnitude of D can be monitored by proper treatment of the PbO surface layer, introducing external fractional charges Q in the surface atomic layers by means of virtual crystal approximation. Different excess or deficit surface charges (from Q=±0.05 to Q=±0.15) were considered, corresponding to small values of the polarization (up to ±0.16C/m2) in both directions. The layer-by-layer electric polarization, tetragonality, and the profile of the electrostatic potential were computed, as well as the projected density of states, as a function of electric displacement field. The magnitude of D is preserved across the dielectric layers, which translates into a polarization of the SrTiO3 spacer layer. The tetragonality of the two PbTiO3 layers is different, in good agreement with experimental x-ray diffraction techniques, with the layer closer to the free surface exhibiting a smaller value. This is attributed to the interplay with surface effects that tend to contract the material in order to make the remaining bonds stronger. Our calculations show how the final structure in this complex oxide heterostructure comes from a delicate balance between electrical, mechanical, and chemical boundary conditions.

Highlights

It is well established that epitaxial heterostructures comprising ultrathin perovskite layers may present emergent properties, which may be absent in their individual constituents.[1,2,3] Among the immense number of possible combinations between ABO3 perovskite oxides, (PbTiO3)n=(SrTiO3)m superlattices have received significant interest during the last few years
Going beyond the previous models[20] in this system, here we report first-principles simulations on the structural and electronic properties of a 14 unit cell PbTiO3 thin film grown on a SrRuO3 metallic electrode where, following the spirit of the previous experiments, a spacing layer of three unit cells of SrTiO3 has been intentionally introduced at the center of the ferroelectric
Using state of the art first-principles calculations, where we have been able to carefully control the electrical, mechanical, and chemical boundary conditions of a complex oxide heterostructure, we have clearly identified how the final structure comes from a delicate balance between all of these interactions

Summary

Introduction

It is well established that epitaxial heterostructures comprising ultrathin perovskite layers may present emergent properties, which may be absent in their individual constituents.[1,2,3] Among the immense number of possible combinations between ABO3 perovskite oxides, (PbTiO3)n=(SrTiO3)m superlattices (where n and m are, respectively, the number of unit cells of each perovskite combined in the heterostructure) have received significant interest during the last few years. Different ground states with a large variety of functional properties can be accessed by playing with the periodicity, layer sequence at the heterostructures, the strain conditions imposed by the substrate, the temperature,[4] or the electric field[5] applied between two external electrodes. The driving force for this transition is the depolarizing field: the divergence of the polarization

Methods

Results

Conclusion