First Principle Study Of Perovskite RTiO₃/AVO₃ Superlattices

ABO₃ perovskite transition metal oxides have become a hot topic in condensed matter physics and materials science due to their own extensive physical properties,such as ferroelectricity,ferromagnetism,multiferroicity,high-temperature superconductivity,and colossal magnetoresistance.With the increasing demand of miniaturized and multifunctional devices,the preparation and regulation of materials at nanometer scale and the realization of multi-functionalization have become one of the efforts of researchers.With the rapid development of oxide film preparation technology,the fabrication of transition metal perovskite oxide superlattices at high atomic scale can be realized.Superlattices are able to couple the materials with different properties together,using the recombination of lattice,charge,orbit and spin degrees of freedom at the interface to produce richer or even different physical properties from the parent material,and to realize the multifunctionalization of the material.Therefore,the in-depth study of transition metal perovskite oxide superlattices has important guiding significance in the design and application of materials and devices.In this paper,the polar metal and metal-insulated transitions in transition metal perovskite oxide superlattices are investigated at the atomic scale using the first-principles calculation method based on density functional theory.The main contents and conclusions of this study are as follows:?1?Itinerant electrons can screen the electric fields and inhibit the electrostatic forces in ferroelectric materials,so the metallicity and ferroelectricity in materials are considered incompatible.To realize the coexistence of metallicity and ferroelectricity in materials,that is,polar metals,the SrVO₃with metallicity and the PbTiO₃with ferroelectricity are selected as materials to form two-color superlattices,and the magnetic ground state and electronic structure of the superlattices at the[001],[110],[101]and[111]stacking orientations are investigated.Firstly,it is found that in the case of all stacking orientations,the PbTiO₃/SrVO₃superlattice interface formed two-dimensional electron gas with spin polarization,which provided the metallicity at the interface;secondly,the polar displacement of the PbTiO₃is not sensitive to the appearance of metallicity,and the ferroelectricity of the PbTiO₃ layer is shared at the interface.Hence,2D polar metals with spin polarization in all oriented PbTiO₃/SrVO₃ superlattices are realized theoretically.?2?As one of the interesting phenomena generated by the electron reconstruction of the superlattice interface,the metal–insulator transition has a very powerful application prospect in new electronic devices.Herein,three kind of physical avenues,i.e.,stacking orientation,epitaxial strain and thickness periods,have been used to tune the metal-insulator transition.Because of the dimensional effect of stacking orientation,the degree of charge transfer at the interface is different.The[001]and[110]oriented?LaTiO₃?₁/?CaVO₃?₁superlattices on SrTiO₃substrate are insulating,while[111]oriented case is metallic.Since the epitaxial strain changes the lattice distortion and adjusts the energy level of 3d orbitals,when the SrTiO₃substrate is replaced La Al O₃the substrate,a metal–insulator transition occurs in[111]oriented case.Besides the structural orientation and strain effect,it is found that the thickness of the[001]oriented?LaTiO₃?_m/?CaVO₃?_nsuperlattice is also highly probable to induce a metal-insulation transition.