First-principles investigation of the electronic states at perovskite and pyrite hetero-interfaces

摘要

Oxide heterostructures are attracting huge interest in recent years due to thespecial functionalities of quasi two-dimensional quantum gases. In this thesis, theelectronic states at the interface between perovskite oxides and pyrite compoundshave been studied by first-principles calculations based on density functional theory.Optimization of the atomic positions are taken into account, which is considered veryimportant at interfaces, as observed in the case of LaAlO3/SrTiO3.The creation of metallic states at the interfaces thus is explained in terms ofcharge transfer between the transition metal and oxygen atoms near the interface.It is observed that with typical thicknesses of at least 10-12 °A the gases still extendconsiderably in the third dimension, which essentially determines the magnitude ofquantum mechanical effects. To overcome this problem, we propose incorporation ofhighly electronegative cations (such as Ag) in the oxides. A fundamental interest isalso the thermodynamic stability of the interfaces due to the possibility of atomicintermixing in the interface region. Therefore, different cation intermixed configurationsare taken into account for the interfaces aiming at the energetically stablestate.The effect of O vacancies is also discussed for both polar and non-polar heterostructures.The interface metallicity is enhanced for the polar system with thecreation of O vacancies, while the clean interface at the non-polar heterostructureexhibits an insulating state and becomes metallic in presence of O vacancy. The Ovacancy formation energies are calculated and explained in terms of the increasingelectronegativity and effective volume of A the side cation.Along with these, the electronic and magnetic properties of an interface betweenthe ferromagnetic metal CoS2 and the non-magnetic semiconductor FeS2 is investigated.We find that this contact shows a metallic character. The CoS2 stays quasihalf metallic at the interface, while the FeS2 becomes metallic. At the interface,ferromagnetic ordering is found to be energetically favorable as compared to antiferromagneticordering. Furthermore, tensile strain is shown to strongly enhancethe spin polarization so that a virtually half-metallic interface can be achieved, forcomparably moderate strain.Our detailed study is aimed at complementing experiments on various oxide interfacesand obtaining a general picture how factors like cations, anions, their atomicweights and elecronegativities, O vacancies, lattice mismatch, lattice relaxation, magnetismetc play a combined role in device design.