Oxide heterostructures are attracting huge interest in recent years due to the
special functionalities of quasi two-dimensional quantum gases. In this thesis, the
electronic states at the interface between perovskite oxides and pyrite compounds
have been studied by first-principles calculations based on density functional theory.
Optimization of the atomic positions are taken into account, which is considered very
important at interfaces, as observed in the case of LaAlO3/SrTiO3.
The creation of metallic states at the interfaces thus is explained in terms of
charge 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 extend
considerably in the third dimension, which essentially determines the magnitude of
quantum mechanical effects. To overcome this problem, we propose incorporation of
highly electronegative cations (such as Ag) in the oxides. A fundamental interest is
also the thermodynamic stability of the interfaces due to the possibility of atomic
intermixing in the interface region. Therefore, different cation intermixed configurations
are taken into account for the interfaces aiming at the energetically stable
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 the
creation of O vacancies, while the clean interface at the non-polar heterostructure
exhibits an insulating state and becomes metallic in presence of O vacancy. The O
vacancy formation energies are calculated and explained in terms of the increasing
electronegativity and effective volume of A the side cation.
Along with these, the electronic and magnetic properties of an interface between
the ferromagnetic metal CoS2 and the non-magnetic semiconductor FeS2 is investigated.
We find that this contact shows a metallic character. The CoS2 stays quasi
half metallic at the interface, while the FeS2 becomes metallic. At the interface,
ferromagnetic ordering is found to be energetically favorable as compared to antiferromagnetic
ordering. Furthermore, tensile strain is shown to strongly enhance
the spin polarization so that a virtually half-metallic interface can be achieved, for
comparably moderate strain.
Our detailed study is aimed at complementing experiments on various oxide interfaces
and obtaining a general picture how factors like cations, anions, their atomic
weights and elecronegativities, O vacancies, lattice mismatch, lattice relaxation, magnetism
etc play a combined role in device design.
|Date of Award||Sep 2012|
- Physical Science and Engineering
|Supervisor||Udo Schwingenschloegl (Supervisor)|
- two dimensional electron gas
- perovskite oxides
- nanoscale devices