Two dimensional electron gas at oxide interfaces

Authors

Karolina Janicka, University of Nebraska-LincolnFollow

Date of this Version

Fall 12-2-2011

Document Type

Article

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Physics and Astronomy, Under the Supervision of Professor Evgeny Y. Tsymbal. Lincoln, Nebraska: December, 2011

Copyright (c) 2011 Karolina Janicka

Abstract

Extraordinary phenomena can occur at the interface between two oxide materials. A spectacular example is a formation of a two-dimensional electron gas (2DEG) at the SrTiO₃/LaAlO₃ interface. In this dissertation the properties of the 2DEG are investigated from first principles.

The spatial extent of the 2DEG formed at the SrTiO₃/LaAlO₃ n-type interface is studied. It is shown that the confinement of the 2DEG is controlled by metal induced gap states formed in the band gap of SrTiO₃. The confinement width is then determined by the attenuation length of the metal induced gap states into SrTiO₃ which is governed by the lowest decay rate evanescent states of bulk SrTiO₃ which in turn can be found from the complex band structure of bulk SrTiO₃.

Magnetic properties of the 2DEG formed at the n-type interface of the SrTiO₃/LaAlO₃ superlattices are investigated. It is found that for a thin SrTiO₃ film the interface is ferromagnetic but for a thicker SrTiO₃ film the magnetic moment decreases and eventually disappears. This is a result of delocalization of the 2DEG that spreads over thicker SrTiO₃ film which leads to violation of the Stoner criterion. Further, it is shown that inclusion of the Hubbard U interaction enhances the Stoner parameter and stabilizes the magnetism.

The effect of the 2DEG and the polar interfaces for the thin film ferroelectricity is investigated using both first principles and model calculations. Using a TiO₂-terminated BaTiO₃ film with LaO monolayers at the two interfaces it is shown that the intrinsic electric field produced by the polar interface forces ionic displacements in BaTiO₃ to produce the electric polarization directed into the interior of the BaTiO₃ layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. It is found that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric.

Adviser: Evgeny Y. Tsymbal