Theoretical study of polar complex oxide heterostructures

The wide range of physical properties exhibited in complex oxides make them an attractive option for scientific investigation and device applications. Interfaces between these oxides can introduce novel phenomena absent in the bulk of each material. An example is the electrically conducting interface between LaAlO₃ (LAO) and SrTiO₃ (STO), both insulators in bulk form. The interface conductivity is quasi-two-dimensional (a 2DEG) and under certain conditions magnetic and/or superconducting, however its origin is still debated. This thesis aims to provide helpful guidelines, using basic theoretical principles, towards understanding the possible origins of the 2DEG in LAO/STO. First principles calculations of a model system, elucidate the importance of polarisation and electrostatics in these heterostructures. The net charge at the LAO-STO interface is argued to be precisely as what one would obtain by taking the formal charges of the ions, irrespective of covalency, and despite both materials being centrosymmetric. The interfacial net charge, or equivalently the LAO formal polarisation, requires charge screening which calculations show can be possible via an electronic transfer between neighbouring interfaces. It is additionally predicted that the LAO polarisation can induce one-dimensional electron gasses at the step edges of vicinal LAO-STO systems. The polarisation and electronic screening arguments for these perovskites, are shown to extend to very different materials with different symmetries, specifically two dimensional honeycomb compounds. Finally the effects of surface defects are explored in thin films of LAO, and a prototypical ferroelectric, BaTiO₃ (BTO). Surface redox processes such as oxygen vacancies, induced by the electric field within the film, can donate electrons to the buried interface, screening the polarisation charges. This process is found to be favourable for both systems, and is argued to be the likely origin of the 2DEG at the LAO-STO interface, and the origin of ferroelectric stability and tunnelling electroresistance at a BTO heterostructure in agreement with experiments.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2012

Author dc:creator

• Bristowe, Nicholas

• Complex oxides

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/13576e7a-b626-4755-9485-3339d1a64fb1/download
• https://www.rioxx.net/licenses/all-rights-reserved/

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.109689

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/370178