Abstract

Perovskite materials exhibit many interesting structural and electronic effects such as ferroelectric domains, tilting octahedral oxygen cages and two-dimensional electron gas (2DEG) formation at interfaces between different materials [1]. The standard theories which describe these effects are uncoupled and thus cannot describe the competition between them. In this work, we introduce theories which couple these effects and study their competition and coexistence. By coupling the theories for ferroelectric domains and 2DEGs in PbTiO3 (PTO) thin films [2] and PbTiO3/SrTiO3 (PTO/STO) superlattices, we find that there are situations in which both can be observed simultaneously. The formation of the 2DEG at the LaAlO3/SrTiO3 (LAO/STO) is regarded as continuous, as described by the standard theory [3]. We show that by coupling this 2DEG to tilts in the materials, it is possible to observe a first order phase transition, and even two phase transitions, with the application of an external electric field.

I will also discuss our first-principles calculations to observe these phase transitions using density functional theory.

[1] A. Ohtomo and H. Hwang, Nature 427, 6973 (2004) [2] P. Aguado-Puente et al., Physical Review B 92 , 035438 (2015) [3] N. Bristowe et al., Journal of Physics: Condensed Matter 26, 143201 (2014)