Role of surface defects on the formation of the 2-dimensional electron gas at polar interfaces

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Leader Name: Emilio Artacho

Leader Institution: CIC nanoGUNE Consolider and University of Cambridge

The discovery of a 2-dimensional electron gas (2DEG) at the interface between two insulators, LaAlO3 and SrTiO3, has fuelled a great research activity on this and similar systems in the last years. The polar catastrophe model, typically invoked to explain the formation of the 2DEG, while being intuitive and successful on predicting the critical thickness of LaAlO3 for the formation of the 2DEG, fails to explain many other observed phenomena on polar interfaces. Oxygen vacancies, on the other hand, are known to affect dramatically the physical behaviour of this system, but their role at the atomic level is far from understood. Here we perform ab initio simulations in order to assess whether the formation of O vacancies can account for various recent experimental results that defy the current theoretical understanding of these interfaces.

In this work we have take full advantage of the RES supercomputing capabilities to perform first principles simulations of LaAlO3/SrTiO3 interfaces with explicit vacancies. Previously only small concentration of defects had been simulated due to the size scaling of the system as larger concentrations are studied, but here, using efficient DFT codes (SIESTA) and the resources provided by RES we have been able to calculate heterostructures with dopings ranging from 0.5 to 0.13 electrons per surface unit cell. With these calculations we have analysed the interplay between the formation of defects and the intrinsic instabilities of the system, such as the rotations of the oxygen octahedra. We have observed that the orientation of the rotation axis in the SrTiO3 substrate plays an important role in the formation of oxygen vacancies in the LaAlO3 film. In particular, we have found that the formation energy of defects can indeed be fairly different for different domains of the SrTiO3. In turn the presence of vacacies at the surface create a distinctive tilting pattern in the oxygen sublattice of the perovskites, what can be used for the detection of defects at polar interfaces.



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