ABO3 perovskite oxides constitute a fascinating class of compounds, which can exhibit, and eventually combine, various interesting properties like ferroelectricity, piezoelectricity, exotic metal-insulator transitions, giant magnetoresistance ... During the last 25 years, many advanced have been performed in the fundamental understanding of the properties of these compounds [1,2]  thanks to first-principles calculations based on density functional theory (DFT) and various theoretical predictions have even been made that have been further confirmed experimentally. Still, calculations are typically restricted to relatively small systems and zero Kelvin. Recently, a new second-principles scheme (i.e. effective models directly fitted on first-principles) including explicitly strain, ionic [3] and electronic [4] degrees of freedom has been proposed to access the properties of these systems at finite temperature and operating conditions (i.e under finite stresses and fields). The purpose of this PhD project is to pioneer the field and validate this new approach approach on vanadate perovskite systems.

The vanadate compounds, with general formula R1-xAxVO3 (R: trivalent cation; A: divalent cation), have attracted much attention due to the coexistence of orbital degrees of freedom, a transparent metallic state when x is large [5] and a Mott transition for small x [6]. In particular, RVO3 systems display a rich phase transition diagram with temperature [6-8].

 

Figure 1.- Scheme showing some orbital orderings in RVO3 perovskites associated to the  of the t2g(xz/yz) orbitals (adapted from [4])

The coexistence and interplay between various structural (polar, antiferrodistortive), spin, orbital and charge degrees of freedom make them an ideal playground  to test the new second-principles scheme. This large-scale method allows capturing the accuracy of a few underlying DFT simulations through a model combining a detailed force-field and electronic structure simulations including band structure, electron-correlation and electron-lattice couplings.

The project will proceed step by step, focusing first on the properties of bulk compounds and extending then the investigations to thin films and superlattices in order to unravel unexpected phenomena. This work will be performed in close collaboration with the experimental group of Jean-Marc Triscone at the University of Geneva.
The work will include first-principles DFT calculations on the reference systems, fitting of the second-principles parameters and second-principles simulations at finite temperature. It might also require some small code developments.

Project Partners

Pablo García-Fernández
Departamento de Ciencias de la Tierra y Física de la Materia Condensada
Universidad de Cantabria

Philippe Ghosez
Theoretical Materials Physics
Université de Liège

The group at Université of Liège (ULg) has extended previous experience in the first-principles simulation of RVO3 vanadates and the development of lattice models in perovskite oxides. The group at Universidad de Cantabria (UC) has developed the electronic structure code for the second-principles simulations and has a strong background on the Jahn-Teller effect.

The PhD candidate was hired by the University of Cantabria and is expected to spend one year in the group of Ph. Ghosez at the University of Liège (Belgium) and two years in the group of P. Garcia-Fernandez at Universidad de Cantabria (Spain).

References

1.    J. Varignon et al. C. R. Physique 16, 153 (2015);
2.    N. Benedeck et al. Dalton Trans. 44, 10543 (2015)
3.    J. Wojdel et al., J. Phys. Condens Mater 25, 305401 (2013).
4.    P. García-Fernández et al. http://arxiv.org/abs/1511.07675
5.    L. Zhang et al. Nat. Materials 15, 204 (2016)
6.    C. Ulrich et al. Phys. Rev. Lett. 91, 257202 (2003)
7.    M. Skoulatos et al. Phys. Rev. B 91, 161104(R) (2015)
8.    J. Varignon et al. Sci. Rep. 5, 15364 (2015)

For more information on the work of the UC and ULg groups, please visit:

UC
https://scholar.google.es/citations?user=D0cph20AAAAJ&hl=es
https://scholar.google.com/citations?user=tcWZ31EAAAAJ&hl=en
ULG
http://www.phythema.ulg.ac.be
https://scholar.google.com/citations?user=bKwqdp4AAAAJ&hl=en 

The report on evaluation of candidates (in Spanish) can be downloaded here: