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Séminaire MCBT

Mardi 5 septembre à 11h00,
Salle Louis Weil, E424

Orateur : Julien Varignon (UMPhy CNRS-Thalès)
"First-principles study of strongly correlated oxide perovskites"


Transition metal oxides with a ABO3 perovskite structure have attracted widespread interest over the last decades, both from an academic and industrial point of view. This is ascribed to their wide range of functionalities going from superconductivity, ferroelectricity, magnetism, orbitalorderings or thermoelectricity for instance. This diversity in their physical behaviour comes from the interplay between lattice, charge, orbital and magnetic degrees of freedom allowed by the transition metal [1]. Among all perovskites, a special emphasis has been dedicated to systems with a 3d transition metal element on the B site. Indeed, partly filled d shells allow for strong electronic correlations and/or ionic (lighter elements) or covalent (heavier elements) effects that in turn can have a dramatic influence on the properties of the materials. In the context of understanding the microscopic mechanism underlying these phenomenon, as well as engineering novel properties and functionalities with perovskites, Density Functional Theory (DFT) has already demonstrated its efficiency and appears nowadays as an essential tool in solid state physics.
In this seminar, I will present two different studies involving correlated and ionic or covalent systems and based on first-principles calculations. In the first part, on the basis of universal symmetry arguments, I will show how to couple lattice mode distortions through the recent “hybrid improper ferroelectricity” mechanism [2,3,4] and enable an electric field control of orbitalorderings and related electronic properties. This concept will be illustrated in rare- earth titanates or rare-earth vanadates based superlattices [5,6] and in highly strained bulk phases of popular perovskites such as BiFeO3 or SrTiO3 [7], where orbital orders can be produced irrespective of electronic degeneracies. In the second part, I will address the problem of rare-earth nickelates and I will evidence that these systems sit at the border line of ionic and covalent characters [8]. Then, I will highlight that covalence can be a powerful lever to control and engineer electronic and magnetic phases both in bulk and at oxide interfaces [9].

[1] Zubko et al, Annu. Rev. Condens. Matter Phys. 2, 41 (2011).
[2] Varignon et al, C.R. Physique 16, 153 (2015).
[3] Bousquet et al, Nature 452, 732 (2008).
[4] Rondinelli et al, Adv. Mater. 24, 1928 (2012).
[5] Bristowe, JV et al, Nat. Commun. 6, 6677 (2015).
[6] Varignon et al, Sci. Rep. 5, 15364 (2015).
[7] Varignon et al, Phys. Rev. Lett. 116, 057602 (2016).
[8] Varignon et al, npj Quantum Materials 2, 21 (2017).
[9] Grisolia, JV et al, Nat. Phys. 12, 484 (2016).

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