I am a theoretical and computational physicist working at the Luxembourg Institute of Science and Technology, and also an affiliate professor of Physics at the University of Luxembourg. I use various theoretical and simulation methods to study materials properties. Most of my work has to do with applications of Density Functional Theory, an approach that offers unique insight into the microscopic origin of the most diverse phenomena, as well as the predictive power necessary for the design of novel systems optimized for applications. My current research focuses on functional oxides, especially ferroelectrics and magnetoelectric multiferroics. I also develop new tools for large-scale simulations within the SCALE-UP project.

Job Opportunities

We often have PhD and post-doc positions available. If you are interested in what we do, and have a good and suitable CV, I encourage you to contact me at any time. In particular, if you are a student looking for a PhD position, please send an academic record as complete as possible.

On occasion, we are willing to sponsor grant applications of excellent candidates. This may be a prestigious way to join us for your PhD (e.g., through the AFR scheme of the Luxembourg National Research Fund) or post-doc work (e.g., through the Marie Skłodowska-Curie program of the European Commission). Feel free to contact me if you think you are a good candidate to get one of those!

Latest highlights

Of rotations and tilts

Perovskite oxides share the same generic chemical formula (ABO3) and basic structure: a lattice or corner-sharing rather-regular oxygen octahedra, with A and B cations located at the inter-octahedral spaces and octahedral interiors, respectively. Despite these structural similarities, subtle changes in perovskite composition may yield drastically different physical properties; further, even a given compound can present a wide variety of behaviors depending on the external conditions (e.g., temperature) or applied fields (e.g., electric, elastic). This richness is the result of a feature that makes perovskites rather unique: the large number of degrees of freedom (structural and electronic, including magnetic ones) competing for attention in these compounds. Among those, there is a structural order parameter that is all but omnipresent in perovskites, and which strongly influences all others: the quasi-rigid concerted rotations (or tilts) of the oxygen octahedra, typically in long-range patterns displaying perfectly in-phase or antiphase spatial modulations. Because of their central importance to the physics of these oxides, understanding the driving forces that favor different tilt orders, and how to control them, has been recognized as a critical issue, albeit one that is much complicated by the fact that a detailed experimental characterization of the network of octahedra (oxygens being nearly invisible to most diffraction and microscopy techniques) is very challenging. Recently, in a series of theoretical papers with Hongjun Xiang (Fudan), Laurent Bellaiche (Arkansas) and other colleagues, we have contributed to the on-going efforts to shed some light on these matters, explaining the mechanisms that favor some tilt polymorphs over others [Chen et al., Phys. Rev. B 97, 024113 (2018)], elucidating their behavior under pressure [Xiang et al., Phys. Rev. B 96, 054102 (2017)], or revealing surprising aspects of their interaction with ferroelectricity [Gu et al., Phys. Rev. Lett. 120, 197602 (2018)]. Beyond their specific merits and impact in the field of perovskites, these works illustrate the usefulness of simulation to understand critical structural aspects of functional materials.

Polar metals and metallic ferroelectrics

Can a metal present a polar distortion that breaks the inversion symmetry of the lattice? (In spite of the fact that the usual driving force for polarity are Coulomb dipole-dipole interactions that all but vanish in metals.) If so, would we be able to switch such a polar distortion with an external bias? (Despite the fact that metals are not supposed to sustain voltage drops.) The interest in these questions is many-fold, from fundamental to applied; indeed, it is always good to understand what's possible and what's not, plus non-centrosymmetric (polar) metals are rare creatures, potentially useful in various contexts. With A. Filippetti, V. Fiorentini (Cagliari) and others, we have studied these problems and stumbled upon a few surprises in the way. We have found that the answer to the first question is an affirmative one; as a matter of fact, we have identified a previously unnoticed, seemingly-universal "meta-screening" effect that favors polar order in metallized ferroelectrics. As for the second question, we are now convinced that, in well-chosen conditions, it is possible to apply an external electric field to a metal and switch its polar distortion, which yields a genuine ferroelectric metal! As a result, we think we have access to a new and fairly exciting family of materials that are likely to surprise us with additional unexpected behaviors. You can read more in Zhao et al., Phys. Rev. B 97, 054107 (2018) and Filippetti et al., Nat. Comm. 7, 11211 (2016).