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

Control heat

There is a keen interest in engineering the lattice thermal conductivity of materials, either to reduce it (as required to obtain good thermoelectric properties) or to enhance it (e.g. for efficient cooling down in electronics, or to maximize heat transfer in caloric applications). More visionary is the possibility that such a control, could it be exerted in a dynamic way, would allow to use heat currents in the same way we use electronic ones today, to do electronics with phonons (aka phononics). Some years ago, Riccardo Rurali (ICMAB-CSIC) and I thought that such a dynamic control of the lattice thermal conductivity could be achieved in ferroelectrics, as in these materials one can use electric fields to write and erase obstacles to the propagating phonons. Indeed, our scatterers would be the so-called domain walls, that is, the boundaries that separate regions with equivalent, but differently oriented, electric polarizations. Our second-principles simulations proved that the concept works, with a twist: we found that ferroelectric domain walls may act as highly-selective phonon filters!, rather than as simple resistors [Royo et al., Phys. Rev. Mats. 1, 051402 (2017); Seijas-Bellido et al., Phys. Rev. B 96, 140101 (2017)]. Further, we also realized that, in highly-polarizable compounds like ferroelectrics and incipient ferroelectrics, the response of the lattice to applied electric fields can be as strong as to cause dramatic changes in the lattice thermal conductivity, an effect we call electrophononic. Interestingly, the strongest electrophononic effects yield a (much) reduced conductivity and rely on an universal mechanism, namely, the field-induced symmetry breaking and attendant increase of three-phonon scattering processes. This effect, which we have predicted to occur in PbTiO3 [Torres et al., Phys. Rev. Lett. 123, 185901 (2019); Seijas-Bellido et al., Phys. Rev. B 97, 184306 (2018)] and SrTiO3 [Torres et al., Phys. Rev. Mats. 3, 044404 (2019)], suggests a specific and promising route to obtain a thermal transistor. In addition to the great fun and learning it is bringing us, this multi-year work suggests that ferroelectric and related materials might prove unique and useful for future applications involving heat management.

Let there be electric skyrmions!

Magnetic skyrmions are mesmerizing spin structures of unique topology and particle-like properties. Can there be an electric analogue, with electric dipoles instead of spins? Scientists have been pondering this question for a few years, and some interesting theoretical proposals have appeared. However, the strategies suggested so far to stabilize skyrmion-like electric states are difficult to implement experimentally and by-construction preclude some of the most desirable properties of skyrmions, e.g. their mobility. The situation has turned around this year. In a second-principles investigation led by M.A.P. Gonçalves (LIST), and with collaborators J. Junquera and P. García-Fernández (Cantabria), we have shown it is possible to harness the non-trivial structure of the ferroelectric domain walls of PbTiO3 to create skyrmion bubbles by simply writing column-like nano-domains in an homogeneously polarized matrix [Gonçalves et al., Science Advances 5, eaau7023 (2019)]. Further, we find that these bubbles undergo topological and isotopological transitions when subject to external fields or heating, offering us a great deal of control. But that's not all: Most importantly, together with collaborators in Berkeley, Penn State and Cornell, we have been able to prove experimentally that such skyrmion bubbles can be stabilized at ambient conditions in PbTiO3-based heterostructures! [Das et al., Nature 568, 368 (2019)] While many challenges lay ahead (how to move the bubble domains, and thus the skyrmions, in a controlled way; how to switch their chirality), these works constitute a major breakthrough towards the obtention and manipulation of topological structures in strong ferroelectric materials, and effectively inaugurate a field that promises to be as exciting as it gets.