Séminaire DMONS-DSI, axes 4 et 5 présenté par Jorge Iñiguez

Jorge Iñiguez (Materials Research and Technology Department, LIST, Department of Physics and Materials Science, University of Luxembourg)

Résumé :
Ferroelectric materials have been around for many decades, and yet they continue to challenge our imagination with unexpected behaviors. The most recent and important ferroelectric revival features nanostructures displaying exotic properties that seemed all but impossible not long ago but are now revealed by modern characterization techniques. For example, transmission electron microscopy has allowed us to visualize mesmerizing dipole vortexes and skyrmions in superlattices combining ferroelectric (PbTiO3) and dielectric (SrTiO3) layers [1], confirming the kind of electrostatic/frustration effects predicted earlier by some theory groups [2] and doubted by most.

In this talk I will review the theoretical models [3] that were used to anticipate the rich behaviors that currently generate so much excitement and describe how they allowed us to predict the occurrence of ferroelectric skyrmions [4] that were experimentally confirmed shortly after [5]. I will also discuss some of the most surprising properties of these frustrated ferroelectric states, in particular their negative capacitance behavior, which leads to a miraculous-sounding voltage amplification [6,7]. I will conclude by commenting on the most exciting opportunities in the field.

My main collaborators in these works were M.A.P. Gonçalves and Mónica Graf (formerly at LIST, now at the Czech Academy of Sciences), and Hugo Aramberri (LIST). Collaborators at the University of Cantabria (Junquera), UC Berkeley (Ramesh) and elsewhere were involved in some of the projects. Work in Luxembourg funded by the Luxembourg National Research Fund through projects FNR/C15/MS/10458889/NEWALLS, C18/MS/12705883/REFOX and INTER/RCUK/18/12601980.

[1] Observation of polar vortices in oxide superlattices, A.K. Yadav et al., Nature 530, 198 (2016).
[2] Unusual phase transitions in ferroelectric nanodisks and nanorods, I.I. Naumov, L. Bellaiche and H. Fu,
Nature 432, 737 (2004).
[3] First-principles model potentials for lattice-dynamical studies: general methodology and example of
application to ferroic perovskite oxides, J.C. Wojdel et al., J. Phys. Condens. Matt. 25, 305401 (2013).
[4] Theoretical guidelines to create and tune electric skyrmion bubbles, M.A.P. Gonçalves et al., Science
Advances 5, eaau7023 (2019).
[5] Observation of room-temperature polar skyrmions, S. Das et al., Nature 568, 368 (2019).
[6] Negative capacitance in multidomain ferroelectric superlattices, P. Zubko et al., Nature 534, 524 (2016).
[7] Giant voltage amplification from electrostatically-induced incipient ferroelectric states, M. Graf, H.
Aramberri, P. Zubko and J. Íñiguez, Nature Materials (2022) https://doi.org/10.1038/s41563-022-01332-z

contact : Riccardo HERTEL : riccardo.hertel@ipcms.unistra.fr

Séminaire DMONS : Maximilien BARBIER

Orateur : Maximilien BARBIER / Lecturer in Theoretical Physics/Applied Mathematics University of the West of Scotland, United Kingdom

Résumé : Thermodynamics in its standard form applies to systems that are at equilibrium. It hence says very little about the processes that may drive a system from one equilibrium state to another. Describing such processes is the goal of nonequilibrium thermodynamics.

The study of nonequilibrium systems has originally been restricted to phenomena that occur near equilibrium. This approach yielded important results such as the celebrated Onsager-Casimir reciprocity relations or the fluctuation-dissipation theorem. Close to equilibrium, the response of the system depends linearly on the constraints that drive it out of equilibrium. This linear behavior breaks down as the system is driven farther away from equilibrium. Alternative methods are thus required to treat such nonlinear regimes. To this regard, exact results have been obtained in the form of the so-called fluctuation relations (or fluctuation theorems) whose main strength is to remain valid arbitrarily far from equilibrium.
In this talk, I will introduce some of the main ideas that underly nonequilibrium thermodynamics and discuss how fluctuation relations can be used to access the nonequilibrium properties of general systems. In particular, I will quantify the impact of a fluctuation relation on the full statistics of the nonequilibrium currents that take place in the system.

Pour tout contact :

Rodolfo JALABERT : rodolfo.jalabert@ipcms.unistra.fr

Dietmar WEINMANN : dietmar.weinmann@ipcms.unistra.fr