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 DON : présenté par Daniele Brida

Daniele Brida, Department of Physics and Materials Science, University of Luxembourg

Many fundamental and ubiquitous physical phenomena have origin at the ultrafast timescale. The possibility to investigate various primary processes on their intrinsic timescales relies on the generation of ultrashort pulses with widely tunable carrier frequency, from ultraviolet to mid- and far-infrared. These optical waveforms allow the investigation of microscopic light-matter interactions in a wide variety of condensed material systems to unveil the deep origin of their optoelectronic properties. 

A novel idea consist in exploiting the optical field itself to control the properties of crystals and nanostructures. With this approach, it becomes possible to access phenomena occurring within a oscillation of light as benchmarked by three experiments: i) optical response of semiconducting nanostructures by impulsively exciting a plasma frequency in mid-infrared range that establishes a plasmonic resonance; ii) quasi-instantaneous localization of electronic wavefunctions in GaAs by non-resonant bias with intense THz radiation; iii) ultrafast electron transport driven by the peak electric field of a single-cycle optical pulses focused on nanostructured gold circuits.