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 AXE 1 – Sciences et Matériaux Quantiques présenté par Francesco FOGLIANO

Francesco FOGLIANO (University of Basel, Switzerland)

In recent years nano-optomechanical systems have proven to be a powerful resource to detect ultra-weak forces, thus providing new insights on fundamental interactions. I will present recent advancements that extend the experimental range of ultrasensitive force measurements based on optically readout vibrations of suspended silicon carbide nanowires. Novel experimental regime will be discussed: first the operation at ultralow optical power at dilution temperatures; second in the ultrastrong coupling regime of cavity optomechanics.

Operating force sensors at dilution temperatures permits to reduce their thermal noise and further benefit from an increased mechanical coherence. However this requires eliminating the sources of unwanted vibrations, such as electrical or mechanical noises, and operating at ultralow optical powers to avoid unwanted laser heating. I will expose the experimental developments that lead to observe a nanowire featuring a noise temperature measured at the 32 mK level, while exploiting novel optical readout schemes operating in the photon counting regime. I will briefly discuss their mechanical and thermal properties at low temperatures and report on enhanced force sensitivities of a few tens of zN/√Hz. In the second part I will describe a novel cavity nano-optomechanical experiment at room temperature that consists in inserting the vibrating extremity of a suspended nanowire with sub-wavelength sized diameter, in a high finesse fiber micro-cavity. The combination of its small mode volume, of the extreme force sensitivity of the nanowires and of the large optomechanical interaction strength demonstrated makes the system very interesting for further explorations of the field of cavity nano-optomechanics. In particular this implementation should allow to enter the so-called ultrastrong coupling regime, where one single intracavity photon can displace the oscillator by more than its zero point fluctuations. By scanning the nanowire within the cavity mode volume and measuring its impact on the cavity mode, it is possible to obtain a map of the 2D optomechanical interaction. Vice versa, by using the toolbox of nanowire-based force-sensing protocols, it is possible to explore the backaction of the optomechanical interaction and map the optical force field experienced by the nanowire. I will conclude with a brief discussion on a more recent work, aimed to implement a hybrid optomechanical system at 4K. The platform is based on a tunable fiber-cavity and on a resonator-in-the-middle configuration. It is suitable for studying a broad range of nanomechanical resonators (including nanowires, nanowires with embedded emitters, carbon nanotube, magnetic 2D material and functionalized membranes), as well as to approach the regime of single-photon cavity optomechanics.

Contact : Arnaud GLOPPE (arnaud.gloppe@ipcms.unistra.fr)

Séminaire DON : présenté par Emilien PROST

Emilien Prost est candidat au poste CDD IR dans l’équipe Femtomag suite au départ de Y.Brelet.

Numerous applications have emerged thanks to the development of terahertz (THz) sources and detection making such pulses an interesting tool for ultrafast science [1]. We introduce a new THz platform relying on THz generation by air-plasma induced by a two-color short laser pulse and on Air Biased Coherent Detection [2]. Using this technique, we measure the THz electric field in the time-domain and can retrieve the spectrum by taking its Fourier transform. Using air as the generation and detection medium allows for the production of high electric field strength in the MV/cm range and smooth broadband spectra, making those pulses interesting to explore ultrafast phenomena. Numerous materials exhibit ro-vibrational bands and phonons in the THz range. We used this THz platform to perform THz time-domain spectroscopy (THz-TDS) [3] of condensed phase materials. The propagation through a sample modifies the temporal profile of the THz pulse. It is thus possible to retrieve the spectral feature of a sample by comparing the Fourier transform of the temporal trace with and without sample.