Seminar DMONS-DSI, axes 4 and 5 presented by 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

PhD defense : Flavien MOUILLARD

This work was carried out under the co-direction of Pr. Adele Carradò (DSI) and Dr. Patrick Masson (DMO).

The defense will take place on Friday, October 21th at 2:00 p.m. in the IPCMS’ Auditorium

Seminar DMONS : Nico Leumer

Speaker : Nico Leumer (IPCMS-DMONS)

Place : Auditorium de l’IPCMS, en présentiel, probablement avec possibilité de suivre en ligne

Abstract : The experimental search for Majorana zero modes, which are exotic modes hosted only by so-called topological superconductors, is despite numerous challenges still an ongoing „hot“ topic. Rather ordinary ingredients, as for instance magnetic fields, s-wave superconductivity and spin-orbit coupling, can turn a „normal“ materiel into such a topological superconductor, as demonstrated for (1d) InAs nanowires [1-3] or carbon nanotubes [4]. Thus, Majorana zero modes (MZM) can be engineered under appropriate conditions. Apart from their exotic nature, MZMs can be also relevant for practical purposes and offer indeed a platform for fault tolerant quantum computation [5].
    Although the underlying mechanisms such as the topological classification of systems and also topological superconductors are well understood from a theoretical point of view, the undeniable experimental detection of MZM is still an open issue. In theory, systems are considered either as infinitely long or treated numerically. Alternatively, one refers to toy models.
    The archetypal toy model of (1d) topological superconductors is the Kitaev chain. Contrary to many other models, the outstanding feature of the Kitaev chain is its apparent simplicity and the opportunity to find exact analytical expression for its spectrum even in the case of open boundary conditions (opc) and finite size [6,7]. In my talk, I will show that opc and finite size are essential for proper theoretical predictions. Further, the proper physical understanding of the Kitaev chain allows directly the analytical treatment of realistic models (finite size and opc) as I will demonstrate in case of the Rashba-nanowires [8]. In order to be appreciated also by non-experts of Majorana fermions in condensed matter, I first introduce the main aspects of this research field and try to sketch important relations within the topic.

[1] Y. Oreg, G. Refael and F. von Oppen, PRL 105, 177002 (2010)
[2] R. M. Lutchyn, J. D. Sau, and S. Das Sarma, PRL 105, 077001 (2010)
[3] J. Alicea, Rep. Prog. Phys. 75, 076501 (2012)
[4] L. Milz, W. Izumida, M. Grifoni and M. Marganska, PRB 100, 155417 (2019)
[5] A. Y. Kitaev, Phys. Usp. 44, 131 (2001)
[6] N. Leumer, M. Marganska, B. Muralidharan, and M. Grifoni, J. Phys.: Condens. Matter 32, 445502 (2020)
[7] N. Leumer, M. Grifoni, B. Muralidharan, and M. Marganska, Phys. Rev. B 103, 165432 (2021)
[8] H. Schmid, MA thesis, University of Regensburg (2020)

The connection links will be as follows :

Zoom access   https://zoom.us/j/7296665252?pwd=YzN4WjlsK1l0T3VmUkNnS0pjUUVhUT09

Meeting ID : 729 666 5252

Secret Code : 123456789