Description du sujet de thèse

Positioning of the scientific project:
Due to their remarkable electrical and optical properties, phase change materials (PCM) have attracted much attention both for fundamental science and for their applications potential. Today a fast-developing field using various PCM is neuromorphic computing combining them as physical artificial neurons and synapses. Their potential to reduce the energy cost and speed up the inference and learning process of artificial intelligence is particularly exciting. The emulation of neurons and synapses is often based on memristors, which allow several resistivity states to be accessed by an external stimulus. Such switching leads to a significant variation of a relevant physical properties such as resistivity, dielectric permittivity or magnetization. The typical time scale for switching in different materials span from hundreds of nanoseconds down to several hundred of picoseconds.
Reducing both characteristic switching times and the energy consumption requires a paradigm change evolving from resistive switching towards other mechanisms such as optical switching. Nowadays, all-optical switching between two magnetic configurations presenting a logical “1” and “0” has been shown magnetic systems such as magnetic metal alloys [1,2]. However, this phenomenom occurs within one established magnetic phase and is not assimilated to a thermodynamic phase transition. Recently, a paraelectric to ferroelectric phase transition has been shown to be possible in quantum-paraelectric Strontium-titanate (SrTiO3) driven by ultrashort optical terahertz pulses [3]. This remarkable result opens the possibility to create a metastable ferroelectric phase from a quantum paraelectric phase in a material under intense terahertz pulses on the picosecond timescale. In the case of quantum-paraelectric SrTiO3, this optically driven phase transition occurs at very low temperature below 50K making it irrelevant for technological applications.
Project:
This thesis project will address the physics underlying photoinduced phase transitions in the femtosecond to nanosecond time scale in a new class of PCM, more precisely lead-free ferro- and antiferroelectric materials.
To meet the above-mentioned challenges, the research will focus on both ferroelectric and antiferroelectric perovskite materials grown as thin films deposited by pulsed laser ablation in the frame of a national ANR research project (=” FASTPHASE”). These materials exhibit several ferroelectric-to-paraelectric and antiferroelectric-to-ferrielectric phase transitions at ambient temperature.
After careful caracterisations of the different stable states by means of optical-and THz absorption spectroscopy and second-harmonic generation, of the thermally driven phase transition, their dynamics from ultrafast photo-induced excitation both using vis-UV excitation and THz excitation will be studied. These time-resolved studies will be performed both through time-resolved optical measurements and time-resolved x-ray diffraction on the femto-to picosecond time scales [4–6].
References :
[1] C.-H. Lambert et al., Science 345, 1337 (2014).
[2] A. V. Kimel et al., Nature 435, 655 (2005).
[3] X. Li et al., Science 364, 1079 (2019).
[4] M. Hervé et al., Nat Commun 15, 267 (2024).
[5] Y. Tokura, J. Phys. Soc. Jpn. 75, 011001 (2006).
[6] T. Kampfrath, K. Tanaka, and K. A. Nelson, Nature Photon 7, 680 (2013).

Contexte de travail

Supervision:
This PhD thesis will be supervised by N. Keller and R. Bertoni for the research focusing on time-resolved light-matter interaction and by C. Mariette for the time-resolved X-ray diffraction studies, essentially performed at large scale infrastructures such as the European Synchrotron Radiation Facility (ESRF).
This thesis aims at training a PhD student in time-resolved optical spectroscopy and x-ray diffraction techniques.
The thesis work will be carried out at the Institut de Physique de Rennes and partially at the ESRF. The material and light team has built over the years an extended ultrafast laser laboratory, including several time-resolved absorption spectroscopy setups and optical setup for IR and THz generation as well as time-resolved optical studies.
Daily work involves strong interactions with all team members working in the laser lab.
Skills and knowledge required
Candidates must have deep knowledge in optics and solids-state physics. Also, a strong background in optical spectroscopy and the related experimental techniques is recommended. Good skills in programming, interfacing, data treatment and analysis (if possible by using Python software) are strongly appreciated. The PhD candidate will be strongly encouraged to present the scientific results in national and international conferences. English is the working language in the team. We are looking for candidates with a taste for experimental work in an intercultural team with good communication skills and developing a meticulous approach with an eye for detail.
Employment conditions:
The contracts established by the CNRS comprises the monthly salary, retirement contributions and give right to full social security coverage and unemployment benefits. PhD students have no teaching obligations at University of Rennes. Rennes is a medium size French city less 1h30 train ride from Paris and 1h from the sea coast, offering a relaxing lifestyle with many cultural and sport activities.
The Team:
The candidate will work inside the “Materials and Light Department” at the Institute of Physics of the University of Rennes. Our research is focused on ultrafast out-of-equilibrium phenomena in materials and molecules using optical and X-ray techniques with femtosecond to picosecond time resolution. The team is now part of a newly establish International Research Laboratory (IRL-DYNACOM) involving French and Japanese Universities (University Tokyo, Tohoku University).
More information can be found on the website:
https://ipr.univ-rennes.fr/en/materials-and-light-departement

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The thesis work will be carried out at the Institut de Physique de Rennes and partially at the ESRF. The material and light team has built over the years an extended ultrafast laser laboratory, including several time-resolved absorption spectroscopy setups and optical setup for IR and THz generation as well as time-resolved optical studies.
Daily work involves strong interactions with all team members working in the laser lab.