Imaging structure and property in 2D van der Waals ferroelectric materials using 4D scanning transmission electron icroscopy

This PhD aims to develop analytical techniques enabling to access information on both atomic structure and polarization in two-dimensional van der Waals (2D vdW) ferroelectronic materials. Recently, 2D vdW ferroelectric systems have been discovered with novel polarization mechanisms explained by subtly tuned atomic structural configurations. To understand the mechanisms that drive spontaneous polarization, the ability to probe atomic position and induced properties in atomically thin 2D layers is essential but still a big challenge. Thanks to improvements in detector efficiency and new numerical data treatment capability, transmission electron microscopy (TEM) is becoming an invaluable tool for the study of 2D materials. Aberration correctors allow low voltage operation to avoid damage and provide direct atomic structure, and EELS can provide chemical composition at the atomic level. More recently, 4D-STEM can be used to determine the local electric fields and charges generated in atomically thin 2D materials with sub-angstrom accuracy. The PhD project will use all these new developments recently realized in MEM-LEMMA to explore the feasibility of accessing information on local polarization in innovative 2D vdW ferroelectric materials developed by the collaborators; SPINTEC (CEA) and NCSR (Greece).

In situ and real time characterization of nanomaterials by plasma spectroscopy

The objective of this Phd is to develop an experimental device to perform in situ and real time elemental analysis of nanoparticles during their synthesis (by laser pyrolysis or flame spray pyrolysis). Laser-Induced Breakdown Spectroscopy (LIBS) will be used to identify the different elements present and to determine their stoichiometry.

Preliminary experiments conducted at LEDNA have shown the feasibility of such a project and in particular the acquisition of a LIBS spectrum of a single nanoparticle. Nevertheless, the experimental device must be developed and improved in order to obtain a better signal to noise ratio, to decrease the detection limit, to take into account the different effects on the spectrum (effect of nanoparticle size, complex composition or structure), to automatically identify and quantify the elements present.

In parallel, other information can be sought (via other optical techniques) such as the density of nanoparticles, the size or shape distribution.

Multi-fidelity model for describing the thermophysical properties of mixed actinide oxides, nuclear fuels for fast neutron reactors

In the context of the revival of the nuclear industry, the objective of this thesis is to contribute to the R&D (Research and Development) on fast neutron reactors, which offer the possibility of efficiently using mixed oxide (MOx) fuels. This type of fuel indeed allows for a better utilization of nuclear resources and a reduction in high-level nuclear waste. Numerical simulations are an extremely beneficial resource for modeling the thermomechanical and physico-chemical behavior of reactor fuel. Scientific computational tools used to simulate this behavior are based, among other factors, on material property behavior laws derived from experimental measurements that are challenging to obtain at high temperatures, sometimes resulting in a lack of data in important application areas.
The objective of this thesis project is to propose more precise behavior laws using machine learning and a multi-fidelity model. This mathematical model will be developed by combining data from atomic-scale calculations, which can be more easily obtained at high temperatures, and experimental data. This will be a major scientific advancement, as this model will integrate data from different sources for the first time. Thermodynamic properties, especially thermal conductivity and specific heat, will be at the heart of this study. The multi-fidelity model will also guide future experiments to improve these behavior laws by identifying areas where they are less accurate.
The thesis will be conducted within the Department of Fuel Studies (IRESNE-CEA Cadarache Institute), and the candidate will join a team of experts in multiscale material modeling. The work will benefit from several collaborations with experts in applied mathematics. The candidate will use various generic techniques applicable to numerous materials science domains. The research will lead to participation in national and international conferences and the preparation of publications.

Attosecond high reprate spectroscopy of ultrafast photoemission of gases

Summary :
The student will develop attosecond spectroscopy techniques making use of the new high reprate Ytterbium laser sources. The ultrafast photoemission dynamics will be studied to reveal in real time the processes of electron scattering/rearrangement as well as electron-ion quantum entanglement, using the charged-particle coincidence techniques.

Detailed summary :
In recent years, there has been spectacular progress in the generation of attosecond (1 as=10-18 s) pulses, rewarded by the 2023 Nobel Prize [1]. These ultrashort pulses are generated from the strong nonlinear interaction of short intense laser pulses with gas jets [2]. A new laser technology based on Ytterbium is emerging, with stability 5 times higher and reprate 10 times higher than the current Titanium:Sapphire technology. These new capabilities represent a revolution for the field.
This opens new prospects for the exploration of matter at the electron intrinsic timescale. Attosecond spectroscopy thus allows studying in real time the quantum process of photoemission, shooting the 3D movie of electronic wavepacket ejection [3,4], and studying quantum decoherence resulting from, e.g., electron-ion entanglement [5].
The first objective of the thesis work is to develop on the ATTOLab laser platform the attosecond spectroscopies using the new Ytterbium laser sources. The second objective is to take advantage of charged particle coincidence techniques, enabled by the high reprate, to study the dynamics of photoemission and quantum entanglement with unprecedented precision.
The student will be trained in ultrafast optics, atomic and molecular physics, quantum optics, and will acquire a broad mastery of XUV and charged-particle spectroscopy techniques.

References :
[2] Y. Mairesse, et al., Science 302, 1540 (2003)
[3] V. Gruson, et al., Science 354, 734 (2016)
[4] A. Autuori, et al., Science Advances 8, eabl7594 (2022)
[5] C. Bourassin-Bouchet, et al., Phys. Rev. X 10, 031048 (2020)

Simulations of radiolysis in organic phases with plutonium

Molecular simulations of ionizing irradiation of plutonium extractant solutions

The CEA is developing separation processes for the multi-recycling of plutonium in spent nuclar fuel. The preferred technology for separating plutonium is solvent extraction. In solvents, radiolysis phenomena generated by the presence of alpha radiation emitters such as plutonium are numerous. A better understanding of these phenomena is essential to develop and control these processes. The aim of this thesis will be to understand the mechanisms by which organic solutions are damaged by radiolysis, using numerical simulations. On a microscopic scale, the irradiation of matter by alpha particles begins with a deposition of energy in the electron cloud, leading to the excitation or ionization of the molecules in the medium. This process takes place on the attosecond time scale. The energy thus deposited is then dissipated in nuclear vibration modes, leading to the localization of charges on certain molecular fragments, the weakening of chemical bonds or even their rupture, and the production of reactive chemical species. The latter are the precursors of chemical reactions occurring at later times.
To simulate these ultrafast processes on the basis of first principles, we will adopt the Time-Dependent Density Functional Theory (TD-DFT) methods [1]. TD-DFT simulations consist in explicitly propagating in time the evolution of the electronic cloud subjected to a perturbation such as a collision by an alpha particle. These simulations give access to the amount of energy delivered to the system at atomic resolution, and to the dynamics of the electron cloud. Coupling the TD-DFT simulations with the Newtonian molecular dynamics simulations of atomic nuclei, then gives access to the simulation of ultrafast chemistry taking place on femto- and picosecond timescales. Hybrid QM/MM (Quantum Mechanics/Molecular Mechanics) schemes will be used to account for environmental effects (solvent, counter-ions)[1,2]. The PhD student will be trained in a wide range of methods in the field of theoretical chemistry.
The successful candidate will have a good background in physical and/or quantum chemistry, be motivated and hard-working. Previous experience in numerical simulation, acquired for example during Master's research internships, will be an advantage. The thesis will be carried out under the joint supervision of D. Guillaumont (CEA) and A. de la Lande (Université Paris Sud), requiring the PhD candidate to be located for long periods on each of the two sites of CEA Marcoule and Université Paris Sud.

[1] X Wu, JM Teuler, F Cailliez, C Clavague´ra, DR Salahub, A de la Lande, J. Chem. Theor. Comput. 2017,13, 3985-4002.
[2] K. A. Omar, F. A. Korsaye, R. Tandiana, D. Tolu, J. Deviers, X. J. Wu, A. Parise, A. Alvarez-Ibarra, F. Moncada, J. N. Pedroza-Montero, D. Mejía-Rodriguez, N. T. Van-Oanh, F. Cailliez, C. Clavaguéra, K. Hasnaoui, A. de la Lande, European Physical Journal-Special Topics 2023.