Growth of 2D Ferromagnetic Chalcogenide Materials for Spintronics
Chalcogenide materials, particularly Ge-Sb-Te (GST) alloys, are essential for phase-change memory (PCMs).
Although high-performance, these memories consume a great deal of energy, which
is driving the search for alternative solutions. GST alloys offer unique opportunities in the field of spin-orbitronics as spin-charge conversion materials or as sources of spin-polarized current. Two-dimensional ferromagnetic alloys such as Fe-Ge-Te or Ge-Mn-Te offer promising avenues as sources of spin current for new types of more efficient memory devices. For efficient spin injection, we are seeking a material that not only exhibits a high Curie temperature (TC) and significant spin polarization, but is also fully compatible with existing silicon-based CMOS technology.
The aim of this thesis is to develop and master, on an industrial scale on 300 mm Si substrates, the van der Waals epitaxial growth of 2D ferromagnetic films based on Fe-Ge (Ga)Te2 (n=3, 5) or Ge_(1-x)Mn_xTe, for example to integrate them in situ with spin-charge conversion chalcogenide layers such as ferroelectric layers (a-GeTe(111)) or topological insulators (Bi_(2-x)Sb2Te3).
Enhancing Faradaic Efficiency in Protonic Ceramic Electrolysis Cells (PCCELs) through Electrolyte and Electrode–Electrolyte Interface Engineering
Proton conducting ceramic electrolysis cells (PCCELs), an advanced variant of solid oxide electrolysis cells (SOECs), enable the direct production of hydrogen through steam electrolysis using proton-conducting electrolytes. Unlike conventional SOECs, which rely on oxygen ion (O²?) conductors, PCCELs operate at lower temperatures (~400–600?°C vs. 750–850?°C for SOECs) due to their higher proton conductivity. This lower operating temperature helps reduce material degradation and overall system costs. While SOEC technology has reached industrial maturity, with large-scale deployment projects underway, the development of PCCELs remains limited by several scientific challenges. These include the difficulty of densifying electrolytes (such as BaCeO3–BaZrO3) without barium volatilization during high-temperature sintering; the proton transport limitations posed by grain boundaries; and the poor control of electrode–electrolyte interfaces. This thesis aims to improve the faradaic efficiency of PCCELs by optimizing the microstructure of the electrolyte and engineering high-quality interfaces through targeted surface treatments. The methodology includes cell fabrication, interface engineering, and electrochemical evaluation. The ultimate goal is to establish robust and scalable processing protocols that enable PCCELs to achieve faradaic efficiencies above 95%, compatible with industrial-scale deployment.
Behavior of matter under isothermal dynamic compression: displacement of chemical reactivity; synthesis of new metastable materials; phase transition mechanisms.
The Diamond Anvil Cell equipped with piezoelectric actuators, or d-CED, is an innovative device that can generate dynamic compressions and decompressions over a wide range of pressure variation rates. The d-CED thus enables finely controlled dynamic stresses to be applied, with (de)compression rates that can vary over several orders of magnitude along isothermal paths. This paves the way for the creation of reference databases for the validation of microscopic mechanisms. Furthermore, the compression or decompression rates can be equated to ultra-fast heating or cooling rates of the sample, offering the possibility of exploring, in a highly controlled manner, certain phenomena still debated in the literature, such as the maximum stability of a solid beyond its melting point.
The objective of this thesis is to exploit the new possibilities offered by d-CED to demonstrate new phenomena or gain a detailed understanding of certain effects discussed in the literature, by performing ultra-fast temperature variations. A first application will consist of studying the nucleation kinetics of rare gases (Ar, Ne, Kr) as a function of the compression rate, and comparing them with recent measurements made at the XFEL in cryogenic jets. A second objective will be to study chemical changes, with an initial study focusing on the modification of the reactivity of nitromethane, a reference explosive. Another area of study will concern the synthesis of new molecular compounds from mixtures of dense molecular fluids (N2, H2, O2).
Measurement of the speed of sound in H2 and He, key components of gas giant interiors
The goal of this thesis is to study hydrogen-helium mixtures in the fluid phase under high pressure and high temperature using Raman and Brillouin spectroscopy. The experiments will be conducted in a diamond anvil cell with laser heating, allowing exploration of a wide range of pressure and temperature conditions representative of the interiors of gas giant planets (1-300 GPa, 300-4000 K). Raman spectroscopy will be used to probe possible chemical changes occurring under extreme conditions, while Brillouin spectroscopy will provide access to the adiabatic sound velocity and the equations of state of these fluid mixtures. These data will be particularly useful for improving the modeling of Jupiter and Saturn’s interiors.
Next-Gen Surface Analysis for Ultrathin Functional Materials
Advanced nanoelectronics and quantum devices rely on ultrathin oxides and engineered interfaces whose chemical composition, stoichiometry and thickness must be controlled with sub-nanometer precision. LETI is installing the first 300-mm multi-energy XPS–HAXPES tool with angle-resolved capability, enabling quasi in situ chemical metrology from deposition to characterization.
This PhD will develop quantitative, multi-energy and angle-resolved XPS/HAXPES methodologies for ultrathin oxides and oxynitrides, validate measurement accuracy, and establish robust protocols for quasi in situ transfer of sensitive layers. Applications include advanced CMOS stacks and quantum Josephson junctions, where sub-2 nm AlOx barriers critically determine device performance.
The project directly supports the development of next-generation quantum technologies, advanced photonics and energy-efficient microelectronics by improving the reliability and stability of nanoscale materials. The work will be carried out within a strong multi-partner framework.
Development of 4D-STEM with variable tilts
The development of 4D-STEM (Scanning Transmission Electron Microscopy) has profoundly transformed transmission electron microscopy (TEM) by enabling the simultaneous recording of spatial (2D) and diffraction (2D) information at each probe position. These so-called “4D” datasets make it possible to extract a wide variety of virtual contrasts (bright-field imaging, annular dark-field imaging, ptychography, strain and orientation mapping) with nanometer-scale spatial resolution.
In this context, 4D-STEM with variable beam tilts (4D-STEMiv) is an emerging approach that involves sequentially acquiring electron diffraction patterns for different incident beam tilts. Conceptually similar to precession electron diffraction (PED), this method offers greater flexibility and opens new possibilities: improved signal-to-noise ratio, faster two-dimensional imaging at higher spatial resolution, access to three-dimensional information (orientation, strain, phase), and optimized coupling with spectroscopic analyses (EELS, EDX).
The development of 4D-STEMiv thus represents a major methodological challenge for the structural and chemical characterization of advanced materials, particularly in the fields of nanostructures, two-dimensional materials, and ferroelectric systems.
Magnetic Tunnel Junctions at Boundaries
Spin electronics, thanks to the additional degree of freedom provided by electron spin, enables the deployment of a rich physics of magnetism on a small scale, but also provides breakthrough technological solutions in the field of microelectronics (storage, memory, logic, etc.) as well as for magnetic field measurement.
In the field of life sciences and health, giant magnetoresistance (GMR) devices have demonstrated the possibility of measuring the very weak fields produced by excitable cells on a local scale (Caruso et al, Neuron 2017, Klein et al, Journal of Neurophysiology 2025).
Measuring the information contained in the magnetic component associated with neural currents (or magnetophysiology) can, in principle, provide a description of the dynamic, directional and differentiating neural landscape. It could pave the way for new types of implants, thanks to their immunity to gliosis and their longevity.
The current bottleneck is the very small amplitude of the signal produced (<1nT), which requires averaging the signal in order to detect it.
Tunnel magnetoresistances (TMR), in which a spin-polarised tunnel current is measured, offer sensitivity performance that is more than an order of magnitude higher than GMR. However, they currently have too high a level of low-frequency noise to be fully beneficial, particularly in the context of measuring biological signals.
The aim of this thesis is to push back the current limits of TMRs by reducing low-frequency noise, positioning them as break sensors for measuring very weak signals and exploiting their potential as amplifiers for small signals.
To achieve this objective, an initial approach based on exploring the materials composing the tunnel junction, in particular those of the so-called free magnetic layer, or on improving the crystallinity of the tunnel barrier, will be deployed. A second approach, consisting of studying the intrinsic properties of low-frequency noise, particularly in previously unexplored limits, at very low temperatures where intrinsic mechanisms are reached, will guide the most promising solutions.
Finally, the most advanced structures and approaches at the state of the art thus obtained will be integrated into devices that will provide the building blocks for going beyond the state of the art and offering new possibilities for spin electronics applications. These elements will also be integrated into systems for 2D (or even 3D) mapping of the activity of a global biological system (neural network) and for evaluating capabilities for clinical cases (such as epilepsy or motor rehabilitation).
It should be noted that these improved TMRs may have other applications in the fields of physical instrumentation, non-destructive testing, and magnetic imaging.
Toughening random lattice metamaterials with structure heterogeneities
To reduce the environmental and/or the energetic impact of vehicles, a favored method is to decrease the mass of prime materials used to build them, that being done without hindering their mechanical performances. In this field, the use of mechanical metamaterials has been a major breakthrough. These metamaterials, generally created using additive manufacturing techniques, have a microscopic truss structure. They are porous by design, and thus very lightweight, and the distribution of their microscopic beams or tubes (i.e. their architecture) can be chosen to make them as stiff as possible, making them choice candidates for high technology applications where the rigidity-density ratio is paramount, such as aerospatial research (https://en.wikipedia.org/wiki/Metallic_microlattice).
For the most part however, metamaterials that have been designed up to now present periodical architectures. As a consequence, their mechanical behavior is inherently anisotropic, which makes them difficult to model using material mechanics conventional approaches, and strongly limits their usage in various possible fields of applications. In recent works, we have developped a new class of microlattice metamaterials with a random spatial distribution of beams, generated with a combination of random close packing and Delaunay triangulation algorithm then 3D-manufactured. These metamaterials show an isotropic mechanical behavior, and their stiffness-density ratio reaches the theoretical limit for porous materials. They are neverheless still fragile and subject to fracture and yielding.
The aim of this PhD project is to toughen these metamaterials based on techniques and mechanisms from polymer and soft matter physics. Our hypothesis is that including in a controlled statistical way structure heterogeneities, at the node level by modulating the connectivity or at the beam level by changing their section or shape, can allow toughening of the metamaterial. Indeed, localized heterogeneities can introduce mechanical dissipations in the network at various scales. The work of this project will consist in experiementally characterizing the mechanical properties of the metamaterials and to compare them to their homogeneous equivalent, and to describe their fracture resistance. Mechanical tests will be performed on an experimental setup conceived in the SPHYNX group. Analysis of the local and global deformation will be performed using different experiemental methods, in order to detect micro crack events with precision. An additionnal theoretical approach completed by numerical simulations based on fuse network and random beam models can also be discussed.
A strong interest for instrumentation and teamwork is requested for this project with a major experimental component. Proficiencies in experimental mechanics, material sciences and/or statistical physics are desirable. Some knowledge in modelization and numerical simulations are a bonus without being required. This project has both fundamental and applied interests and can help the student find prospects both in academia and in industrial opportunities.
Triplet superconductors: from weak to strong spin-orbit coupling
Since the 1980s, several unconventional superconductors have been discovered, some of which exhibit triplet pairing (total spin S=1) that may lead to interesting topological properties. Unlike singlet superconductors, their order parameter is a vector depending on the spin components (S_z=-1,0,1) and is strongly influenced by the crystal symmetry and the spin–orbit coupling (SO).
The thesis aims to study the transition between weak and strong spin–orbit coupling in a triplet superconductor, using a minimal multiband model inspired by the material CdRh2As3, where a field-induced triplet phase was recently observed. This research will enable the calculation of the dynamic spin susceptibility and the identification of possible collective spin resonances, similar to those seen in superfluid He3.
The project will mainly rely on analytical field-theoretical methods applied to condensed matter. It is intended for candidates with a solid background in quantum mechanics, statistical physics, and solid-state physics.
Experimental study and numerical simulation of deformation mechanisms and mechanical behavior of zirconium alloys after irradiation
The cladding of nuclear fuel rods used in Pressurized Water Reactor, made of zirconium alloys, is the first barrier for the confinement of radioactive nuclei. In-reactor, the cladding is subjected to radiation damage resulting in a change of its mechanical properties. After in-reactor use, the fuel rods are transported and stored. During these various steps, the radiation damage is partially annealed, leading to another evolution of the material properties. All these evolutions are still not well understood.
The objective of this PhD work is to better understand the deformation mechanisms and the mechanical behavior of zirconium alloys after irradiation, and after a partial annealing of the radiation damage. This will help to better predict the behavior of the cladding tube after use and thus guaranty the confinement of radioactive nuclei.
In order to achieve this goal, original experimental methods and advanced numerical simulations will be used. Ion irradiations will be conducted in order to reproduce the radiation damage. Heat treatments will then be done on the specimens after irradiation. Small tensile samples will be strained in situ, after annealing, inside a transmission electron microscope, at room temperature or at high temperature. Deformation mechanisms observed at nanometer scale and in real time will be simulated using dislocation dynamics, at the same time and space scales. Large scale dislocation dynamics simulations will then be conducted in order to deduce the single-crystal behavior of the material. In parallel with this study at the nanometric scale, a study will also be conducted at the micrometric scale. Nanoindentation and micropillar compression tests will be performed to assess the mechanical behavior after irradiation and annealing. The results of mechanical tests will be compared with large-scale dislocation dynamics numerical simulations.
This study will allow a better understanding of the special behavior of zirconium alloys after irradiation and annealing and then help to develop physically based predictive models. In a future prospect, this work will contribute to improve the safety during transport and storage of spent nuclear fuel.