Thermally activated glide of screw dislocations in bcc metals
Thermally activated glide of dislocation is a key point for understanding the plastic deformation of metals. The screw dislocation in bcc metals is an archetypical case for which a large quantity of experimental data has been published in the scientific literature. It is then possible to compare these data to the theoretical predictions realized from the Vineyard statistical theory [1,2]. Such a theory is an essential tool allowing to perform a scale transition from atomistic computations toward macroscopic scale at which are realized the deformation tests.
The aim of our research will be to test Vineyard theory in comparison with molecular dynamics simulations [3]. Some preliminary computations have shown a significant discrepancy that is not present when we repeat the comparison for point-like defect as vacancies or self-interstitial atoms.
[1] Vineyard G.H., J. Phys. Chem. Solids 3, 121 (1957).
[2] Proville L., Rodney D., Marinica M-C., Nature Mater. 11, 845 (2012).
[3] Proville L., Choudhury A., Nature Mater. TO BE PUBLISHED.
Quantum device integration on Ge/SiGe heterostructures
Spin qubits in semiconductors quantum dots offer a fast scaling perspective of quantum processors by leveraging the manufacturing techniques of the microelectronics industry. To explore this approach, industrial research teams implemented qubits directly on their existing routes (e.g. FDSOI at CEA-Leti or FinFET at Intel). However, these devices suffer from an important electrostatic disorder stemming from the presence of an Si/SiO2 interface next to the qubits.
An alternative way consists in using semiconductor heterostructures based on Ge/SiGe stacks. They allow the charge confinement between crystalline interfaces, thus drastically reducing the electrostatic disorder. Besides the low effective mass of carriers in Ge allows more relaxed dimensions, while the spin-orbit coupling of holes in Ge allows spin manipulations without integration of any external control element.
The PhD thesis aims at developing a Ge/SiGe-based platform at CEA-Leti. The work will consist in fabricating test structures such as Hall bars on different substrate coupons, perform low temperature characterization and provide feedback to help optimizing the substrates quality. In parallel a 200mm route based on eBeam lithography will be set up for the fabrication of one- and two-dimensional arrays of quantum dots.
Phase field modeling of porosity transport by evaporation and condensation in MOX fuel for Sodium Fast Reactors
The aim of the proposed thesis is to develop a phase-field model describing the migration of fabrication porosity in uranium-plutonium mixed oxides under irradiation in sodium fast reactors. The model will take into account evaporation and condensation phenomena as the microscopic driving force moving porosity where the necessary local temperature and temperature gradient conditions are reached to establish this transport mechanism. To provide a consistent estimate of the vapor pressure differences governing the rate of evaporation (and condensation), the phase field model will be coupled to a thermodynamic description of the U-Pu-O ternary system. The candidate will work on the development of the theoretical formulation of the phase field model, as well as its inclusion in the framework of a finite element solver. The computational tool will be applied to study the conditions encountered in prototypical fast neutron reactors, in order to derive new pore migration rate laws for inclusion in the scientific fuel behavior calculation tools (OCS) used at industrial level, such as PLEIADES/GERMINAL V3, the OCS developed in the host department at CEA IRESNE.
The results obtained in the thesis will be the subject of presentations at international conferences (e.g. NuMat - The Nuclear Materials Conference, MMM - Multiscale Materials Modeling Conference, MiNES - Materials in Nuclear Energy Systems, CALPHAD), as well as publications in international scientific journals (Computational Material Science, Journal of Nuclear Materials, Journal of Applied Physics, Acta Materialia).
At the IRESNE institute (CEA Cadarache), the candidate will join a multi-disciplinary materials science community (solid state physics, thermics, mechanics, applied mathematics, thermochemistry, thermodynamics) open to both international research and the industrial world, will have the opportunity to interact with experts involved in the various stages of nuclear reactor design, and will use state-of-the-art numerical simulations on both small and large scales. The candidate will have the opportunity to spend part of his thesis in his thesis supervisor's university laboratory at the Ecole Polytechnique, where he/she will be able to complement the experience gained in an applied research institute such as the CEA with the more academic exposure offered by a university institution. Finally, the candidate will have the opportunity to familiarize himself/herself with and become an expert of two of the most widely used techniques for analyzing the evolution of microstructures in many industrial fields: phase field and thermodynamic calculations using the CALPHAD technique.
Thermal transport in non-abelian quantum hall states of graphene
Even-denominator states of the fractional quantum Hall effect (e.g. ??=5/2) are expected to host excitations that have non-abelian anyonic statistics, making them promising candidates for the realization of topological quantum computing [1]. While the demonstration of these non-abelian statistics has long been an extremely challenging endeavor, recent experiments in GaAs semiconductor heterostructures have shown that the edge thermal conductance of the ??=??/?? state is quantized in half-integer values of the thermal conductance quantum [2,3]. This half-integer quantization is known to be an universal signature of non-abelian statistics, including of Majorana fermions [4]. However, many of the suspected candidates for the ground state of ??=5/2 have complex edge structures exhibiting counterpropagating neutral modes, which can modify the edge thermal conductance and give them non-integer values similar to that of a non-abelian edge. A very recent experiment [3] has circumvented the issue by finding a way to separate the contributions of the different channels at the edge, confirming the existence of a non-abelian channel with half-integer quantized electrical and thermal conductance. The next obvious interrogation is whether this result is truly universal: does it hold for different material, and different even-denominator states?
In this project, we propose to address these questions by performing heat transport measurements in fractional quantum Hall states in bilayer graphene. Bernal-stacked bilayer graphene (BLG) has recently shown to host a large variety of robust even-denominator fractional quantum Hall states [5-8], both hole- and electron-type. This provides an excellent test-bed on which to probe the thermal conductance, as these fractions are expected to be described by different (possibly non-abelian) ground states; furthermore, the ability to apply electric displacement fields allows a further degree of control over the even-denominator states, which can be investigated in terms of heat transport.
This experimental project relies on ultra-low temperature, high magnetic field thermal transport [9] based on high sensitivity-sensitivity electrical measurements. We are looking for highly motivated candidates whoe are interested in all aspects of the project, both experimental (sample fabrication, low noise measurements, cryogenics) and theoretical.
[1] Nayak, et al., RMP 80, 1083 (2008) [2] Banerjee, et al., Nature 559, 205 (2018)
[3] Dutta, et al., Science 377, 1198 (2022) [4] Kasahara, et al., Nature 559, 227 (2018)
[5] Ki, et al., Nano Letters 14, 2135 (2014) [6] Li, et al., Science 358, 648 (2017)
[7] Zibrov, et al., Nature 549, 360 (2017) [8] Huang, et al., PRX 12, 031019 (2022)
[9] Le Breton, …, & Parmentier, PRL 129, 116803 (2022)