ED Mathématiques et Informatique
Exploring the role of neural network architecture onto decision-making processes with reservoir computing
by Naomi CHAIX-EICHEL (LaBRI - Laboratoire Bordelais de Recherche en Informatique)
The defense will take place at 17h00 - Amphithéâtre Broca Centre Broca Nouvelle Aquitaine 146 Rue Léo Saignat, 33000 Bordeaux
in front of the jury composed of
- Mehdi KHAMASSI - Full professor - Sorbonne Université - Examinateur
- Eleni VASILAKI - Full professor - University of Sheffield - Rapporteur
- Peter DOMINEY - Full professor - Université de Bourgogne - Rapporteur
- Anne COLLINS - Assistant professor - University of Berkeley - Examinateur
- Clément MOULIN-FRIER - Assistant professor - INRIA Bordeaux - Examinateur
- Thomas BORAUD - Full professor - Université de Bordeaux - Directeur de these
- Nicolas ROUGIER - Full professor - Université de Bordeaux - Directeur de these
A striking similarity exists in the organization and structure of certain brain regions across diverse species. For instance, the brain structure of vertebrates, from fish to mammals, includes regions like the cortex, hippocampus, cerebellum and basal ganglia with remarkable similarity. The presence of these structures across a wide range of species strongly suggests that they emerged early in vertebrate evolution and have been conserved throughout evolution. The persistence of these structures raises intriguing questions about their evolutionary origins: are they optimal solu- tions for processing information and controlling behavior, or could alternative brain architectures emerge to achieve similar functional properties? To investigate this question, this thesis explores the relationship between brain architecture and cogni- tive function, with a focus on decision-making processes. We propose to use variants of a recurrent neural network model (echo state network) that is structurally min- imal and randomly connected. We aim to identify whether a minimal model can capture any decision-making process and if it cannot, we will explore whether multi- ple realizable solutions emerge through structural variations. First we demonstrate that a minimal model is able to solve simple decision tasks in the context of spatial navigation. Second, we show that this minimal structure has performance limita- tions when handling more complex tasks, necessitating additional structure. Third, by employing a genetic algorithm to evolve network structures, we discover that multiple architectures can effectively solve the same decision-making task, and that identical architectures can exhibit a range of different behaviors. Finally, we investi- gate additional factors that might contribute to these behavioral differences beyond structural variations. Our analysis of the behavior of 24 monkeys living in a com- munity reveals that social factors, such as social hierarchy, also play a significant role in influencing behavior.
ED Entreprise Economie Société
Emerging countries, monetary policy autonomy and dynamics of international reserves
by Théo LAMAGNERE (BSE - Bordeaux sciences économiques)
The defense will take place at 14h00 - Salle des thèses C1-002 16 avenue Léon Duguit, Pessac 33600, Bâtiment C1
in front of the jury composed of
- Delphine LAHET - Professeur des universités - Université de Bordeaux - CoDirecteur de these
- Cécile COUHARDE - Professeur des universités - Université Paris Nanterre - Examinateur
- Sophie BRANA - Professeur des universités - Université de Bordeaux - CoDirecteur de these
- Camélia TURCU - Professeur des universités - Université d'orléans - Rapporteur
- Grégory LEVIEUGE - Professeur des universités - Banque de France - Rapporteur
This thesis links the dynamics of international reserves accumulation observed in emerging countries over the last two decades with the question of monetary policy autonomy, which has regained interest following Rey's (2013) demonstration of a shift from the trilemma to the dilemma. The first chapter aims to re-examine the drivers of monetary autonomy in emerging countries and measure the role of international reserves in this autonomy. We show that adopting a fixed exchange rate regime still constrains monetary autonomy, except when there is a high level of international reserves, which relaxes the constraints of the trilemma and allows for the achievement of domestic monetary policy objectives. In the second chapter, after analyzing the dynamics of international reserves accumulation in emerging countries, we determine a threshold of international reserves beyond which monetary autonomy increases. This threshold, defined at 22-23% of the ratio of international reserves to GDP, allows us to challenge the often-attributed "excessive" nature of international reserves accumulation and to demonstrate the partial validity of the Mundell trilemma, which evolves into a quadrilemma beyond this threshold, thereby extending the conclusions of the first chapter. In the final chapter, we estimate whether a more autonomous monetary policy goes hand in hand with greater financial stability. While our results show a positive relationship, a more autonomous monetary policy alone is not sufficient to reduce financial imbalances due to the “financial channel of exchange rates.” In this context, we demonstrate that foreign exchange interventions motivated by international reserves accumulation are a credible alternative to managing financial imbalances.
ED Sciences Physiques et de l'Ingénieur
Study of the Influence of the Buried-Gate Power GaN MOS-HEMT Architecture on the Physical Properties and Electrical Performance of the Devices
by Clémentine PIOTROWICZ (Laboratoire de l'Intégration du Matériau au Système)
The defense will take place at 10h00 - Salle 205 en face de l'arrêt de tram/bus Marie-Louise Paris – CEA au 17 Av. des Martyrs, 38000 Grenoble
in front of the jury composed of
- Nathalie MALBERT SAYSSET - Professeure des universités - Université de Bordeaux - Directeur de these
- Bassem SALEM - Directeur de recherche - Laboratoire LTM - Examinateur
- Philippe GODIGNON - Ingénieur - CEA LETI - Examinateur
- Loïc THEOLIER - Maître de conférences - Laboratoire IMS - Examinateur
- Dominique PLANSON - Professeur des universités - Laboratoire AMPERE - Rapporteur
- Daniel ALQUIER - Professeure des universités - Laboratoire GREMAN (UMR 7347) - Rapporteur
The lateral MOS-HEMT architecture with etched gate ("normally-off") in gallium nitride (GaN), currently under development at CEA-Leti on 200mm silicon substrates, aims to meet the growing electrification needs in response to the high demand for medium-to-high voltage power converters (>200V). The targeted applications include phone chargers, laptop chargers, onboard chargers for electric vehicles, photovoltaic micro-inverters, etc., which require more efficient converters than those currently based on silicon technology. The objective of this thesis is to study the influence of the MOS-HEMT architecture on the physical properties and electrical performance. The focus is on optimizing the on-state resistance of the transistors (RON), with a global perspective on maintaining blocking properties (RON/BV) and the "normally-off" behavior of the components (RON/VTH). Two main areas are studied. The first concerns the gate channel resistance, which depends on the etching, influencing the morphology and the channel mobility, along with the contribution of the dielectric/GaN interface quality (µ≤ 250 cm2.V−1.s−1). Variations in gate morphology, such as angle (90° to 60°), depth (50nm to 350nm), channel crystallographic orientation, and the impact of fabrication processes (ICP-RIE, ALE, cleaning) were studied through electrical measurements IDS(VG), C(VG), IDS(VDS) in blocking mode, and were then simulated and modeled using TCAD. A mobility extraction method around the gate was proposed, showing better mobility at the bottom of the gate (≤190 cm2.V−1.s−1 at 25°C) compared to the sidewalls (≤ 93 cm2.V−1.s−1 at 25°C). Additionally, correlations between electrical parameters (RON, VTH, SS) and design were established, showing a better RON/leakage compromise with an intermediate etching depth of 150nm and a gate length of 0.5µm (RON=8.1Ω.mm at 25°C and 15.2 Ω.mm at 150°C). The second area focuses on the transistor's access resistance, linked to the properties of the AlGaN/AlN/GaN heterojunction and polarization mechanisms responsible for the 2D electron gas (2DEG) at the AlN/GaN interface. Several technological variations of the heterojunction, including the AlN layer thickness (0.7nm to 1.5nm), AlGaN barrier (7nm to 24nm), aluminum percentage (24% to 60%), and doping, were electrically characterized using 5-point I(VG) and C(VG) measurements on Van der Pauw structures at 25°C and 150°C. 1D Poisson-Schrödinger simulations were also used to assess the interface polarization charge and identify improvement levers for this 2DEG resistance. A minimum value of 213Ω/◻ was obtained at 25°C for 60% aluminum (µ=1591 cm2.V−1.s−1 and nS =18.3×1012cm−2) and 455Ω/◻ at 150°C (µ=774 cm2.V−1.s−1 and nS = nS =17.7×1012cm−2), reducing RON by 2.1Ω.mm at 150°C. In conclusion, the results of this thesis open up new perspectives for future generations of GaN transistors, both in terms of performance optimization and manufacturing process improvements.
Heat transfer modeling in charring composites
by Jérémy CHEVALIER (I2M - Institut de Mécanique et d'Ingénierie de Bordeaux)
The defense will take place at 14h00 - Amphi 2 batiment A9 351 Cours de la Libération, 33400 Talence ; Batiment A9 amphi 2
in front of the jury composed of
- Jean LACHAUD - Maître de conférences - Université de Bordeaux - Directeur de these
- Bruno DUBROCA - Directeur de recherche - LCTS - CoDirecteur de these
- Franck RICHARD - Maître de conférences - Université de Poitiers - Examinateur
- Alessandro TURCHI - Ingénieur de recherche - Agenzia Spaziale Italiana - Examinateur
- Gérard VIGNOLES - Professeur - Université de Bordeaux - Examinateur
- Alexis COPPALLE - Professeur émérite - INSA Rouen - Rapporteur
- Alexandre MARTIN - Professeur - University of Kentucky - Rapporteur
- Antonio COSCULLUELA - Ingénieur de recherche - CEA CESTA - CoDirecteur de these
Access to space and the design of modules able to support atmospheric re-entry to Earth or Mars often require the use of ablative materials. The materials considered in this study are carbon fiber composites with a polymer matrix. As the temperature rises, these materials are subject to high-gradient thermal phenomena coupled with oxidation, pyrolysis, sublimation and thermomechanical cracking. The aim of this work is to improve understanding and modelling of the pyrolysis phenomenon. A generalized pyrolysis model has been developed that allows complex reaction mechanisms, particularly competitive ones, to be taken into account. This model was calibrated on thermogravimetric analysis data obtained at various heating rates. The production of gas by pyrolysis leads to a rise in pressure in the material. A multi-scale study of the mechanical behavior was carried out. At the microscopic level, decohesion at the fiber-matrix interface and pressure build-up in the pores were studied. This approach enabled the origin of cracks during heating of the material to be identified. Volume averaging was used to obtain a poromechanical model consisting of a mechanical behavior law and a conservation of momentum equation on a macroscopic scale, taking into account the gas pressure in the pores. The model
ED Sociétés, Politique, Santé Publique
First uses of alcohol, tobacco, e-cigarette, and cannabis: Issues related to social media use among middle school students and romantic relationships among college students
by Mathilde JANOTA (Bordeaux Population Health Research Center)
The defense will take place at 10h00 - Amphi Sigalas 3ter Place de la Victoire Bâtiment C 33000 BORDEAUX
in front of the jury composed of
- Mathilde HUSKY - Professeure - Université de Bordeaux - Directeur de these
- Lucia ROMO - Professeure des universités - Université Paris Nanterre - Rapporteur
- Abdel Halim BOUDOUKHA - Professeur des universités - Université de Nantes - Rapporteur
- Stacey CALLAHAN - Professeure des universités - Université de Toulouse 2 Jean Jaurès - Examinateur
- Laurent KARILA - Professeur des universités - praticien hospitalier - Université Paris-Saclay - Examinateur
Alcohol, tobacco, cannabis and, more recently, e-cigarettes are among the most widespread psychoactive substances in France. The first uses occur mostly at an early age, starting in middle school, and increase through to early adulthood. However, early initiation of consumption is associated with an increased risk of developing substance use disorders, leading to significant negative consequences for both the individual and their daily functioning. These consequences can also arise from lower levels of consumption and affect various areas of life, including interpersonal relationships. These relationships are particularly important during emerging adulthood, a period also marked by an increase in alcohol use. Therefore, the first study of this thesis aimed to explore the links between risky alcohol use and romantic relationships in a sample of students, first cross-sectionally, and then at one-year follow-up. The data were collected as part of the French portion of the international World Mental Health College Student Survey, an online survey with annual follow-up, aimed at characterizing the mental health needs of students. Risky alcohol use was associated with unstable romantic relationships, as well as high stress about one's love life. However, the long-term associations were no longer significant after adjustment for lifetime mental health disorders. These results highlight the importance of considering consumption levels below the diagnostic threshold for substance use disorders. Indeed, while the majority of the population will experiment with at least one psychoactive substance over their lifetime, usually alcohol, only some individuals will develop a related substance use disorder. Thus, focusing on the different levels of use before the onset of a disorder, and more specifically on the transitions between these levels, appears particularly relevant for understanding consumption trajectories and preventing substance use disorders. In this regard, identifying factors associated with these transitions seems essential, starting with the transition from non-use to experimentation. The second study presented in this thesis examined the links between social media use and substance experimentation in early adolescence. The exploration of this environment seemed particularly relevant due to its popularity with adolescents and its propensity to share content involving substances. Accordingly, data collected from the multi-center Cyberlife survey (2019-2023) were used to explore these links. Based on middle school students' self-reported responses, the study highlighted that: (1) having multiple accounts on different platforms; (2) using certain social media sites such as Instagram and TikTok; and (3) following content creators who had previously posted about substances, led to an increased risk of experimenting with at least one of the substances studied. This thesis highlights the need to focus on levels of use below the diagnostic threshold for substance use disorders. Further exploration of the transitions between these levels, and the factors associated with them, could be an important avenue for future research. Improving the current understanding of consumption trajectories is crucial for guiding prevention efforts. Additionally, the study of social media in relation to substance experimentation in adolescence is of particular interest and deserves further exploration. However, better measurement of social media use is essential, as is the integration of other factors into the analyses that could contribute to the observed associations.