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(M/F) Coupling phenomena between thermal and electronic transport at the origin of exotic thermoelectric effects.

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Date Limite Candidature : dimanche 1 août 2021

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General information

Reference : UMR5306-STEPAI-003
Date of publication : Thursday, July 22, 2021
Scientific Responsible name : Stéphane Pailhès
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 4 October 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

We propose a PhD thesis in fundamental physics on the studies of microscopic coupling mechanisms between electronic and thermal transport as part of a research line on thermoelectricity. Thermoelectric effects make it possible to design devices that can either operate in generator mode by converting heat into electrical energy (Seebeck effect) or, when supplied electrically, be used in heat pump mode to cool or heat a surface (Peltier effect). ). Thus for many energy-related issues (reducing the use of fossil fuels, electrical autonomy of systems, thermal regulation of elements, etc.), thermoelectric devices might provide technological solutions. However, despite remarkable developments over the past three decades, the performance of thermoelectric modules needs to be significantly increased to allow them to deploy massively in the society. The main limitation is at the level of the intrinsic material performances. A fundamental research effort is needed to identify new concepts and launch technological breakthroughs.

In crystalline systems, the thermoelectric effects are determined by the properties of elementary excitation (or quasi-particle) gases associated with the collective movements of electrons and atoms, phonons, and by the exchanges of energies and particles in contact with heat and charge reservoirs. The knowledge of the individual properties of theses excitations (their energies, polarizations, statistics and lifetimes) makes it possible to describe the thermodynamic properties at equilibrium and out-of- equilibrium. The thermoelectric effects result from the transport phenomena of these gases placed out-of-equilibrium and subjected to a temperature gradient and / or a difference in electric potential. Modern ab initio numerical tools and spectroscopic experiments approaches make it possible to map these states in momentum and in energy which is necessary for solving the charge and phonon transport equations from which the thermoelectric coefficients and thermoelectric material efficiency, called figure of merit "ZT", can be determined. It is therefore at this level that new strategies can be developed. The electron-phonon interactions correspond to a diffusion process which, in transport equations, usually leads to a reduction in the mean free paths. However, in some materials and at low temperatures, strong electron-phonon coupling is responsible for giant thermoelectric effects, the physical mechanism of which remains to be elucidated. The objective of the thesis is precisely to study the microscopic origin of this effect in different massive and nanostructured systems by coupling the experimental and theoretical approach. Experimentally, the student will be required to make macroscopic thermoelectric measurements in laboratories and spectroscopic measurements mainly in large scale research facilities (synchrotron, neutrons and /or XFEL). Theoretically, the microscopic mechanisms of electron-phonon coupling will be studied and modeled on the basis of ab initio techniques (DFT) and using the Green function formalism.

This thesis project is funded by a collaborative research program between the University of the Witwatersrand in South Africa and the Institut Lumière Matière in Lyon, France. The student will be mainly based in Lyon and will be required to stay between 10 and 20% of his thesis time in South Africa. He will work within two teams that gather their expertise and know-how for this new research program. The profile sought is a condensed matter physicist, highly motivated by fundamental research, willing to engage in a project combining theory and experience and who is comfortable with programming.

Work Context

This thesis project is funded by a collaborative research program between the University of the Witwatersrand in South Africa and the CNRS at the Institut Lumière Matière (ILM) in Lyon, France. The student will be mainly based in Lyon and will be required to stay between 10 and 20% of the thesis time in South Africa. ILM is a joint CNRS and UCBL research unit. At ILM, the student will join the "(Nano) Materials for Energy" team on the thermoelectricity theme led by S. Pailhès. He will do theoritical work (ab initio) and experimental work in the laboratory and in large scale research facilities (Neutrons, Synchrotron and XFEL). The frequency of the later is of the order of a few annual experiences over a few days with extended working hours (day / night shifts and weekends)

Constraints and risks

The student will be led to do neutron and X-ray experiments in a synchrotron

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