Detailed subject

Understanding the parameters which determine the magnitude of thermal conductivity (Κ) in solids is of both fundamental and technological interests. Κ is sensitive to all quasiparticles carrying energy, whether charged or neutral. Foremost among these are phonons, the collective vibrations of atoms in crystals. In terms of applications, thermal properties of solids are at the heart of major social and environmental issues. The need, for instance, for highly efficient thermoelectric and thermal barrier devices to save energy has driven the quest for low thermal conductors. Over time, a range of strategies has thus been suggested to hinder phonon velocities and/or mean free paths: use of weak interatomic bonds, strong anharmonicity, nanoscale designs, or complex or disordered unit cells. Another promising concept to further impair the phonon mean-free path is based on magneto-elastic coupling.

This concept has emerged from the observation of a spin-phonon coupling in some rare-earth perovskites, pyrochlores or garnets. The magnetic excitations involved in the magneto-elastic coupling at play in those compounds are low energy crystal field excitations (CEF). Those are local electronic excitations, thus they do not contribute to κ ; they can significantly reduce the phonon lifetime, however, through new phonon scattering mechanisms.

The aim of this M2 internship will be therefore to investigate experimentally the thermal conductivity of a rare-earth garnet with high disorder using the high-entropy approach.

Location

CEA Saclay, (91) Essonne, France

Internship conditions

• Durée du stage : 6 mois
• Niveau d’étude requis : Bac+5
• Formation : Master 2
• Poursuite possible en thèse : Oui
• Date limite de candidature : 2 mars 2026

Méthods, technics :
During this internship you will:

• Synthesize a high-entropy crystalline rare-earth garnet single crystal, with high entropy on the A-site (rare-earth site) of the garnet crystal structure, in collaboration with ICMMO, a solid-state chemistry laboratory at Université Paris-Saclay,
• Perform comprehensive thermal conductivity and specific heat measurements down to sub-kelvin temperature on state-of-the art experimental setups, at ICMMO or at ESPCI (Paris),
• If time allows, carry out neutron scattering experiments in a large-scale facility such as ILL, to study CEF dynamics

Responsable du stage

Francoise Damay
Tél. : 01 69 08 49 54
Email :

Responsable LLB / NFMQ

Francoise Damay

Detailed subject:

Saving energy thanks to highly efficient thermoelectric and thermal barrier devices has driven the quest for low thermal conductors. In insulators, heat is carried by phonons, collective vibrations of atoms. Over time, a range of strategies has been suggested to hinder phonon velocities and/or mean free paths: use of weak interatomic bonds, strong anharmonicity, nanoscale designs, complex or disordered unit cells.

Another promising concept to further impair the phonon mean-free path has emerged from experimental observations in a variety of rare-earth compounds. Indeed, there is a suspicion that local low energy electronic excitations, so-called crystal field excitations (CEF), significantly reduce the phonon lifetime by virtue of a resonant coupling with phonons. Strikingly, tuning the CEF by applying a magnetic field for instance allows one to strongly modify the heat conductivity.

CEF excitations, however, are also sensitive to magnetic excitations between rare earths. As a result, these local modes become collective and acquire a delicate dispersion. In this context, the aim of the proposed M2 internship is to study numerically this dispersion using a model Hamiltonian. Simulations will then be compared with existing experimental data, in this case the magnetic excitations spectra of KTb₃F₁₀.

Location:

CEA Saclay, (91) Essonne, France

Internship conditions

• Internship duration: 6 months
• Required level: Bac+5 (5 years of higher education)
• Education: Master’s degree
• Possibility of continuing with a PhD: Yes
• Application deadline: March 2, 2026

Méthods, technics :
During this internship you will:

• Perform calculations of the excitation spectrum of KTb₃F₁₀, using a code developed in the LLB laboratory,
• If times permits, expand the model by introducing a magneto-elastic coupling term and compare it with the inelastic neutron scattering data of KTb₃F₁₀ at low temperature and various magnetic fields which are already available.

Internship supervisor

Sylvain Petit
Tél. : 01 69 08 60 39
Email :

Head of LLB / NFMQ

Francoise Damay