Low energy electronic excitations and heat conductivity

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