Thesis

Understanding the parameters which determine the magnitude of thermal conductivity (k) in solids is of both fundamental and technological interests. k is sensitive to all quasiparticles carrying energy, whether charged or neutral. Foremost among these are phonons, the collective vibrations of atoms in crystals. Measurements of k, however, have also identified more exotic carriers like spinons in the antiferromagnetic Heisenberg chain. 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.
Still in its infancy, this concept has emerged from the observation of a spin-phonon coupling in a variety of rare-earths based materials. The magnetic excitations involved in the magnetoelastic coupling at play in those compounds are not standard magnons, but low energy crystal field excitations (CEF). Since the latter are local electronic excitations, they do not disperse and thus cannot be associated with propagating quasiparticles. In other words, they are not potential heat carriers hence do not contribute to k, in contrast with dispersive magnetic quasiparticles like magnons. However, they can significantly reduce the phonon lifetime by opening a new scattering mechanism.
The aim of the PhD thesis is therefore to investigate, both experimentally and theoretically, magnetoelastic coupling and its impact on thermal conductivity. The systems to be studied will be (but not restricted to) Tb perovskites, and will include high-entropy or entropy stabilized compositions, displaying glass-like thermal conductivity.