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.

In recent years, progress in the field of frustrated magnets have led to the emergence of innovative concepts including new phases of matter. The latter’s do not show any long-range order (no symmetry breaking), but, in classical systems, exhibit a highly degenerate ground state made of classical configurations. An emblematic example is spin ice in pyrochlores : in this case, the construction of those configurations relies on a simple rule, which states that the sum of the four spins in any tetrahedron of the magnetic lattice must be zero. This so-called “ice rule” can be understood as the conservation rule of an emergent gauge field. Experimental evidence of this physics was provided by the observation of singular points in the spin-spin correlation function by elastic neutron scattering experiments. Such singular points, called pinch points, arise because the correlations of the emergent divergence free field are dipolar in nature, with
algebraic spin-spin correlations.
The origin of this physics lies in the conjunction between lattice connectivity, anisotropy and magnetic interactions, which collude to select configurations where a local constraint between spins is preserved. Recently, several authors have proposed a generalization of this concept to other geometries and other constraints, as for instance the “octochlore” lattice, formed by corner sharing octahedra.
Depending on the chosen constraint, different spin liquids have been theoretically predicted.
An experimental realization of the octochlore lattice can be found in rare earth fluorides KRE3F10, as their crystal structure forms a “breathing” network of small and large RE octahedra. Very little is known about the physics of KRE3F10 compounds, apart from magnetization measurements performed two decades ago. The goal of the PhD work will be to characterize the ground state of two Kramers members of the KRE3F10 system (RE = Dy3+, Er3+), to identify in particular any signature of the spin liquid physics suggested by recent theoretical works, and better understand the constraints leading to it.