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: 
• In metals, the contribution of electrons to κ is directly related to electrical conductivity [1].
• In contrast, in insulating materials, acoustic phonons (collective vibrations of atoms) are the main carriers of heat. Peierls described heat transport in crystalline solids using Boltzmann's transport equation [2] taking into account phonon-phonon scattering phenomena
• Magnetic excitations can also contribute significantly to heat transport, as has been observed in the Haldane spin chains AgVP2S6 and Y2BaNiO5.
• Measurements of κ, however, have also identified more exotic carriers: emergent quasiparticles such as non-Abelian states that manifest in the fractional quantum Hall effect [3], or spinons in the antiferromagnetic spin ½ Heisenberg chain, the best-known examples of which are the cuprates KCuF3, Sr2CuO3, and SrCuO2 [4]. 

Thermal conductivity is therefore an intrinsic property of materials that indirectly allows one to trace the physical characteristics (velocity, mean free path) of excitations in matter, from conventional phonons to magnetic excitations, and above all macroscopically highlight the couplings that may exist between the two. The take-home message is that thermal conductivity is a highly relevant complement to the neutron spectroscopy experiments which have always been the LLB trademark, as neutron spectroscopy allows to directly image the dispersion of quasiparticles, phonons, spin waves, spinons or CEF levels, thereby tracking traces of the microscopic coupling between them. The NFMQ group has strong experience in this field, but being able to analyze macroscopic data such as κ with the microscopic approach offered by neutron spectroscopy has become one of the challenging research topics of the group.

The aim of this project is to develop a set up allowing measurement of κ to be confronted with measurements of the dispersion of quasiparticles by inelastic neutron scattering done at ILL. Experiments are done / will be done with a Quantum Design PPMS electronic puck (300K – 3K) and with a special sample holder in a dilution refrigerator (4K – 17mK) at LLB Saclay.

This project is a collaboration between CEA/LLB in Saclay and ESPCI in Paris.

References
[1] C. Kittel, Introduction to Solid State Physics, Wiley, 1996.
[2] M. Bannerjee et al., Observation of half-integer thermal Hall conductance, Nature 559, 205 (2018)
[3] R. Peierls, The kinetic theory of thermal conduction in crystals, Annalen der Physik 3, 1055 (1929).
[4] C. Hess, Heat conduction in low-dimensional quantum magnets, EPJ-special Topics 151, 73–83 (2007).

Keywords
set up|PPMS|dilution fridge|thermal conductivity|MMR3/MGC3