Mesophon project is devoted to the study of thermal transport in nano-materials where the characteristic sizes are comparable to the phonon wavelength Lph (100nm at 3K & 1nm at 300K). In order to reach this goal a multidisciplinary research project is proposed. It lies on the elaboration of specific nano-structures and on the study of their thermal properties by using state of the art measurement and modeling. The main asset of the project is to develop an original approach based on very low temperature measurements and numerical simulations to observe wave effects on phonon transport. In this low temperature limit, thermal transport in nano-structured semiconductor has been scarcely studied, as measurements are very challenging at these length scales. Nevertheless, recent experimental works pointed out significant contributions of intermediate phonon wavelength to heat transport at the nanoscale, indeed questioning existing textbook thermal models. Furthermore, there is an increasing interest for understanding phonon scattering in newly developed phononic devices that could be relevant for several applications, particularly in the fields of electronic and optic. Within this collaborative research program, specialists in the fields of elaboration, thermal measurement, theory and numerical simulation will work together. The scientific program involves 3 workpackages that address specific issues. • First, unique thin Ge films with Ge:Mn or Ge:Sn:Mn nano-inclusions will be grown at SiNaPS. Their design, concerning size and dispersion of clusters, make them the most suitable objects to be used as model materials to study wave effects due phonon-cluster interaction. • Second, thermal properties measurements of the elaborated thin films, from 300K to 4K, will be held at Institut Néel. Through dedicated devices, thermal conductivity and phonon drag effect on Seebeck coefficient are going to be measured. Collected data will help to develop a theoretical model for phonon scattering by nano-inclusions as a function of their wavelength. • Third, at the same time within the LEMTA, models and numerical simulations will be developed to appraise thermal transport properties and compare them with measurements. Feedback to the growing procedure will improve the targeted window in terms of size and dispersion of nano-inclusions. New numerical models targeting wave effect in the phonon transport will be investigated in order to appraise properties such as a phonon scattering phase function. This work is strongly collaborative and interdependent. From the material viewpoint, thin films elaboration for new devices with a controlled tailoring of the nano-structures is very challenging. Thus, developing characterization tools (experiments, models and simulations) that can measure or predict optimal properties give the necessary feedback for improving materials. From the advanced characterization standpoint, high sensitivity metrology development requires model materials with well defined properties as well as theoretical and numerical model to extract and appraised reliability of measured properties. Lastly, in what concerns modeling and numerical simulation at the atomic and mesoscales, realistic materials as well as accurate properties measurements are extremely valuables to assess models and tools. In this frame, the Mesophon project will provide useful knowledge for several research communities. Furthermore, as it is exposed in the detailed scientific document, preliminary studies including fabrication of Ge:Mn films, thermal conductivity measurements at 300K and simulations on nano-structured Ge were already conducted by the consortium members to appraise the project feasibility. The obtained promising results demonstrate that there are interesting scientific and technological challenges to tackle. Results of this study should foster the development of “phononic” science and its applications (thermal management, heat recovery, heat diodes, etc).