Thermal transport at the nanoscale has long been the subject of research, due to its importance in both fundamental physics and practical applications. On the one hand, there is still a significant knowledge gap in this field, particularly in nanoscale systems at very low temperatures (down to 1 K or less). Under these conditions, the mean free path (Λ) and dominant wavelength (λ_DOM) of phonons become comparable to or larger than the sample size, allowing access to ballistic and coherent phonon transport regimes. On the other hand, the miniaturization of electronic devices and new trends in nanoscale integrated circuits have introduced significant challenges in thermal management. This is becoming increasingly important for applications in quantum computers or the development of cryo-CMOS technologies. The Hanibal project aims to deepen knowledge of the different thermal transport regimes and study some interesting phenomena observed in this field, such as thermal rectification, thermal conductance reduction in phononic crystal systems, and thermal transport quantization.

Being able to demonstrate these counterintuitive heat transfer mechanisms means converting tiny amounts of energy into a measurable quantity. To achieve these goals, silicon-based suspended thermal devices have been fabricated. These devices are produced from silicon-on-insulator (SOI) wafers using micro/nanofabrication techniques, primarily electron beam lithography and dry etching (RIE, ICP-RIE). NbTi superconducting wires ensure that there is no parasitic heating during device control, while detection is provided by thin layers of NbN, achieving sensitivities in the femtowatt range (10⁻¹⁵ W). Various samples (nanowires and 2D patterned membranes) have been fabricated for the planned studies. This work is being carried out in collaboration with CEA-LITEN (for theoretical simulations) and the Néel Institute (for sample fabrication and dilution refrigerator measurements). At the Institut Néel, SiN-based samples were also produced in order to compare the behavior of amorphous SixNy with that of crystalline Si. Contrary to expectations, asymmetric phononic-patterned membranes based on SiN exhibit a reproducible rectification rate of approximately 20% below 4 K, demonstrating directional heat transport with potential applications.

The results for our silicon-based samples will be shown. In summary, the high sensitivity of these thermometry devices and their ability to accommodate multiple sample types between reservoirs will advance experimental capabilities in low-temperature heat transport measurements. In this thesis defense, we will discuss the main scientific and technological challenges of the project, highlight the fully finished devices and the measurement method. The first thermal rectification data will be presented. Finally, we will outline the future prospects for sample characterization and avenues for research.