Thesis defended December 12, 2025

Abstract
The superconductivity discovered a few years ago in UTe₂ combines several original and fascinating aspects. It is extremely robust under magnetic field, hinting that UTe2 may be a spin-triplet superconductor. This implies that it could also be a topological superconductor, a very popular topic with applications in quantum computing. More recently, the very interesting effect of pressure or magnetic field on this system, with the appearance of multiple superconducting phases, has been highlighted. The overall objective of the thesis is therefore to fully investigate the properties of UTe₂ by combining both pressure and intense magnetic field. These properties are very sensitive to the orientation of the applied magnetic field relative to the crystal lattice as well. To the end of studying this effect, a miniaturised pressure cell was designed and built. One of the objectives of the thesis was to test this cell and then validate its viability by performing calorimetry measurements under pressure on UTe₂, allowing in-situ rotation of the cell. At the same time, numerous calorimetric measurements were carried out in standard pressure cells in order to establish the thermodynamic phase diagram of UTe₂ under pressure and field. A major result is the connection we were able to prove between the two superconducting phases induced by pressure or a field along the b-axis, which in reality form a single high-pressure, high-field superconducting phase. This result adds strong constraints on theoretical models seeking to explain the stabilisation of the different superconducting condensates in UTe₂. ​