Revealed by RIXS*

UTe₂ is a remarkable heavy fermion f-electron superconductor with properties including a large upper critical field H_c2(T) of 40 T along the b crystalline axis[1], an intermediate valence[2], and a mysterious re-entrant superconducting phase when a large magnetic field (40–60T) is applied between the b and c crystalline axis within the angle of 23 to 45[1,3]. Further clarity on the properties of UTe₂ can come from studying the electron correlations between 5f orbital and conduction electrons. For this purpose, we studied the 5f-electron occupancy of UTe₂ as a function of pressure using Resonant Inelastic X-ray Spectroscopy (RIXS)[4] with incident energy at the U L3 edge while detecting the 3d_5/2 to 2p_3/2 emission. The 5f-electron configuration weight was then calculated from the fitted emission peaks to study the valence electron modification with changing pressure. Additionally, Partial Fluorescence Yield X-Ray Absorption Spectroscopy (PFY-XAS) data taken with changing pressure revealed the intermediate U³⁺ and U⁴⁺ valence veer towards U⁴⁺ but then back to U³⁺ up to 52 GPa.