Investigating the electronic states of UTe2 using X-ray spectroscopy

UTe2 is a strong candidate for spin-triplet superconductivity; however, to fully understand its superconducting properties a more complete understanding of its general electronic structure is needed. Here, the authors perform two types of X-ray spectroscopy to investigate the 5 f electronic states of the uranium atom, finding unusual features for the pressure dependence of the 5 f electron count. The recent discovery of superconductivity in paramagnetic UTe2 turns spotlight on a serious candidate for spin-triplet state. To draw a complete picture of the superconducting state in UTe2 precise knowledge of the electronic properties of the 5 f states of Uranium is missing. We report on x-ray absorption and magnetic circular dichroism experiments performed at the U M-4,M-5 edges at 2.7 K. At ambient pressure the 5 f electron count is found to be in-between 2.6 and 2.8. Partial delocalization of the 5 f electrons is further confirmed by the reduced value of the U orbital to spin magnetic moment ratio. The 5 f count is reduced by as large as 7 percent at the transition to a magnetically ordered state at P-c approximate to 1.5 GPa. At pressures above 4 GPa, the 5 f count increases back towards U3+ in the tetragonal phase. The observed "valence instabilities" and their interplay with magnetism seem to be important ingredients to understand the electronic structure in UTe2 in different phases.