5f electronic states in the UTe2 superconductor from XANES and XMCD studies

Abstract

Abstract The recent discovery of superconductivity in paramagnetic UTe2 turns spotlight on a serious candidate for spin-triplet state. It exhibits highly unusual transport and magnetic properties which are extremely sensitive to pressure. To draw a complete picture of the superconducting state in UTe2 precise knowledge of the electronic properties of the 5f states of Uranium is missing. We report on x-ray absorption and magnetic circular dichroism experiments performed at the U- M4,5 edges at 2.7 K. In the normal phase the 5f electron count is found to be close to 2.8 at ambient pressure. Partial delocalization of the 5f electrons is further confirmed by the reduced value of the U orbital to spin magnetic moment ratio deduced from the XMCD spectra. X-ray absorption measurements under applied pressure show that the 5f count is reduced by 0.2 electrons pointing out an increase of the valency towards U4+ at the transition to a magnetically ordered state at Pc ≈ 1.5 GPa. Surprisingly, at pressures above 4 GPa, the 5f count increases towards U3+. This points out a change of the electronic structure in the tetragonal phase observed by x-ray diffraction above 4 GPa with a new superconducting state. The observed valence instabilities and their interplay with magnetism seem to be important ingredients to understand the electronic structure in UTe2 in different phases.