Observing magnetic and charge instabilities in a UTe2 superconductor by X-ray spectroscopy

The coexistence of magnetism and superconductivity is one of the most enigmatic problems in condensed matter physics, especially with the discovery of multiple superconducting phases in UTe₂ under magnetic fields and under high pressure, making it a prime candidate for spin-triplet and topological superconductivity [1]. At ambient pressure, UTe₂ is paramagnetic and becomes superconducting below T_sc = 1.6-2 K. Applying a modest pressure of 1.5 GPa leads to a suppression of superconductivity and to a magnetically ordered state. This is most likely due to a singularity of the crystal structure and instability of electronic properties at the Fermi level, formed predominantly by the uranium 5f states. To draw a complete picture of the superconducting state in UTe₂, a precise knowledge of the electronic properties of the 5f states of uranium is a prerequisite.

X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) at the uranium M_4,5-edges provide direct information about the 5f electronic and magnetic properties [2]. The XANES spectra allow the occupation of the 5f levels to be extracted, whereas the XMCD spectra reveal the orbital and spin magnetic moments carried by the 5f electrons. Beamline ID12 offers unique spectroscopic capabilities for studies under multiple extreme conditions of high magnetic field, low temperature and high pressure. Together with scientists from the CEA in Grenoble, the ID12 team was able to record high-quality XANES and XMCD spectra at the uranium M_4,5-edges in UTe₂ single crystals at magnetic fields up to 17 Tesla, temperatures as low as 2.7 K, and applied pressures as high as 5 GPa.

Unambiguously, the team established that the uranium 5f occupancy at ambient pressure is between 2.6 and 2.8, which is significantly different from the 5f ³ (U³⁺) or 5f ² (U⁴⁺) configurations predicted by resonant inelastic X-ray scattering [3] or angle-resolved photoelectron spectroscopy [4] measurements. This finding is further supported by the observed reduction of the uranium 5f orbital to spin moment ratio, deduced from XMCD measurements.

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Fig. 1: Pressure dependence of the U-M_4,5 white lines. Shape of the M₅ (full lines) and M₄ white lines for selected pressures at 2.7 K. The inset shows the enlarged part of the top of the M₅ white line. E₀, the energy of the maximum of the M₅ white line at 0.75 GPa, was set to zero.

The most surprising finding is the strong variation of the 5f electron count under pressure, as determined from the M_4,5 XANES spectra. Using a specific diamond anvil cell, it was possible to follow the evolution of the XANES spectral shape under pressures up to 5 GPa at 2.7 K (Figure 1). The overall results of the changes in the 5f count as a function of pressure are summarised in Figure 2.

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Fig. 2: Variation of the 5f electron count versus pressure. P_c represents the critical pressure of the transition from the paramagnetic state (PM) towards a magnetically ordered (MO) state. The blue dashed curve is a guide to the eyes. The shaded part indicates the pressure region where the crystal structure changes from orthorhombic to tetragonal.

When UTe₂ transitions to a magnetically ordered normal state at a pressure of roughly 1.5 GPa, a drop in the 5f occupancy of 0.2 e- is observed. The change towards the 5f ² (U⁴⁺) electronic configuration is not unexpected if one considers the values of the ionic radii of U³⁺ and U⁴⁺ as well as the observed large volume shrinking close to 3% at 1.7 GPa with respect to ambient pressure. The rather low bulk modulus of UTe₂ (57 GPa) is typical for highly compressible materials, which are generally prone to “valence instability”. The observed drastic decrease of the 5f occupation is much larger than the values reported for other uranium intermetallic compounds under pressure.

The results also exhibit a sudden increase of the 5f count towards U³⁺ above 4 GPa, reaching the value at ambient pressure and even higher at 4.75 GPa. This is certainly associated with a change of the electronic structure induced by the structural transition (orthorhombic to tetragonal) observed in X-ray diffraction experiments [5]. In the tetragonal phase, the shortest U–U distance increases by about 8%, causing a much smaller overlap among 5f orbitals of neighbouring uranium atoms.  This increase in localisation of the 5f electrons leads to a change of the 5f count towards U³⁺.

To conclude, element-specific and orbital-selective studies revealed that the magnetic and charge instabilities in UTe₂ are strongly coupled to the occupancy of the 5f levels. This coupling could play an important role in the superconductivity mechanism of UTe₂.

Principal publication and authors
Investigating the electronic states of UTe₂ using X-ray spectroscopy, F. Wilhelm (a), J.-P. Sanchez (b), D. Braithwaite (b), G. Knebel (b), G. Lapertot (b), A. Rogalev (a), Comm. Phys. 6, 1  (2023), https://doi.org/10.1038/s42005-023-01220-1
(a) ESRF
(b) University Grenoble Alpes, INP, CEA, IRIG-Pheliqs, Grenoble (France)

References
[1] S. Ran et al., Science 365, 684 (2019).
[2] F. Wilhelm, J.-P. Sanchez & A. Rogalev, J. Phys. D: Appl. Phys. 51, 333001 (2018).
[3] S. Liu et al., Phys. Rev. Lett. 106, L241111 (2022).
[4] L. Miao et al., Phys. Rev. Lett. 124, 076401 (2020).
[5] F. Honda et al., J. Phys. Soc. Jpn. 92, 044702 (2023).