Insights into the Thermodynamics of Superconductivity and Metamagnetism in UTe₂

Spin-triplet superconductors have recently attracted considerable attention, as they are natural candidates for topological superconductivity. Spin-triplet superconductivity is rare in bulk materials and best established in the ferromagnetic superconductors URhGe and UCoGe. Evidence for topological spin-triplet superconductivity exists also in the recently discovered paramagnetic superconductor UTe2 (superconducting transition at Tc = 1.6 − 2.1 K) [1]. The superconductivity in UTe2 shows an extraordinary reenforcement of its upper critical field for the hard magnetization axis above 15 T and up to 35 T, where a second superconducting phase appears which abruptly ends by a metamagnetic transition [2]. Moreover, a re-entrant superconducting phase appears at very high magnetic fields > 40 T for particular field directions and multiple superconducting phases occur in UTe2 under the application of hydrostatic pressure [3,4].

The resulting complex phase diagrams and connections between the different superconducting phases are presented in this talk. The aim of the presented PhD project was to fully establish and identify the different superconducting phases induced by magnetic fields in UTe2. Using the combined expertise existing at CEA/PHELIQS and CNRS/LNCMI, we set up linear thermal expansion, ultrasound spectroscopy and specific heat measurements in high magnetic fields on UTe2. We explore in detail the different phase transitions inside the superconducting reenforced state and determine the precise phase diagram for various different sample qualities. The thermodynamic connection between the reenforced superconductivity and magnetic fluctuations arising from the metamagnetic transition is also made, wherefore the metamagnetism is mapped out in details in the normal state of UTe2. Additionally, bulk evidences for the field re-entrant superconducting phase and a possible interplay with the metamagnetic transition are presented [5]. These experimental results, mostly unpublished to date, provide a detailed characterization of the exotic superconductivity in UTe2. Together with our findings on its metamagnetism, they yield a deeper understanding of this compound that can be extended to other unconventional superconductors.

[1] Ran, S. et al. Science365,684-687 (2019)
[2] Rosuel, A. et al. Phys. Rev. X 13, 011022 (2023)
[3] Ran, S. et al. Nat. Phys. 15, 1250–1254 (2019)
[4] Braithwaite, D. et al. Comm. Phys. 2, Nr. 1:147. (2019)
[5] Marquardt, N. et al. arXiv.2512.17691 (2025)