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Adrien Rosuel

Tuning the spin-triplet superconducting state by a magnetic field in UTe2 and UCoGe

Published on 15 May 2023
Thesis presented May 15, 2023

In 2018, superconductivity was discovered in the paramagnetic compound UTe2 between 1.5 and 2 K. The possibility of spin-triplet superconductivity, possibly induced by ferromagnetic fluctuations, and of topological superconductivity, has triggered a lot of study. Nevertheless, the pairing symmetry and mechanism remain unknown.
A striking property of UTe2 at ambient pressure is its resilience to magnetic field. For a certain range of angles in the crystal, superconductivity is detected up to 60 T. Also, for a field along the b-axis of hard magnetisation, superconductivity is enhanced above 15 T up to a metamagnetic transition occurring at Hm = 34.5 T.
A major result of this work is the discovery of a thermodynamic transition line between two superconducting phases in the H-T phase diagram along the b axis. These measurements reveal the emergence of a second superconducting phase above 15 T when a field is applied along the b-axis. Moreover, they show that the two phases are of a different nature, not corresponding to a simple change of symmetry of the superconducting order parameter as proposed in previous theoretical studies. They confirm, as suggested by the pressure studies, that there are two competing pairing mechanisms in UTe2. A likely scenario would be a low-field spin-triplet superconducting phase, possibly driven by ferromagnetic fluctuations, and a high-field spin-singlet phase, possibly driven by anti-ferromagnetic fluctuations as the metamagnetic transition approaches.
Specific heat measurements were also performed on the ferromagnetic superconductor UCoGe. It is a strong candidate for spin-triplet superconductivity. Despite its discovery sixteen years ago (2007), few thermodynamic measurements have been made. Our specific heat measurements have allowed us to establish a first complete thermodynamic phase diagram. A more detailed analysis of the results allows to demonstrate in a “direct” way the suppression of the pairing mechanism for magnetic fields applied along the easy magnetisation axis, and also shows a very unusual behaviour of the order parameter at low fields in the directions of hard magnetisation​.

corralated electrons, unconventional superconductivity, spin-triplet superconductivity, specific heat, heavy fermions

On-line thesis. ​