Interplay between ferromagnetic quantum criticality, Fermi surface and unconventional superconductivity in UCoGe

Orateur : Soutenance de Thèse de Gaël BASTIEN (CEA/INAC/Pheliqs)

The microscopic coexistence between ferromagnetism and unconventional superconductivity was observed at ambient pressure in the two Uranium based heavy-fermion compounds URhGe and UCoGe. In this thesis we concentrated on the interplay of magnetic quantum criticality, Fermi-surface properties and unconventional superconductivity in these ferromagnetic superconductors. This presentation will be focused on UCoGe. We determined the pressure –temperature phase diagram up to 10.5 GPa. The Curie temperature decreases almost linearly with pressure from Tc = 2.7 K and vanishes at a critical pressure : pc 1 GPa. Superconductivity was observed both below pc and above pc with a maximum of Tsc just below the critical pressure. Superconductivity can be observed up to 4GPa and non-Fermi liquid up to 6GPa. The temperature and pressure dependence of the upper critical field can be explained from the strong influence of magnetic field on the pairing interaction. The Fermi surface properties of UCoGe was studied by quantum oscillations under high magnetic field up to 34T and under hydrostatic pressure up to 2 .5 GPa. Ambient pressure measurements showed several Fermi surface instabilities and reconstructions at H=16T and H=21T. The measurement of quantum oscillations under pressure up to 2.5 GPa shows a smooth evolution of the Fermi surface with pressure from the ferromagnetic to the paramagnetic ground state.

Télécharger dans mon agenda