Fermi surface instabilities with and without magnetic transitions : study of URhGe, UPd2Al3, UCoGe and CeIrIn5

Orateur : Soutenance de Thèse de Adrien GOURGOUT

In this thesis, we have studied the interplay of Fermi surface instabilities, magnetic orders and quantum criticali ty in highly correlated electron systems. This presentation will be devoted to the cases of the ferromagnetic superconductors UCoGe and URhGe. In UCoGe, several successive anomalies were observed under magnetic field (along the easy magnetization c-axis) in resistivity, Hall effect and thermoelectric power, without any thermodynamic transition. The direct observation of quantum oscillations showed that these anomalies are related to topological changes of the Fermi surface, also known as Lifshitz transitions. In URhGe with the field applied along the hard magnetization b-axis, a drastic change in the Fermi surface at the spin reorientation field (HR = 11.75 T) has been observed through thermoelectric power measurements. Additionally, this study allows us to confirm the first order character of the transition and to locate precisely the tricritical point in this compound. This work appears as a pioneering work to understand the effect of polarization on the Fermi surface topology in highly correlated electron systems.

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