Thesis presented January 09, 2017

Abstract:
This thesis is concentrated on the ferromagnetic superconductors UCoGe and URhGe andon the hidden order state in URu₂Si₂. In the first part the pressure temperature phase diagram of UCoGe was studied up to 10.5 GPa. Ferromagnetism vanishes at the critical pressure pc≈1 GPa. Unconventional superconductivity and non Fermi liquid behavior can be observed in a broad pressure range around p_c The superconducting upper critical field properties were explained by the suppression of the magnetic fluctuations under field. In the second part the Fermi surfaces of UCoGe and URhGe were investigated by quantum oscillations. In UCoGe four Fermi surface pockets were observed. Under magnetic field successive Lifshitz transitions of the Fermi surface have been detected. The observed Fermi surface pockets in UCoGe evolve smoothly with pressure up to 2.5 GPa and do not show any Fermi surface reconstruction at the critical pressure p_c. In URhGe, three heavy Fermi surface pockets were detected by quantum oscillations. In the last part the quantum oscillation study in the hidden order state of URu₂Si₂ shows a strong g factor anisotropy for two Fermi surface pockets, which is compared to the macroscopic g factor anisotropy extractedfrom the upper critical field study.

Keywords:
Superconductivity, Magnetism, Correlated electrons, Heavy fermions, Quantum oscillations, Resistivity

On-line thesis.