Thesis presented October 26, 2010

Abstract:
In this thesis two heavy fermion compounds have been studied under pressure. URu₂Si₂ has a mysterious ground state below T₀ = 17.5 K at ambient pressure. The order parameter has not been identified yet, which led to the name “hidden order” (HO). Below 1.5 K the system becomes additionally superconducting. With pressure, the ground state switches from the hidden order phase to an antiferromagnetic (AF) phase at a critical pressure and superconductivity is concomitant suppressed. Shubnikov-de Haas measurements under pressure show that the Fermi surface doesn't change between the two phases. The folding of the Fermi surface which happens in the high pressure AF phase therefore already happens in the HO phase, indicating a unit cell doubling. Our measurements of the complete angular dependence of the oscillation frequencies test the electronic structure and support new theoretical band structure calculations with rather itinerant 5f electrons. The second part of my research focuses on another uranium compound, UCoGe. It is one of the few known materials where superconductivity (T_sc = 0.6 K) coexists with ferromagnetism (T_Curie = 2.8 K). Precise studies of the pressure phase diagram by resistivity, ac calorimetry and ac susceptibility show that the ferromagnetic phase is suppressed at a pressure of about 1 GPa and the superconducting phase extends into the paramagnetic phase induced by pressure. Ferromagnetism is rapidly suppressed when superconductivity appears first on cooling. Thus, its pressure phase diagram is unique in the class of ferromagnetic superconductors.

Keywords:
heavy fermion systems, ferromagnetic superconductors, pressure phase diagram, quantum oscillations

On-line thesis.