Thesis presented October 27, 2006

Abstract:
As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of their coexistence in UGe₂ and URhGe has attracted a lot of interest. The mechanism to explain such a state has, however, not yet been fully elucidated. In these compounds superconductivity may be unconventional: Cooper pairs could be formed by electrons with parallel spins and magnetic fluctuations might be involved in the pairing mechanism.
This thesis deals with the study of the ferromagnet superconductor URhGe. URhGe becomes ferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to the c-axis of its orthorhombic crystal structure. For temperatures below 260 mK and fields lower than 2 Tesla, superconductivity was first observed in 2001. The thesis reports the discovery of a second pocket of superconductivity. This new pocket of superconductivity appears in single crystals at higher fields applied close to the b-axis, enveloping a sudden magnetic moment rotation transition at HR=12 Tesla. Detailed studies of the field induced metamagnetic transition and superconductivity are presented. This work suggests that there is a quantum critical point in the magnetic phase diagram. The role of the magnetic fluctuations emerging from this quantum critical point to the pairing mechanism giving superconductivity is discussed.

Keywords:
Ferromagnet superconductor, Metamagnetic transition, Quantum critical point, Unconventional superconductivity, Field induced superconductivity

On-line thesis.