Thesis presented February 11, 2011

Abstract:
The subject of this thesis is the analyze of the superconducting upper critical field (H_c2) and the interaction between superconductivity and quantum critical points (QCP), for the compounds CeCoIn₅, URhGe and UCoGe. In CeCoIn₅, study by mean of resistivity of the Fermi liquid domain allows us to localize precisely the QCP at ambient pressure. This analyze rule out the previously suggested pinning of H_c2(0) at the QCP. In a second part, the evolution of H_c2 under pressure is analyzed. The superconducting dome is unconventional in this compound with two characteristic pressures: at 1.6GPa, the superconducting transition temperature is maximum but it is at 0.4GPa that physical properties (maximum of H_c2(0), maximum of the initial slope dHc2/dT, maximum of the specific heat jump DC/C,... ) suggest a QCP. We explain this antagonism with pair-breaking effects in the proximity of the QCP. With these two experiments, we suggest a new phase diagram for CeCoIn₅. In a third part, measurements of thermal conductivity on URhGe and UCoGe are presented. We obtained the bulk superconducting phase transition and confirmed the unusual curvature of the slope dH_c2/dT observed by resistivity. The temperatures and fields dependence of thermal conductivity allow us to identify a non-electronic contribution for heat transport down to the lowest temperature (50mK) and probably associated with magnon or longitudinal fluctuations. We also identified two different domains in the superconducting region, These domains are compatible with a two bands model for superconductivity. Thermopower measurements on UCoGe reveal a strong anisotropy to current direction and several anomaly under field applied in the b direction. We suggest a Lifshitz transition to explain our observations in these two compounds.

Keywords:
Heavy fermions, Thermal conductivity, Superconductivity, CeCoIn₅

On-line thesis.