Thesis presented December 18, 2013

Abstract:
Thermal conductivity measurements have been performed at low temperatures and under field in the superconducting ferromagnet UCoGe and in the weak antiferromagnet YbRh₂Si₂. In both systems, the magnetic fluctuations have an important role in their properties, and it appeared that they contribute as a heat channel, seen by thermal conductivity at low temperatures. In UCoGe, the extra contribution due to the magnetic fluctuations have the same field dependence as the one measured by NMR, and, unexpectedly, a new heat channel appears at very low temperatures. Furthermore, thermal conductivity measurements in the superconducting state have confirmed the multigap superconductivity of UCoGe. XMCD measurements have also been performed in UCoGe. The electronic states are very sensitive to the magnetic field, which gives another evidence for its small Fermi energy. In YbRh₂Si₂, the very low temperature thermal conductivity measurements have shown that an extra contribution appears at very low temperature, which avoids to conclude definitively about the violation or the validation of the Wiedemann-Franz law at the quantum critical point, even if the results can be interpreted supposing its validation.

Keywords:
Unconventionnal superconductivity, Ferromagnetism, Thermal conductivity, Very low temperature, UCoGe, YbRh2Si2

On-line thesis.