Thesis presented January 17, 2006

Abstract:
The thesis illustrates the combined influences of gap and crystal anisotropies on superconducting properties under a magnetic field. In order to describe the multiband superconductivity in MgB₂, we derive the Ginzburg-Landau functional for a two-gap superconductor from a weak coupling BCS model. The interaction between the two condensates is thus described by a unique Josephson-type coupling. The two-gap theory then enables to explain the curvature and the anisotropy of the upper critical field, as well as the 30°-rotation for the vortex lattice which follows an increase of the magnetic field applied along the c-axis. Besides we investigate the vortex lattice geometry in the heavy fermion PrOs₄Sb₁₂. When taking into account non local corrections for an s-wave T_h-tetrahedral superconductor, we can explain the observed deformation of the lattice by the crystal symmetry effect. Ab initio results of the band structures confirm quantitatively our analysis.

Keywords:
Multiband, multigap superconductivity, vortex lattice, upper critical field, Ginzburg-Landau theory, non local corrections, PrOs₄Sb₁₂, T_h-tetrahedral symmetry

On-line thesis.