Quantum oscillations in the superconducting state of LuNi₂B₂C

We have studied the de Haas-van Alphen (dHvA) effect of the borocarbide superconductor LuNi₂B₂C both in the normal and in the superconducting state by use of the field-modulation method at high magnetic fields up to 15 T and at low temperatures down to 0.5 K. Starting in the normal state we were able to observe dHvA oscillations deep inside the superconducting state with only a minor additional damping of the oscillation amplitudes. Only close to the upper critical field we find a slightly stronger damping. However, in this region we also observe a strong peak effect which hampers the analysis and complicates the interpretation. We compare our results with recent theories and discuss the possibilities of determining the magnetic-field-dependent gap for different bands with this method. Nevertheless, the apparent correlation between the occurence of the peak effect and the extra damping might be attributet to suppression of vortex-lattice order associated with the enhanced flux-line pinning in this region.