Magnetic quantum oscillations in the normal and superconducting state of YNi₂B₂C

The de Haas-van Alphen (dHvA) effect, or quantum oscillations of the magnetization, is the most direct method to study Fermi-surface properties in metals.It is remarkable, that these magnetic quantum oscillations persist deep into the vortex state of many type-II superconductors. The damping of the oscillation amplitude below the upper critical field can be related to the magnitude of the superconducting gap. For YNi₂B₂C, however, quite controversial results for the dHvA signal in the superconducting state have been reported. We will present dHvA measurements of YNi₂B₂C single crystals prepared by different methods. The flux-grown crystals exhibit dHvA oscillation in the superconducting state down to 3 T as reported in literature. However, we observed an unexpectedly sudden vanishing of the dHvA signal in the mixed state for crystals grown by a zone-melting method. The very quick disapppearance of the oscillating signal below B _c2 suggests an unexpected fast opening of a large superconducting gap. At high magnetic fields six different dHvA frequencies could be detected.