de Haas-van Alphen study in the superconducting state of YNi2B2C

In the borocarbide superconductor Y${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$C the de Haas-van Alphen (dHvA) oscillations have been observed continuously at the upper critical field $B_{c2\mathrm{}}$ for the transition from the normal into the superconducting state. In the normal state above $B_{c2\mathrm{}}=10.5\mathrm{}$ T six different frequencies were found in magnetic fields up to 23 T and for $\Theta =14.4\mathrm{}°$. The smallest orbit with $F_{\alpha }=511\mathrm{}$ T is present down to about 4 T, but with an additional attenuation of the dHvA signal in the superconducting state relative to an extrapolation of the standard Lifshitz-Kosevich formula. The observation of magnetoquantum oscillations over such a wide field range and very close to $B_{c2\mathrm{}}$ enables a detailed discussion of the damping in the context of recent theories of these oscillations in the superconducting state.