Small superconducting gap on part of the Fermi surface of YNi₂B₂C from the de Haas-van Alphen effect

The de Haas-van Alphen oscillation coming from a small electron pocket in the borocarbide superconductor YNi₂B₂C (H_c2 = 8.8 T) has been observed in the vortex state down to a surprisingly low field of 2 T. The oscillation amplitude is strongly suppressed in a field region immediately below H_c2 but recovers at lower fields. The observed amplitude at low fields is much larger than theoretical predictions based on a literature value of the superconducting gap. It is argued that the gap opening on this particular Fermi surface is much smaller than the gap on other parts of the Fermi surface.