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 obser ved in the vortex state down to a surprisingly low field of 2 T. The o scillation amplitude is strongly suppressed in a field region immediat ely below H-c2 but recovers at lower fields. The observed amplitude at low fields is much larger than theoretical predictions based on a lit erature value of the superconducting gap. It is argued that the gap op ening on this particular Fermi surface is much smaller than the yap on other parts of the Fermi surface.