Electronic band structure and superconducting gap of YNi₂B₂C and LuNi₂B₂C

We present de Haas-van Alphen (dHvA) measurements of the nonmagnetic borocarbides YNi2B2C and LuNi2B2C. The measurements were carried out on high-quality crystals by the torque method in magnetic fields up to 33 T. The Fermi-surface topology has been measured in the normal state and compared with band-structure calculations. In the superconducting state below the upper critical field, Bc2, the dHvA-signal is strongly damped due to the opening of the superconducting gap, which allows to study the evolution of the superconducting gap. In order to investigate the symmetry of the order parameter we performed angle-dependent measurements
of this damping. We compare results from two differently grown kinds of crystals. The crystals produced by a zone-melting method show an abrupt vanishing of the dHvA signal at Bc2. In contrast, flux-grown crystals display a broader phase transition combined with a continuous decrease of the oscillation amplitude. The observed anisotrop of the dHvA-signal damping points to an anisotropic energygap in these materials.