Anisotropic many-body effects in nonmagnetic borocarbides

We present de Haas{van Alphen measurements of the nonmagnetic borocarbide superconductors LuNi2B2C and YNi2B2C. The measurements have been performed by use of the torque method in high magnetic fields up to 32 T and at temperatures down to 50 mK. The Fermi-surface cross sections are extracted from the magnetic quantum oscillations in the normal state. In accordance with previous investigations we find a complex band structure with dfferent open and closed Fermi-surface sheets. From the temperature dependence of the oscillations amplitude the effective mass of the single bands can be determined. We observe sizeable and anisotropic enhancements of the effective masses compared to full-potential-localorbital calculations which is due to anisotropic many-body interactions.