Magnetic quantum oscillations in borocarbide superconductors

We report systematic de Haas–van Alphen (dHvA) investigations in the normal and superconducting state of RNi₂B₂C (R = Y and Lu). The observed rich frequency spectrum of the dHvA signals results from a rather complex electronic band structure with different open and closed Fermi-surface sheets. From our data in combination with full-potential local-orbital calculations we are able to extract the angular-resolved mass-enhancement factors, lambda, for different bands. We find a strong anisotropy and band dependence of lambda, clearly reflecting the multiband character of the superconductivity in RNi₂B₂C. We further were able to resolve dHvA oscillations deep into the superconducting state. The observed additional damping of the dHvA amplitudes is much less than expected from most theories. This hints at a reduced or even zero superconducting gap for the detected Fermi surface