de Haas-van Alphen investigations on the nonmagnetic superconductors LuNi₂B₂C and YNi₂B₂C

We present de Haas-van Alphen (dHvA) investigations on the nonmagnetic borocarbide
superconductors LuNi2B₂C and YNi₂B₂C. The measurements were carried out by use of a cantilevertorque method on a rotator set up in high magnetic fields up to 32 T and at low temperatures down to 50 mK. From the normal-state oscillations we determine the electronic band structure of the material. From comparison with full-potential local-orbital calculations we assigned the measured dHvA frequencies to the different bands identified the expected spherical, cubic and ‘cushion’-like closed Fermi surfaces. Additionally, we found frequencies that probably belong to a complicated multipleconnected Fermi-surface sheet. Further, we performed temperature-dependent measurements that allowed the extraction of the effective band masses of the different Fermi-surface sheets. We find mass enhancements in comparison to the calculated values that are due to electron-phonon coupling.
Finally, we are able to determine the angular dependences of the electron-phonon coupling for the different Fermi-surface sheets.
Below the upper critical field, Bc2, an additional damping of the oscillation amplitude appears. This decrease might be related to the increase of the superconducting gap parameter which could be calculated from quantitative measurements of this effect. However, the analysis is hampered by additional influences originating form changes in the flux-line lattice that appear close to the superconducting phase transition. Therefore, we compare two differently grown crystals. The one grown by a zone-melting method shows a rather abrupt vanishing of the oscillation amplitude in the superconducting state. In contrast, for the flux-grown crystals we observe a smooth damping
combined with a broad phase transition.