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Zusammenfassung:
We present evidence that at the superconducting transition temperature Tc of the Heavy-Fermion-Superconductors CeCu2Si2 and UBe13, the order parameter is formed only on part of the Fermi-surface. In UBe13, at least part of the Fermi-surface that does not participate in superconductivity at Tcsime0.9K develops an order parameter in the accessible temperature range, i.e. at 0.55K. The effect can be sharply enhanced by doping UBe13 with a few at% Th, leading to a second phase transition within the superconducting state. In CeCu2Si2, those parts of the Fermi-surface which do not participate in superconductivity down to the lowest temperatures carry a small effective mass. The influence of impurities on the Fermi-surface anisotropy is discussed.
