SCALING BEHAVIOR OF POINT CONTACTS BETWEEN A TUNGSTEN TIP AND THE HEAVY-FERMION SUPERCONDUCTORS

We investigated metallic point contacts between a tungsten tip and the superconducting heavy-fermion compounds. On lowering the temperature to below T-c the contact resistance drops by delta R. Usually this cha nge in resistance is interpreted by the Andreev-reflection process at the normal-superconducting interface. However, we found that delta R d oes not scale with the inverse area of the orifice-as expected for And reev reflection-but with the inverse radius and the specific resistivi ty in the normal state. Such a behaviour is characteristic for Maxwell 's resistance being suppressed in the superconducting heavy-fermion ph ase. Additionally, the residual contact resistance at zero-bias is str ongly enhanced compared to the ballistic Sharvin contribution. We disc uss the possible microscopic origin for the reduced or even absent And reev-hole current and the enhanced residual resistance.