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 change in resistance is interpreted by the Andreev-reflection process at the normal-superconducting interface. However, we found that delta R does not scale with the inverse area of the orifice-as expected for Andreev reflection-but with the inverse radius and the specific resistivity in the normal state. Such a behaviour is characteristic for Maxwell's resistance being suppressed in the superconducting heavy-fermion phase. Additionally, the residual contact resistance at zero-bias is strongly enhanced compared to the ballistic Sharvin contribution. We discuss the possible microscopic origin for the reduced or even absent Andreev-hole current and the enhanced residual resistance.