Additional sub-gap conductance enhancement in nanoscale Andreev point contact junctions

We demonstrate an enhancement in the sub-gap conductance in Nb/Al/Cu nanoscale pinhole junctions, significantly larger than the conductance doubling expected from the point contact Andreev reflection process. Such an enhancement is not observed in standard micrometre-driven NbTi/Cu wire-tipped junctions where the expected sub-gap conductance is regained. This additional sub-gap conductance arises due to regions of the pinhole junction being driven into the normal state due to current-induced pair breaking, causing the Andreev reflection process to occur at different interfaces within the contact structure as the bias is increased. This work demonstrates, again, that a great deal of care must be taken in interpreting Andreev reflection measurements in nanoscale junctions, in particular with regard to extracting accurate spin polarization