Andreev Reflection Enhanced Aharonov-Bohm Oscillations in Conductance of a Mesoscopic Normal Metal Ring Connected to Normal and Superconducting Leads

We investigate the conductance through an Aharonov-Bohm normal metal ring connected to normal and superconducting leads. It is shown that, due to the Andreev reflection at the normal superconducting phase boundary, the conductance maxima could be enhanced to be twice as large as that for the ring connected with normal conducting leads. The voltage bias may suppress this conductance enhancement. However, the periodicity of the oscillating conductance in flux is not altered by the presence of the superconducting contact. The discussion is also extended to include the impurity scattering effect and the multichannel case. Quite interestingly, the period halving could be induced by the impurity scattering even in one-dimensional normal superconducting hybrid structures.