  A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor
is predicted to harbor a topological superconducting phase, the elementary
excitations of which (i.e. Majorana fermions) can form topological qubits upon
non-Abelian braiding operations. A recent transport experiment interprets the
half-quantized two-terminal conductance plateau as the presence of chiral
Majorana fermions in a millimeter-size QAH-Nb hybrid structure. However, there
are concerns about this interpretation because non-Majorana mechanisms can also
generate similar signatures, especially in a disordered QAH system. Here, we
fabricated QAH-Nb hybrid structures and studied the QAH-Nb contact transparency
and its effect on the corresponding two-terminal conductance. When the QAH film
is tuned to the metallic regime by electric gating, we observed a sharp
zero-bias enhancement in the differential conductance, up to 80% at zero
magnetic field. This large enhancement suggests high probability of Andreev
reflection and transparent interface between the magnetic topological insulator
(TI) and Nb layers. When the magnetic TI film is in the QAH state with
well-aligned magnetization, we found that the two-terminal conductance is
always half-quantized. Our experiment provides a comprehensive understanding of
the superconducting proximity effect observed in QAH-superconductor hybrid
structures and shows that the half-quantized conductance plateau is unlikely to
be induced by chiral Majorana fermions.
