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A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductoris predicted to harbor a topological superconducting phase, the elementaryexcitations of which (i.e. Majorana fermions) can form topological qubits uponnon-Abelian braiding operations. A recent transport experiment interprets thehalf-quantized two-terminal conductance plateau as the presence of chiralMajorana fermions in a millimeter-size QAH-Nb hybrid structure. However, thereare concerns about this interpretation because non-Majorana mechanisms can alsogenerate similar signatures, especially in a disordered QAH system. Here, wefabricated QAH-Nb hybrid structures and studied the QAH-Nb contact transparencyand its effect on the corresponding two-terminal conductance. When the QAH filmis tuned to the metallic regime by electric gating, we observed a sharpzero-bias enhancement in the differential conductance, up to 80% at zeromagnetic field. This large enhancement suggests high probability of Andreevreflection and transparent interface between the magnetic topological insulator(TI) and Nb layers. When the magnetic TI film is in the QAH state withwell-aligned magnetization, we found that the two-terminal conductance isalways half-quantized. Our experiment provides a comprehensive understanding ofthe superconducting proximity effect observed in QAH-superconductor hybridstructures and shows that the half-quantized conductance plateau is unlikely tobe induced by chiral Majorana fermions.

Non-Majorana Origin of the Half-Quantized Conductance Plateau in Quantum Anomalous Hall Insulator and Superconductor Hybrid Structures