Transparent Contacts between Quantum Anomalous Hall Insulators and Superconductors*

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor a chiral topological superconducting phase, the elementary excitations of which (i.e. chiral Majorana fermions) upon non-Abelian braiding operations can form topological quantum qubits. Here, we fabricated the QAH/Nb hybrid heterostructures and first studied their two-terminal conductance. We found that the two-terminal conductance constantly shows a half-quantized value in the entire range of the magnetic field where the magnetization is well-aligned. Next, we studied the contact transparency between the QAH and Nb films. When the QAH layer is tuned to the metallic regime by gating, we observed Andreev reflections, i.e., a large enhancement of the resistance when a DC bias voltage across the magnetic TI/Nb junction is increased above the Nb superconducting gap. This observation indicates a transparent interface between the QAH and Nb layers. Our study provides a more comprehensive understanding of the relation between the superconducting proximity effect and the observation of the half quantized two-terminal conductance in the QAH/Nb hybrid structure.