Fig. 3

Numerical calculations of dI/dV and the superconducting proximity effect for the Nb/Cd₃As₂ hybrid structure. a A schematic sketch showing the Andreev reflections and the superconducting proximity effect in the Nb/Cd₃As₂ junction. When superconducting Nb layer is on the top of Cd₃As₂ with a thickness of a, proximity-induced cooper pairs exist in both bulk states and surface states. The red and blue spheres represent the electrons and holes with opposite spin directions. The dashed line displays the Andreev reflection process. b, c Numerical calculations of the normalized Andreev reflection amplitude in Nb/Cd₃As₂ junction: b Andreev reflection amplitude for Fermi level lying at the Dirac points (E_F = 0 meV). In this case, the bulk density of states is negligible and Fermi-arc states dominate the Andreev reflections. Clearly, only a flat conductance plateau is found in the dI/dV spectrum; c Andreev reflection amplitude for Fermi level lying high above the Dirac points (E_F = 70 meV), where the bulk density of states cannot be ignored. A broad zero-bias peak emerges on top of the flat plateau in the dI/dV spectrum. d Spectral density on the topmost surface of Cd₃As₂ for E_F = 70 meV, with the contribution of superconducting Nb integrated out and projected on the surface of Cd₃As₂. The color bar shows the local density of states on the top Cd₃As₂ surface on a logarithmic scale. In the color scale, red (blue) color indicates high (low) density. Evidently, two pairing gaps are induced on the surface. The larger proximity gap Δ_s originates from Fermi-arc states (with high local density on the surface). Notably, 2Δ_s corresponds to the width of the flat conductance plateau due to surface Fermi arcs in (b) and (c). In contrast, the smaller gap Δ_b originates from bulk states (with low density on the surface). Consistently, 2Δ_b roughly measures the width of the zero-bias peak due to bulk states in (c)