Andreev reflection in graphene–superconductor junctions with line defects

We investigate Andreev reflection and valley-dependent transport in a graphene/line-defect/superconductor junction. Using a tight-binding model combined with the Bogoliubov–de Gennes formalism and a scattering matrix approach, we show that the line defect acts as a universal valley filter for both electrons and holes, independent of their band index. This universal behavior originates from the invariance of the pseudospin structure governing transmission across conduction and valence bands. Based on this property, we identify two distinct transport mechanisms associated with different Andreev reflection regimes. In the retro-Andreev reflection regime, where electrons and holes reside in the same band, the defect induces a double-filtering mechanism that enhances valley selectivity and produces strongly asymmetric angular distributions. In contrast, in the specular regime, the interband nature of Andreev reflection leads to a complementary filtering mechanism, which suppresses transport at large incident angles and restores angular symmetry. These features are quantitatively captured by an analytical equation for coherent multiple scattering, which accurately reproduces the zero-bias differential conductance. Our results establish a direct connection between microscopic valley filtering and macroscopic transport observables, demonstrating that differential conductance measurements provide a clear signature of both the Andreev reflection regime and the efficiency of valley filtering.