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Cross-over from retro to specular Andreev reflections in bilayer graphene

Ongoing experimental progress in the preparation of ultra-clean graphene/superconductor (SC) interfaces enabled the recent observation of specular interband Andreev reflections (AR) at bilayer graphene (BLG)/NbSe$_{2}$ van der Waals interfaces [Nature Physics 12, (2016)]. Motivated by this experiment we theoretically study the differential conductance across a BLG/SC interface at the continuous transition from high to ultra-low Fermi energies $E_{F}$ in BLG. Using the Bogoliubov-deGennes equations and the Blonder-Tinkham-Klapwijk formalism we derive analytical expressions for the differential conductance across the BLG/SC interface. We find a characteristic signature of the cross-over from intra-band retro- (high $E_{F}$) to inter-band specular (low $E_{F}$) ARs, that manifests itself in a strongly suppressed interfacial conductance when the excitation energy $|\varepsilon |=|E_{F}|<Δ$ (the SC gap). The sharpness of these conductance dips is strongly dependent on the size of the potential step at the BLG/SC interface $U_{0}$.