Probing Andreev edge state with shot noise

The existence of Andreev edge states (AESs) is one of the prerequisites to realize emergent excitations like Majorana fermion at the interface of normal or anomalous quantum Hall (QH) insulators and superconductor (SC). At a QH-SC interface the hybridized electron-hole states called AESs carry the current in the zero bias limit, whereas normal quasiparticles carry the current when the bias energy is higher than the superconducting gap. Here, we report on measurements of both electrical conductance and shot noise in a graphene QH and SC junction at integer filling ν=2. Remarkably, the Fano factor of the shot noise approaches to half when the bias energy is less than the superconducting gap, whereas it is close to zero above the superconducting gap. This is striking, given that, at the same time the electrical conductance remains at 2e²/h within and above the superconducting gap. These observations are in accord with our theoretical analysis based on non-equilibrium Green’s function (NEGF) calculation with disorders, pointing to complete de-phasing of interference of AESs along the QH-SC interface. Our results with unambiguous evidence of AESs at the QH-SC interface pave the way forward to explore the exotic topological excitations in QH-SC hybrids.