The Andreev reflection at the graphene quantum Hall state and superconductor

Superconductivity and quantum Hall effect (QHE) are distinct states of matter arising in complementary physical conditions. Recent theoretical developments suggest that coupling at the interface of a QHE edge state with a superconductor (SC) can provide a fertile ground for realizing exotic topological excitations such as non-abelian Majorana, parafermion or Fibbonacci particles. As a step toward that goal, we demonstrate in this letter Andreev reflection (AR) at the junction of a QHE state in a single layer graphene (SLG) and a two dimensional (2D) NbSe2 superconductor. This system allows us to study Andreev effect up to magnetic fields as high as B = 10T when graphene consists of well resolved Landau levels (LL). Our principal result is the observation of an anomalous finitetemperature peak located precisely at the Dirac point, which provides a definitive evidence for inter-Landau level Andreev reflection. We also find other characteristic signatures of AR, such as enhanced conductance inside the superconducting gap and oscillations in the conductance as a function of the magnetic field or the chemical potential. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in coupling SC and QH state.