Andreev reflection in Graphene

Andreev reflection (AR) is the underlying quantum phenomena by which the current flows from a normal region into a superconductor at the normal-superconductorjunction. This process is quite different for relativistic electrons in graphene.Theoretically, it has been predicted for both retro as well as specular type of AR in graphene. Despite extensive search for about a decade, specular Andreev reflection isonly recently realized in bilayer graphene-superconductor interface. In this talk the evolution from the typical retro type Andreev reflection to the unique specular Andreev reflection in a van der Walls interface of single layer graphene and NbSe2superconductor will be discussed. We find that the conductance becomes suppressedas we pass through the Dirac cone via tuning the Fermi level and bias energy. The suppression indicates the blockage of Andreev reflection beyond a critical angle of the incident electron with respect to the normal between the single layer graphene and the superconductor junction. In the second part of talk I will discuss about the AR inquantum Hall edge state of graphene. The coupling of a quantum Hall state and a superconductor has been proposed as a novel route for creating even more exotictopological entities, such as non-abelian Majorana, parafermion or Fibbonacci particles.As a step toward that goal, we demonstrate AR at the junction of a QH state in a singlelayer graphene and a two dimensional NbSe2 superconductor. We see characteristic signatures of Andreev reflection, such as: enhanced conductance inside the superconducting gap; oscillations in the conductance as a function of the magnetic field or the back gate voltage; and also an anomalous finite-temperature peak located precisely at the Dirac point, which provides a compelling evidence for inter-Landau level Andreev reflection.