Tuning Andreev reflection at Graphene superconductor junction from zero field to quantum Hall regime

Abstract

Andreev reflection (AR) is the underlying phenomena that determines the quasiparticle dynamics at the junction of a superconductor with any non-superconducting material, which in turn determines the transport properties of the junction. The tunability of Fermi energy in the two-dimensional (2D) Dirac semimetal graphene by using an electrostatic gate opened up possibility to realize some new and intriguingly different kind of AR. All of my thesis work is on understanding different kind of AR at graphene superconductor interfaces, either by conductance measurements or by combining this with shot noise measurements. In conventional Normal metal - superconductor junctions the AR is always retro type. Interplay of superconductivity and relativistic dynamics gives rise to specular type AR (SAR) at graphene - superconductor junction. We carried out the transport measurements in van der Waals junctions of graphene and the quasi 2D niobium diselenide (NbSe_2) superconductor. We investigate the AR near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy, which reveal the transition from retro to a non-retro type AR dominated transport near the Dirac point. However, the observation of SAR was restricted due to the large Fermi energy broadening in the graphene. The physics of AR is predicted to alter dramatically in the quantum Hall (QH) regime, where electron transport occurs primarily through the chiral edge states, which themselves are topologically robust manifestations of the Landau levels in the interior of the sample. In another interesting work, we observe signature of AR at the junction of QH edge state in graphene with the NbSe_2 superconductor. Our principal finding is the observation of an anomalous finite temperature conductance peak located precisely at the charge neutrality point, providing a definitive evidence for inter-Landau-level Andreev reflection in a QH system... newline