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.

Further, we carried out shot noise measurements in an edge contacted BLG - Niobium superconductor junction at zero magnetic field as well as QH regime. At the Dirac point we observe Fano factor ~1/3 above the superconducting gap and transition to an enhanced Fano factor ~0.5 below the superconducting gap. By changing the carrier density, we find a continuous reduction of Fano factor for both types of carriers; however, the enhancement of Fano factor within the superconducting gap by a factor of ~1.5 is always preserved. The enhancement of shot noise is also observed in the QH regime, where the current is carried by the edge state, below the critical magnetic field and within the superconducting gap. These observations clearly demonstrate the enhanced charge transport at the BLG - superconductor interface. In a most recent effort, we tried to improve the understanding of AR at QH - superconductor junction by using a high B_C superconductor Molybdenum Rhenium (MoRe) as the superconducting contact to hBN encapsulated graphene. We observe the conductance at the QH plateaus to be very close to the value expected for graphene QH with normal contacts. By measuring shot noise, we observe Fano factor close to half when the bias energy is less than the superconducting gap. The observation of close to half Poissonian value of shot noise together with the observed conductance in QH plateaus uniquely demonstrate the Andreev edge state (AES) at the QH - superconductor interface and also shed light on the underlying phase mixing mechanism.