Unconventional Andreev Reflection at Graphene-Superconductor Junctions

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 semi-metal graphene by using an electrostatic gate opened up possibility to realize some new and intriguingly different kind of AR. One such unique theoretically predicted process is specular AR, which appears due to the interplay of relativistic dynamics of Dirac quasiparticles in graphene with superconductivity. Furthermore, graphene-superconductor junctions provide an ideal platform to study the Andreev dynamics in quantum Hall regime, which was not possible in conventional 2D electron gas systems due to technical difficulties in connecting them with a superconductor. Graphene QH-superconductor junctions can host exotic topological excitations such as Majorana Fermions, making it a potential candidate for utilization in future quantum computing. We combine conventional conductance measurements with shot noise to probe these interesting Andreev physics in graphene – superconductor junctions.

Related Publications:

  1. Inter-Landau-level Andreev Reflection at the Dirac Point in a Graphene Quantum Hall State Coupled to a NbSe2 Superconductor, MR Sahu et al., Phys. Rev. Lett. 121, 086809 (2018).
  2. Enhanced specular Andreev reflection in bilayer graphene, A Soori, MR Sahu et al., Phys. Rev. B 98, 075301 (2018).
  3. Andreev reflection near the Dirac point at the graphene – NbSe2 junction, MR Sahu et al., Phys. Rev. B 94, 235451 (2016).

Non-Retro Andreev reflection: Despite extensive search for about a decade, specular Andreev reflection (SAR) has only recently been realized in the bilayer graphene-superconductor interface. The experimental observation of retro to specular Andreev reflection is not only fundamentally important, but also has potential application in quantum computing, etc. We have carried out the transport measurements at the van der Waals interface of single-layer graphene and NbSe2 superconductor. We investigate the Andreev reflection near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy. We find that the normalized conductance becomes suppressed as we pass through the Dirac cone, which manifests the transition from retro to non-retro-type Andreev reflection. 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. However, the observation of SAR was restricted due to the finite Fermi-energy broadening. The results are compared with a theoretical model of the corresponding setup.

(Top-left) AR for a semiconductor-superconductor junction. bottom left: schematic of the corresponding retro Andreev reflection process. top right: In case of graphene, another type of AR appears when the Fermi energy is very close to Dirac point. bottom right: schematic of corresponding specular Andreev reflection process. (Top-right) Optical image of graphene on hBN; right: the device with NbSe2. (Bottom-left) 2D colormap of normalized differential conductance as a function of VBG and VSD. (Bottom-right) Schematic of non-retro type Andreev reflection for the critical angle and resultant normalized conductance has minimum around the Dirac point due to Non-retro type Andreev reflection, which is observed in the 2D colormap.

Inter-Landau level Andreev reflection: Superconductivity and the quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of the quantum Hall effect with a superconductor can provide fertile ground for realizing exotic topological excitations such as non-Abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we study the observation of Andreev reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two-dimensional niobium di-selenide (NbSe2) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point, providing a definitive evidence for inter-Landau-level Andreev reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles in future.

(Top-left) Semi-classical picture of AR at the interface of QH edge state and superconductor based on skipping orbit. The electron and hole orbits have the same chirality for intra-band process. Bottom: optical image of graphene on hBN; the device with NbSe2. (Bottom-left) Schematic of the measurement setup. For the four probe measurements, current is applied between contacts S and D, and voltage is measured between contacts VA and VB. (Top-middle) Quantum Hall response with the superconducting contact. (Bottom-middle) differential conductance as a function of bias energy; showing the signature of co-existence of quantum Hall effect and superconductivity. (Top-right) Anomalous peak at zeroth filling below superconducting transition. Schematic of the inter-Landau Andreev refection process.