QDev Seminar: Dimitri Efetov – University of Copenhagen

QDev Seminar: Dimitri Efetov

Inducing Superconductivity into Graphene :

Graphene holds one last, not yet experimentally discovered prediction, namely exhibiting intrinsic superconductivity. With a vanishingly small Fermi Surface at the Dirac Point, graphene is a semi-metal with very weak electronic interactions. Though, if it is tuned into the metallic regime, where the size of the Fermi Surface becomes comparable to the size of the Brillouin Zone, the DOS becomes sizeable and electronic interactions may be exponentially enhanced resulting in predictions of competing correlated ground states such as superconductivity, magnetism, CDW etc. Following this route, we report on the creation of “metallic” graphene via electrostatic doping techniques based on electrolytic gates. Due to graphenes’ surface only properties, we are capable of inducing record high carrier densities of n > 1014 cm-2 (εf  > 1 eV) into the chemically inert graphene. In this regime we observe fundamentally altered electron-phonon interactions, show the existence of high energy sub-bands in bilayer graphene and demonstrate that this technique can also be used to create graphene derived species with strongly renormalized properties, such as graphite intercalation compounds.

A totally different approach of inducing superconductivity into graphene is by bringing it into proximity with a superconductor. Such devices would allow to study the proximity effect in the ballistic 2D limit, where predictions go as far as specular Andreev Reflections, the formation of Andreev Edge States in strong magnetic fields and the possible formation of Majorana zero modes. Here we present a new route of fabrication of such devices made entirely out of cleanly stacked layered van der Vaals materials BN/Graphene/NbSe2. The formidable electric contact between the type-II superconductor NbSe2 and the high mobility BN/graphene Hallbar allows to perform Andreev Reflection spectroscopy of the fully developed Quantum Hall states. Here, we observe a clear enhancement of the Andreev Reflection probability when Cooper Pairs are injected into the incompressible Quantum Hall states. This finding can be explained as the result of the chiral nature of the topological edge states and the absence of back-scattering. We furthermore tie these finding with the observation of renormalized values of the Quantum Hall plateaus below the upper critical field of NbSe2 of Hc2 = 4T.