High field termination of a Cooper-pair insulator

We conducted a systematic study of the disorder dependence of the termination of superconductivity, at high magnetic fields (B), of amorphous indium oxide films. Our lower disorder films show conventional behavior where superconductivity is terminated with a transition to a metallic state at a well-defined critical field, Bc2. Our higher disorder samples undergo a B-induced transition into a strongly insulating state, which terminates at higher Bs forming an insulating peak. We demonstrate that the B terminating this peak coincides with Bc2 of the lower disorder samples. Additionally we show that, beyond this field, these samples enter a different insulating state in which the magnetic field dependence of the resistance is weak. These results provide crucial evidence for the importance of Cooper-pairing in the insulating peak regime.

Ripples and Charge Puddles in Graphene on a Metallic Substrate

Graphene on a dielectric substrate exhibits spatial doping inhomogeneities, forming electron-hole puddles. Understanding and controlling the latter is of crucial importance for unraveling many of graphenes fundamental properties at the Dirac point. Here we show the coexistence and correlation of charge puddles and topographic ripples in graphene decoupled from the metallic substrate it was grown on. The analysis of interferences of Dirac fermion-like electrons yields a linear dispersion relation, indicating that graphene on a metal can recover its intrinsic electronic properties.

Transport through graphene on SrTiO3

We report transport measurements through graphene on SrTiO3 substrates as a function of magnetic field B, carrier density n, and temperature T. The large dielectric constant of SrTiO3 screens very effectively long-range electron-electron interactions and potential fluctuations, making Dirac electrons in graphene virtually non-interacting. The absence of interactions results in a unexpected behavior of the longitudinal resistance in the N=0 Landau level, and in a large suppression of the transport gap in nano-ribbons. The bulk transport properties of graphene at B=0T, on the contrary, are completely unaffected by the substrate dielectric constant.