Manipulating quantum Hall edge channels in graphene through Scanning Gate Microscopy

We show evidence of the backscattering of quantum Hall edge channels in a narrow graphene Hall bar, induced by the gating effect of the conducting tip of a Scanning Gate Microscope, which we can position with nanometer precision. We show full control over the edge channels and are able, due to the spatial variation of the tip potential, to separate co-propagating edge channels in the Hall bar, creating junctions between regions of different charge carrier density, that have not been observed in devices based on top- or split-gates. The solution of the corresponding quantum scattering problem is presented to substantiate these results, and possible follow-up experiments are discussed.Comment: 10 pages, 12 figure

Electrostatic field-driven supercurrent suppression in ionic-gated metallic Josephson nanotransistors

Recent experiments have shown the possibility to tune the electron transport properties of metallic nanosized superconductors through a gate voltage. These results renewed the longstanding debate on the interaction between intense electrostatic fields and superconductivity. Indeed, different works suggested competing mechanisms as the cause of the effect: unconventional electric field-effect or quasiparticle injection. By realizing ionic-gated Josephson field-effect nanotransistors (IJoFETs), we provide the conclusive evidence of electrostatic field-driven control of the supercurrent in metallic nanosized superconductors. Our Nb IJoFETs show bipolar giant suppression of the superconducting critical current up to $\sim45\%$ with negligible variation of both the critical temperature and the normal-state resistance, in a setup where both overheating and charge injection are impossible. The microscopic explanation of these results calls upon a novel theory able to describe the non-trivial interaction of static electric fields with conventional superconductivity.Comment: 9 pages, 6 figure

Unveiling mechanisms of electric field effects on superconductors by a magnetic field response

We demonstrate that superconducting aluminium nanobridges can be driven into a state with complete suppression of the critical supercurrent via electrostatic gating. Probing both in- and out-of-plane magnetic field responses in the presence of electrostatic gating can unveil the mechanisms that primarily cause the superconducting electric field effects. Remarkably, we find that a magnetic field, independently of its orientation, has only a weak influence on the critical electric field that identifies the transition from the superconducting state to a phase with vanishing critical supercurrent. This observation points to the absence of a direct coupling between the electric field and the amplitude of the superconducting order parameter or 2π-phase slips via vortex generation. The magnetic field effect observed in the presence of electrostatic gating is described within a microscopic model where a spatially uniform interband π phase is stabilized by the electric field. Such an intrinsic superconducting phase rearrangement can account for the suppression of the supercurrent, as well as for the weak dependence of the critical magnetic fields on the electric field