Fig. 1: Device under study and tunnelling spectroscopy of sub-gap states.
From: Microwave-induced conductance replicas in hybrid Josephson junctions without Floquet—Andreev states
a Schematic representation of density of states (DOS) and tunnelling spectroscopy into a superconducting–semiconducting–superconducting (SNS) junction using a superconducting probe (top). Andreev bound states (ABSs) are present in the DOS of the SNS junction at energies ± EA. A current (yellow) flows when the source–drain voltage VSD aligns occupied (red) to unoccupied (grey) states. In a Floquet–Andreev scenario (middle), replicas of Andreev peaks shifted by the photon energy hf emerge in the DOS of the SNS junctions, giving rise to additional tunnelling resonances. In a photon assisted tunnelling scenario (bottom), absorption of a photon (green) induces tunnelling into an ABSs for VSD = EA − hf. Floquet-Andreev modes are represented as replicas of Andreev peaks shifted by hf. b Schematic representation of differential tunnelling conductance G measured in the absence (blue) and presence (green) of microwave irradiation. c False-coloured electron micrograph of a device identical to that under study, composed of InAs (pink) and Al (blue) and controlled via electrostatic gates (yellow). Gate voltages Vα (α ∈ {T, TG, Probe}), bias voltages (VSD, VAC) and currents (IDC), and measured voltages (V1, V2) and currents (I1) are indicated. d Zoom-in of the tunnelling junction before gate deposition. The gates controlling the tunnelling barrier transparency are drawn in yellow. e Differential conductance G of the tunnelling probe as a function of bias VSD and gate voltage VT. Signatures of the supercurrent (SC, light green), multiple Andreev reflections (MAR, light blue) and ABSs (red) are indicated with dashed arrows. The transport gap is indicated as 2Δ/e (white arrow). f Tunnelling spectroscopy of sub-gap states at VT = − 2.11V, as a function of perpendicular magnetic field B⊥. Conductance features labelled with colours defined in e.
