Fig. 5: Hard gap and 2e-periodic Coulomb blockade in InSANe InSb–Al hybrid nanowire devices.

a False-color scanning electron microscopy (SEM) of a typical normal–nanowire–superconductor (N–NW–S) device for illustration purpose, with a schematic of the device cross-section b at the position indicated by the black arrow in a. The InSb nanowire (red) is covered by a 7 nm thick Al layer (green, covered by the etch mask). Part of the Al film on the nanowire is selectively etched before normal metal electrodes deposition (yellow, Cr/Au), and gate-tuneable tunnel barrier. The tunnel- and super-gates are Ti/Au (blue and purple, respectively), deposited on top and separated from the wire by a Si_xN_y dielectric layer as seen in the cross-section schematic on the right. Scale bar is 500 nm. Fridge temperature is 20 mK. c Differential conductance (dI/dV) as a function of bias voltage (V) and super-gate voltage in the tunneling regime, resolving a hard superconducting gap (Δ~250 μeV) with the line-cuts (both linear and logarithmic scale) shown in d at gate voltage indicated by black bar in c. The sub-gap/above-gap conductance suppression reaches two orders of magnitude. e: False-color SEM of the superconducting island device. The Al island on the nanowire is ~1 µm long, with a top plunger gate (purple) to tune the electron density, and two tunnel-gates (blue) to control the tunnel coupling to the two leads. Scale bar is 500 nm. f Differential conductance of the island device as a function of bias and plunger-gate voltage resolving the Coulomb blockade diamonds. The horizontal line-cut at zero bias (black curve) shows 2e-periodic Coulomb oscillations where each peak corresponds to adding/removing two electrons (one Cooper pair), suggesting negligible quasi-particle poisoning. At higher bias voltage where quasi-particle can be excited, the Coulomb oscillations become 1e-periodic. g Magnetic field dependence of the Coulomb oscillations at zero bias voltage with the field direction along the wire. The 2e-peaks split into 1e-peaks at ~0.3 Tesla, indicating a sub-gap state crosses zero energy. h Even (S_e red) and odd (S_o blue) peak spacing extracted from g, with error bars indicated with shaded areas, showing possible Majorana or Andreev oscillations.