Andreev pair injection into a transition metal dichalcogenide monolayer.

Abstract

We demonstrate Andreev pair injection across Nb-WS₂ junction evident as Andreev reflection in differential conductivity spectra below Nb critical temperature $T_c$ . The superconducting- 2D semiconducting junction defined by a focused ion beam, shaped Nb pads, and semi-dry transfer of single layer CVD-grown WS₂ crystals ensured the mechanical integrity of the 2D TMD film, reduced contamination and defects at Nb-WS₂ junction, enabling the pristine study of the interface and facilitating Andreev pair injection. We observed enhanced conductivity in $\mathrm{dI}/\mathrm{dV}$ spectra for junction voltages smaller than the corresponding Nb superconducting gap, which vanishes as the device temperature is increased above the $T_c$ . The position and the temperature dependence of the conductivity peaks suggest proximity effect-related phenomena explained by developed modified BTK theory. The presented results are crucial for the future implementation of proximity-based 2D hybrid devices including quantum light sources and superconducting field-effect transistors based on superconductor-semiconductor junctions.