Andreev pair injection into a transition metal dichalcogenide monolayer

We demonstrate Andreev pair injection across Nb-WS₂ junction evident as Andreev reflection in differential conductivity spectra below Nb critical temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>c</mml:mi></mml:mrow> </mml:msub> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>dI</mml:mi> <mml:mo>/</mml:mo> <mml:mi>dV</mml:mi></mml:mrow> </mml:math> spectra for junction voltages smaller than the corresponding Nb superconducting gap, which vanishes as the device temperature is increased above the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>c</mml:mi></mml:mrow> </mml:msub> </mml:math> . 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.