Phase tuning of multiple Andreev reflections of Dirac fermions and the Josephson supercurrent in Al-MoTe2-Al junctions

Abstract

When an electron is incident on a superconductor from a metal, it is reflected as a hole in a process called Andreev reflection. If the metal N is sandwiched between two superconductors S in an SNS junction, multiple Andreev reflections (MARs) occur. We have found that, in SNS junctions with high transparency (τ → 1) based on the Dirac semimetal MoTe2, the MAR features are observed with exceptional resolution. By tuning the phase difference ϕ between the bracketing Al superconductors, we establish that the MARs coexist with a Josephson supercurrent Is = IA sin ϕ. As we vary the junction voltage V, the supercurrent amplitude IA varies in step with the MAR order n, revealing a direct relation between them. Two successive Andreev reflections serve to shuttle a Cooper pair across the junction. If the pair is shuttled coherently, it contributes to Is. The experiment measures the fraction of pairs shuttled coherently vs. V. Surprisingly, superconductivity in MoTe2 does not affect the MAR features.