Multiple reflections of an injected electron can shuttle a supercurrent.

What is it about?

An electron injected into a metal between two superconductors executes repeated reflections as it “ping-pongs” off the superconductors. At the first reflection, the electron returns as a “hole” but at the next reflection the hole returns as an electron. Two successive reflections shuttle a pair of electrons across the metal. These multiple Andreev reflections are observed with exceptional resolution in the Dirac semimetal MoTe2. By tuning the phase difference between the two superconductors, we show that the multiple reflections are tightly choreographed with a companion Josephson supercurrent that arises from the shuttled pairs. The injected electron may be likened to a bus driver shuttling paired passengers one-way between terminals. If the distance is short, the pairs form a supercurrent.

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Photo by Dave Hoefler on Unsplash

Photo by Dave Hoefler on Unsplash

Why is it important?

The Josephson supercurrent and Andreev reflections are well-investigated, but often regarded as independent phenomena in superconductivity. Applying the phase-tuning technique, we show that they are intimately related. The supercurrent arises from the Cooper pairs shuttled by the injected electron undergoing multiple reflections. In Dirac semimetals we find that the Josephson supercurrent relies on multiple Andreev reflections to be transmitted. In the limit when the junction is dissipationless, the shuttled pairs account for the entire Josephson supercurrent. Our experiment raises the interesting question whether this intimate relationship holds in all junctions.

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