Andreev reflections (AR) play a central role in transport phenomena in Josephson junctions. They shuttle supercurrents in a topologically trivial junction, while an Andreev bound state (ABS) is pinned at zero-energy in a topological junction. Such a state is also known as Majorana zero mode. The different ABSs result in different periodicities of Josephson effect; in a topological junction, the effect is 4π-periodic while it is 2π periodic in a trivial junction. The anomaly in periodicity can be detected by measuring the current-phase relation (CPR) of the junction.

Our lab employs asymmetric SQUIDs to measure the CPR of Josephson junctions that are made of various topological materials. A typical asymmetric SQUID consists of a large auxiliary aluminum tunneling junction and a sample junction of interest. The auxiliary junction carries a much larger supercurrent than the sample junction does, and it is used to fix the phase of entire SQUID. The contribution from the sample junction is then a small perturbation to the fixed phase. By varying the penetrating flux in the SQUID and studying the effect from the sample junction, we can obtain the CPR of the sample junction.

Although only a fraction of AR remain phase coherent to carry supercurrents, in a recent study of Al-MoTe2-Al asymmetric SQUIDs, we observed multiple Andreev reflections (MARs) with exceptional clarity and were able to determine the coherent fraction. [1] Specifically, the singularity in subharmonic gap is experimental evidence for the existence of MARs in the MoTe2 junctions. Utilizing the phase dependence of these MARs, we can probe the phase coherent current even at finite voltage bias in the dissipative regime. Our work paves roads for analyzing critical current and pairing correlation with a more quantitative treatment, especially in unconventional superconducting junctions.

[1] Zheyi Zhu, Stephan Kim, Shiming Lei, Leslie M. Schoop, R. J. Cava, and N. P. Ong, Phase tuning of multiple Andreev reflections of Dirac fermions and the Josephson supercurrent in Al–MoTe2–Al junctions, Proceedings of the National Academy of Sciences of the United States of America 119, 28 (2022).