We theoretically study the Shapiro steps in a hetero Josephson junction made of a single-gap superconductor and a two-gap one. We find that an anomalous DC Josephson current is induced by tuning the frequency of an applied microwave to the Josephson-Leggett mode frequency, which creates an extra step structure in the I-V characteristics besides the conventional Shapiro steps. The step heights at the resonance voltages exhibit an alternate structure of a large and small value reflecting the gap symmetry of the two-gap superconductor. In the $\pm s-wave case in which the two gaps have opposite signs in the two-gap superconductor the steps with odd index are enhanced, whereas in the s-wave case the ones with even index have larger values. The existence of the fractional Shapiro steps is also predicted.
@article{arxiv.1006.2184,
title = {Shapiro Steps as a Direct Probe of $\pm s$-wave Symmetry in Multi-gap Superconducting Josephson Junctions},
author = {Yukihiro Ota and Masahiko Machida and Tomio Koyama},
journal= {arXiv preprint arXiv:1006.2184},
year = {2010}
}