Fractional Shapiro steps without fractional Josephson effect

It is widely believed that superconducting junctions involving topological insulators and hosting Majorana-like bound states may exhibit unusual "fractional" ($4\pi$-periodic) ac Josephson effect. Accordingly, "fractional" Shapiro steps on the current-voltage characteristics of such junctions are expected to occur under external microwave radiation. Here, we microscopically evaluate Shapiro steps in topologically trivial highly transparent superconducting weak links. The key features recovered within our analysis -- including, e.g., the so-called "missing" Shapiro steps -- turn out to be similar to those observed in topological Josephson junctions. Our results demonstrate that caution is needed while interpreting experimental results for superconducting weak links in terms of Majorana physics.
Comment: 19 pages, 9 figures