Leggett Modes in Dirac Semimetals

Since the discovery of symmetry-broken phases hosting multiple order parameters, the condensed matter physics community has pursued control of collective modes corresponding to these order parameters. One example is a Leggett collective mode predicted more than 50 years ago. Since then, Leggett modes have been studied in multiband superconductors such as iron pnictides. However, challenges in controlling and understanding collective excitations in these complicated superconductors involving many degrees of freedom are still outstanding. Here we investigate the co-existence of surface and bulk states in a Dirac semimetal (DSM) with inter- and intraband pairing giving rise to a Leggett collective mode. We study the microwave response of a Leggett mode in a DSM-based SQUID and observe missing even Shapiro steps and a unique response to a magnetic field. We present experimental measurements of a Cd3As2-based SQUID that are consistent with the existence of a Leggett mode.