  In recent years experimentalists have been able to clearly show that several
materials, such as MgB2, iron-based superconductors3, monolayer NbSe2, are
multiband superconductors. Superconducting pairing in multiple bands can give
rise to novel and very interesting phenomena. Leggett modes are exemplary of
the unusual effects that can be present in multiband superconductors. A Leggett
mode describes the collective periodic oscillation of the relative phase
between the phases of the superconducting condensates formed by electrons in
different bands. It can be thought of as the mode arising from an inter-band
Josephson effect. The experimental observation of Leggett modes is challenging
for several reasons: (i) Multiband superconductors are rare; (ii) they describe
charge fluctuations between bands and therefore are hard to probe directly;
(iii) their mass gap is often larger than the superconducting gaps and
therefore are strongly overdamped via relaxation processes into the
quasiparticle continuum. In this work we show that Leggett modes, and their
frequency, can be detected unambigously in a.c. driven superconducting quantum
interference devices (SQUIDs). We then use the results to analyze the
measurements of a SQUID based on Cd3As2, an exemplar Dirac semimetal, in which
superconductivity is induced by proximity to superconducting Al. The
experimental results show the theoretically predicted unique signatures of
Leggett modes and therefore allow us to conclude that a Leggett mode is present
in the two-band superconducting state of Dirac semimetal (DSM) Cd3As2.
