  Superconductors with broken time-reversal symmetry represent arguably one of
the most promising venues for realizing highly sought-after topological
superconductivity that is vital to fault-tolerant quantum computation. Here, by
using extensive muon-spin relaxation and rotation measurements, we report that
the isostructural silicide superconductors (Ta, Nb)OsSi spontaneously break
time-reversal symmetry at the superconducting transition while surprisingly
showing a fully-gapped superconductivity characteristic of conventional
superconductors. The first-principles calculations show that (Ta, Nb)OsSi are
three-dimensional Dirac semimetals protected by nonsymmorphic symmetries.
Taking advantage of the exceptional low symmetry crystal structure of these
materials, we have performed detailed theoretical calculations to establish
that the superconducting ground state for both (Ta, Nb)OsSi is most likely a
nonunitary triplet state.
