  Conventional and unconventional superconductivity, respectively, arise from
attractive (electron-phonon) and repulsive (many-body Coulomb) interactions
with fixed-sign and sign-reversal pairing symmetries. Although heavy-fermions,
cuprates, and pnictides are widely believed to be unconventional
superconductors, recent evidence in one of the heavy fermion superconductor
(CeCu$_2$Si$_2$) indicate the presence of a novel conventional type pairing
symmetry beyond the electron-phonon coupling. We present a new mechanism of
attractive potential between electrons, mediated by emergent boson fields
(vacuum or holon) in the strongly correlated mixed valence compounds. In the
strong coupling limit, localized electron sites are protected from double
occupancy, which results in an emergent holon gauge fields. The holon states
can, however, attract conduction electrons through valence fluctuation channel,
and the resulting doubly occupied states with local and conduction electrons
condense as Cooper pairs with onsite, fixed-sign, $s$-wave pairing symmetry. We
develop the corresponding self-consistent theory of superconductivity, and
compare the results with experiments. Our theory provides a new mechanism of
superconductivity whose applicability extends to the wider class of
intermetallic/mixed-valence materials and other flat-band metals.
