  We explore a new platform for realising excitonic insulators, namely van der
Waals (vdW) bilayers comprising 2D Janus materials. In previous studies,
Type-II heterobilayers have been brought to the excitonic insulating regime by
tuning the band alignment using external gates. In contrast, the Janus bilayers
of the current work represent intrinsic excitonic insulators. We first conduct
ab initio calculations to obtain the quasiparticle band structures, screened
Coulomb interaction, and interlayer exciton binding energies of the bilayers.
These ab initio-derived quantities are then used to construct a BCS-like
Hamiltonian of the exciton condensate. By solving the mean-field gap equation,
we identify 16 vdW Janus bilayers with insulating ground states and superfluid
properties. Our calculations expose a new class of advanced materials that are
likely to exhibit novel excitonic phases at low temperatures, and highlight the
subtle competition between interlayer hybridization, spin-orbit coupling, and
dielectric screening that govern their properties.
