  Two-dimensional (2D) moire materials provide a new solid-state platform with
unprecedented controllability for studies of correlated quantum phenomena. To
date, experimental studies have focused on the correlated electronic states;
the correlated bosonic states in moire materials have remained practically
unexplored. Here, we report a correlated dipolar excitonic insulator, a charge
insulating state driven by the formation of excitons, in a Coulomb-coupled WSe2
monolayer and WSe2/WS2 moire bilayer at total hole doping density equal to the
moire density. The system is a Mott insulator when all the holes reside in the
moire layer. Under an out-of-plane electric field, the holes can be
continuously transferred to the WSe2 monolayer, but remain strongly bound to
the empty moire sites; they form an interlayer exciton fluid in the moire
superlattice under a particle-hole transformation. We identify the phase space
and determine the charge gap energy of the excitonic insulating state by
optical spectroscopy and capacitance measurements, respectively. We further
observe the emergence of local magnetic moments in the WSe2 monolayer induced
by the strong interlayer Coulomb correlation. Our demonstration of an exciton
fluid in a lattice paves the path for realizing correlated bosonic quantum
phenomena described by the Bose-Hubbard model in a solid-state system.
