  We study the physics of hard-core bosons with unfrustrated hopping (t) and
nearest-neighbor repulsion (V) on the three dimensional pyrochlore lattice. At
half-filling, we demonstrate that the small V/t superfluid state eventually
becomes unstable at large enough V/t to an unusual insulating state which
displays no broken lattice translation symmetry. Equal time and static density
correlators in this insulator are well described by a mapping to electric field
correlators in the Coulomb phase of a U(1) lattice gauge theory, allowing us to
identify this insulator with a U(1) fractionalized Mott-insulating state. The
possibility of observing this phase in suitably designed atom-trap experiments
with ultracold atoms is also discussed, as are specific experimental
signatures.
