  Copper oxide high temperature superconductors universally exhibit multiple
forms of electronically ordered phases that break the native translational
symmetry of the CuO2 planes. The interplay between these orders and the
superconducting ground state, as well as how they arise through doping a Mott
insulator, is essential to decode the mechanisms of high-temperature
superconductivity. Over the years, various forms of electronic liquid crystal
phases including charge/spin stripes and incommensurate charge-density-waves
(CDWs) were found to emerge out of a correlated metallic ground state in
underdoped cuprates. Early theoretical studies also predicted the emergence of
a Coulomb-frustrated 'charge crystal' phase in the very lightly-doped,
insulating limit of the CuO2 planes. Here, we use resonant X-ray scattering,
electron transport, and muon spin rotation measurements to fully resolve the
electronic and magnetic ground state and search for signatures of charge order
in very lightly hole-doped cuprates from the RBa2Cu3O7-d family (RBCO; R: Y or
rare earth). X-ray scattering data from RBCO films reveal a breaking of
translational symmetry more pervasive than was previously known, extending down
to the Mott limit. The ordering vector of this charge crystal state is linearly
connected to the charge-density-waves of underdoped RBCO, suggesting that the
former phase is a precursor to the latter as hole doping is increased. Most
importantly, the coexistence of charge and spin order in RBCO suggests that
this electronic symmetry-breaking state is common to the CuO2 planes in the
very lightly-doped regime. These findings bridge the gap between the Mott
insulating state and the underdoped metallic state and underscore the prominent
role of Coulomb-frustrated electronic phase separation among all cuprates.
