  An array of ultracold atoms in an optical lattice (Mott insulator) excited to
a state where single electron wave-functions spatially overlap would represent
a new and ideal platform to simulate exotic electronic many-body phenomena in
the condensed phase. However, this highly excited non-equilibrium system is
expected to be so short-lived that it has eluded observation so far. Here, we
demonstrate the first step toward its realization by exciting high-lying
electronic (Rydberg) states of the atomic Mott insulator with a coherent
ultrashort laser pulse. Beyond a threshold principal quantum number where
Rydberg orbitals of neighboring lattice sites overlap with each other, the
atoms efficiently undergo spontaneous Penning ionization resulting in a drastic
change of ion-counting statistics, sharp increase of avalanche ionization and
the formation of an ultracold plasma. These observations signal the actual
creation of exotic electronic states with overlapping wave functions, which is
further confirmed by a significant difference in ionization dynamics between a
Bose-Einstein condensate and a Mott insulator.
