Not yet reviewed by Pith; the record is open.

Employing extensive cellular dynamical mean-field theory (CDMFT) calculations with exact diagonalization impurity solver, we investigate the ground state phase diagrams and non-magnetic metal-insulator transitions of the half-filled Hubbard model on two plaquette -- the 1/5 depleted and checkerboard -- square lattices. We identify three different insulators in the phase diagrams: dimer insulator, antiferromagnetic insulator, and plaquette insulator. And we demonstrate that the plaquette insulator is a novel fragile Mott insulator (FMI) which features a nontrivial one-dimensional irreducible representation of the $C_{4v}$ crystalline point-group and cannot be adiabatically connected to any band insulator with time-reversal symmetry. Furthermore, we study the non-magnetic quantum phase transitions from the metal to the FMI and find that this Mott metal-insulator transition is characterized by the splitting of the non-interacting bands due to interaction effects.