  We measured the effects of periodic surface holes, created using a focused
ion beam, on the phase diagram of the vortex matter in high-T_c
Bi_2Sr_2CaCu_2O_{8+\delta} crystals. Differential magneto-optical measurements
show that the irreversibility line is shifted to higher fields and
temperatures, with respect to the pristine melting line. The irreversibility
line displays weak field dependence between integer matching fields indicating
multiple-flux-quanta pinning at holes. We find reduced equilibrium
compressibility of the vortex matter at integer matching fields, which is
strong evidence for the existence of thermodynamic Mott insulator phases.
Shaking with a transverse ac field surprisingly reveals first-order melting
that is not shifted with respect to the pristine melting line and that seems to
occur within the Mott insulator regions. This melting is understood to be the
first-order transition in the bulk of the crystal beneath the surface holes.
The transition is visible at the surface, despite the reduced vortex
compressibility in the top layer.
