  The two-dimensional (2D) vortex lattice in the extreme type-II limit is
studied by Monte Carlo simulation of the corresponding 2D Coulomb gas, with
identical pins placed at sites coinciding with the zero-temperature triangular
vortex lattice. At weak pinning we find evidence for 2D melting into an
intermediate hexatic phase. The strong pinning regime shows a
Kosterlitz-Thouless transition, driven by interstitial vortex/anti-vortex
excitations. A stack of such identical layers with a weak Josephson coupling
models a layered superconductor with a triangular arrangement of columnar pins
at the matching field. A partial duality analysis finds that layer decoupling
of the flux-line lattice does not occur at weak pinning for temperatures below
2D melting.
