Decoupling and decommensuration in layered superconductors with columnar defects

We consider layered superconductors with a flux lattice perpendicular to the layers and random columnar defects parallel to the magnetic field B. We show that the decoupling transition temperature Td, at which the Josephson coupling vanishes, is enhanced by columnar defects by an amount δTd/Td~B². Decoupling by increasing field can be followed by a reentrant recoupling transition for strong disorder. We also consider a commensurate component of the columnar density and show that its pinning potential is renormalized to zero above a critical long-wavelength disorder. This decommnesuration transition may account for a recently observed kink in the melting line.