  Bitter decoration and magneto-optical studies reveal that in heavy-ion
irradiated superconductors, a 'porous' vortex matter is formed when vortices
outnumber columnar defects (CDs). In this state ordered vortex crystallites are
embedded in the 'pores' of a rigid matrix of vortices pinned on CDs. The
crystallites melt through a first-order transition while the matrix remains
solid. The melting temperature increases with density of CDs and eventually
turns into a continuous transition. At high temperatures a sharp kink in the
melting line is found, signaling an abrupt change from crystallite melting to
melting of the rigid matrix.
