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.PACS numbers: 74.60. Ec, 74.60.Ge, 74.72.Hs Melting of heterogeneous systems, and in particular of nanocrystals embedded in porous rigid matrices, is a complex process with many uncontrolled parameters. Metal and semiconductor nanocrystals with free surfaces, for example, usually show a decrease in their melting temperature with decreasing size [1], whereas nanocrystals encapsulated in a porous matrix often display an increase in melting temperature [2]. Although the contribution of the different factors is still a matter of debate, the melting process is known to depend on the size, dimensionality, material properties of the nanocrystals and the matrix, as well as the interface energies between the materials [1,2]. In this work we investigate an analogous, but a more controllable composite system, which is a 'porous' vortex matter consisting of vortex nanocrystals encapsulated in a matrix of strongly pinned vortices. As shown below, this system is present in the commonly heavy-ion irradiated superconductors when the vortices outnumber the columnar defects (CDs). The rigid matrix is created by vortices localized on the network of random CDs, while the softer nanocrystals are formed within the 'pores' of this matrix by the interstitial vortices. The size of the nanocrystals can be readily varied from several hundred down to a few vortices by changing the applied field or the density of CDs. We find that this composite vortex matter reveals a number of intriguing mechanisms: Similarly to the metallic nanocrystals in a matrix, we observe for the first time a pronounced upward shift in the vortex melting temperature T m , while preserving the first-order nature of the transition (FOT). With increasing density of CDs, the size of the pores decreases, resulting in a larger shift in T m . We also find a critical point at which the FOT changes into a continuous melting. Moreover, the crystallites can melt while the matrix remains rigid. As a result, at high temperatures we find an abrupt breakdown in the upward shift of T m and a sharp kink in the FOT line, which apparently result from the collapse of the matrix due to vortex depinning from the CDs.The reported findings were obtained using Bitter decoration and differential magneto-optical (MO) [3] techniques. High quality Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals (T c ≈ 89 K) were covered by various patterned masks and irradiated at GANIL by 1 Ge...

Magnetic screening response of the superconductor 2H-NbSe 2 with varying pinning reveals a rich evolution of the peak effect from a history-independent sharp anomaly to a broad and strongly history dependent effect with internal structure. The results display a stepwise disordering of the vortex lattice through transformations affected by both thermal fluctuations and quenched disorder. ͓S0163-1829͑99͒03309-3͔ PHYSICAL REVIEW B