Defect-unbinding and the Bose-glass transition in layered superconductors

The low-held Bose-glass transition temperature in heavy-ion irradiated Bi2S r2CaCu2O8+delta increases progressively with increasing density n(d) of irr adiation-induced columnar defects, but saturates for n(d) = greater than or similar to 1.5 x 10(9) cm(-2). The maximum Bose-glass temperature correspo nds to that above which diffusion of two-dimensional pancake vortices betwe en vortex lines becomes possible, and the "linelike" character of vortices is lost. We develop a description of the Bose-glass line that quantitativel y describes experiments on crystals with widely different track densities a nd material parameters.