Observation of a disordered vortex state in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ single crystals containing columnar defects

We report on the visualization of vortex patterns by means of the high-resolution Bitter-decoration technique in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{x}}$ single crystals containing columnar defects generated via high-energy heavy-ion irradiation. By irradiating only a part of each sample, a simultaneous decoration of irradiated and nonirradiated parts was possible. The columnar defects lead to a complete destruction of translational order, whereas the vortex lattice in the nonirradiated parts exhibits hexagonal ordering. Analyses of the decoration images yield translational correlation lengths of 1.5 to 2 flux spacings in the nonirradiated areas and about 0.5 flux spacings in the irradiated areas.