Stair-step columnar defects in ion-irradiated Bi₂Sr₂CaCu₂O_x crystals

Bi₂Sr₂CaCu₂O_x single crystals irradiated by 75 MeV B and 230-MeV Au ions were investigated using transmission electron microscopy. Apart from columnar and cascade defects, another type of defect with stair-step morphology was observed in all the samples irradiated under the two different irradiation conditions. Such defects usually exist in the area, where the density of the ion energy deposited is lower than that required for producing cascade defects. The analyses of the defect microstructure and the location of the defect in the target material indicate that this kind of defects is neither simple cracks nor ordinary irradiation-induced defects. The mechanism for the formation of this kind of stair-step defect is qualitatively discussed.