Modulated structure and local oxygen reordering induced by high-energy Au-ion irradiation in YBa₂Cu₃O₇δ

We have investigated the damage induced in single crystals of the high-T_csuperconductor YBa₂Cu₃O₇δ by high-energy (2.25GeV) Au-ion irradiation. Transmission electron microscopy characterization reveals that irradiation in the c-axis direction leads to the formation of aligned columnar defects, the diameter of which is modulated along the c axis. The columns are amorphous and the associated volume expansion causes strain in the surrounding matrix. Lobed ‘bow-tie’ contrast is observed in the [001] high-resolution images of the regions surrounding the amorphous columns. It is shown that this contrast is caused by local O reordering in the a-b planes to give a 90° reorientation of the orthorhombic unit cell in the affected area, this allowing partial relaxation of the tensile hoop stress around the amorphous columns.