AC susceptibility in deoxygenated YBCO single crystals

Abstract We measured the first and third harmonic of the complex AC susceptibility ( χ i ′+ jχ i ″; i =1, 3) in YBCO single crystals with various oxygen content (6.5≤ x ≤7). The amplitude of the AC field was varied in the presence of an external DC field, both applied parallel to the c -axis of the crystals. We show evidence that deoxygenation leads to a reduction of bulk pinning strength, and consequently to a stronger contribution of geometrical barriers. These results support recently reported investigations showing that deoxygenation makes the YBCO crystals more anisotropic, reducing the effective pinning barriers for quasi two-dimensional vortices.