Columnar defects acting as passive internal field detectors

It is a well established fact that the presence of columnar defects ~CD’s ! in high-temperature superconductors ~HTSC’s ! enhances the critical current ( Jc) due to the strong pinning and the inhibition of thermal wandering when the flux lines lay into these tracks. 1,2 In the YBa2Cu3O7 ~Y:123! compound, the directional pinning produced by these correlated structures becomes evident when the angular dependence of Jc is studied. 3,4 In the last decade several works have shown that a sharp peak in Jc(Q) appears when the applied field H is aligned with the direction ( Q D) of these linear defects 1,3,5‐ 8 ~here Q and Q D are the angles formed by the crystallographic c axis with H and the CD, respectively!. However, this behavior only holds when H is high enough to ensure that the average vortex direction is parallel to H .A t lower fields, both material anisotropy and geometry effects become relevant and modify the vortex orientation, 9 which consequently may not coincide with that of H. Since maximum pinning occurs when the vortex orientation ~given by the internal field B rather than by H) is aligned with the CD, Jc(Q) should maximize at an angle Q maxfiQD . In other words, any misalignment between B and H manifests itself as a shift in the angular position of the peak in Jc(Q) with respect to the tracks direction.