PINNING BY TWIN BOUNDARIES AND PEAK EFFECT IN YBACUO HIGH-T-C SUPERCONDUCTORS

Measurements of the imaginary part of the ac magnetic susceptibility o f single crystals and melt-textured samples of YBa2Cu3Ox (YBCO) at T=7 7 K in a magnetic held ranging between 1 and 20 kOe are reported. If t he de magnetic field H-dc is rotated in the ab plane of the sample, th e magnetic susceptibility and critical current density j(c) have peaks corresponding to the magnetic field aligned with twin boundaries. Pea ks in the curve of j(c) versus magnetic field are observed at angles c orresponding to these peaks, where j(c) proportional to root H-dc in a wide range of magnetic fields. The results have been interpreted in t erms of the theory describing twin boundaries as a system of quasi-pla nar pinning sites. The pinning is strong if the elastic displacements of flux lines are of the order of the vortex lattice constant d(f). Th ese displacements decrease with the magnetic field because of the decr ease in d(f), and the contribution of the elastic energy to the Gibbs potential is reduced accordingly, which is the cause of the peak effec t. (C) 1997 American Institute of Physics.