Harmonic ac susceptibilities for textured YBa2Cu3O7

Harmonic ac susceptibilities were measured as a function of temperature (T), ac field frequency (f) and amplitude (B-ac,), and de field (B-dc,) for bulk textured YBa2Cu3O7. The higher harmonics consist mainly of odd ones in strong de fields (B-dc, > 50 mT) and depend on B-dc, B-ac, and f. It is confirmed that the origin of harmonic generation for the textured sample is thermally activated flux diffusion over barriers Ci which depend nonlinearly on current density (j). The high nonlinear flux diffusion induces a self-organized Bean critical state in which the straight line approximation is a good description of the field profile except that the current density is j(f) instead of the critical value j(c). Thus the frequency dependent Bean critical state accounts for the origin of harmonic generation and explains the present and other experiments well. (C) 1997 Elsevier Science B.V.