ANISOTROPY IN THE IRREVERSIBLE BEHAVIOR OF POINT-LIKE DEFECTS AND TWINS IN YBA2CU3O7-DELTA SINGLE-CRYSTALS WITH A PEAK EFFECT

In different RBa(2)Cu(3)O(7-delta) (R = Y,Tm) single crystals the peak of the current density j vs magnetic field B is investigated in the a ngular region B parallel and perpendicular to the c: axis of the cryst als. The magnetic moment from the current is measured as a function of angle phi between field direction and c axis with a vibrating sample magnetometer up to B = 7 T. The peak effect which probably results fro m an isotropic pointlike defect structure (oxygen vacancies, impuritie s from doping or crucible) is observed in B parallel to c for the curr ent parallel to the a,b plane (j parallel to a,b) and in B parallel to a,b for both currents j parallel to a,b and j parallel to c. This was verified in twinned and twin-free crystals. For B parallel to c the i nfluence of the twin structure on the j(B) peak results in an increase of j below and above the peak from pinning at twin walls. However, in the peak region the current becomes depressed either from: channeling of vortices along twin walls or from enhanced stiffness the tilt modu lus of the flux lattice. The twinned crystals show a cusplike dependen ce of the irreversibility field Bi-irr vs phi with the maximum al B pa rallel to c in agreement with a Bose-glass transition. Whereas the det ermined crystal reveals a minimum of B-irr(phi) at B parallel to c as expected from an anisotropic but homogeneous crystal. This interferenc e between point-like and correlated disorder in the flux lattice becom es less pronounced with larger impurity contents. In B parallel to a,b intrinsic pinning from CuO layers is observed by an increase of the m agnetic moment vs phi. This is further proved by lock-in oscillations which demonstrate the matching condition between crystal layer and flu x lattice spacing. At higher fields the moment vs phi shows an unusual decrease in an angular region +/- 0.3 degrees from B parallel to a,b. This behavior is related to j parallel to c which is expected to decr ease from shear instability of the flux lattice.