Induced microwave absorption by magnetic modulation in untwinned and twinned YBaCuO crystals and its comparison with resistivity measurements

We studied the magnetic field modulated microwave absorption across T-c for an untwinned and for a twinned YBa2Cu3O7-delta crystal(T-c congruent to 93 K) which were cleaved from the same high quality single crystal. The absor ption signals were analyzed using the Tinkham model. A good fit to the mode l was obtained for the untwinned crystal for B parallel to c and for B > 0. 2 T, while for the twinned crystal we observed pronounced deviations from t he model which we attribute to the presence of twin boundary pinning. The a ngular dependence of the activation energy of vortex motion in the untwinne d crystal shows a sharp increase when the direction of magnetic field is al most parallel to the a-b plane, which is suggestive of a lock-in transition . Although field-dependent resistivity measurements on untwinned crystals c learly demonstrate a vortex melting transition, microwave studies do not re veal any signature of it. This agrees with results of other groups showing that high frequency measurements in YbaCuO(7-delta) single crystals, unlike Bi2Sr2CaCu2Ox single crystals, are rather insensitive to the vortex meltin g transition. (C) 1999 Published by Elsevier Science B.V. All rights reserv ed.