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

Abstract

We studied the magnetic field modulated microwave absorption across T c for an untwinned and for a twinned YBa 2Cu 3O 7− δ crystal ( T c≅93 K) which were cleaved from the same high quality single crystal. The absorption signals were analyzed using the Tinkham model. A good fit to the model was obtained for the untwinned crystal for B‖ c and for B>0.2 T, while for the twinned crystal we observed pronounced deviations from the model which we attribute to the presence of twin boundary pinning. The angular dependence of the activation energy of vortex motion in the untwinned crystal shows a sharp increase when the direction of magnetic field is almost parallel to the a– b plane, which is suggestive of a lock-in transition. Although field-dependent resistivity measurements on untwinned crystals clearly demonstrate a vortex melting transition, microwave studies do not reveal any signature of it. This agrees with results of other groups showing that high frequency measurements in YbaCuO 7− δ single crystals, unlike Bi 2Sr 2CaCu 2O x single crystals, are rather insensitive to the vortex melting transition.