LOW-TEMPERATURE TORSION PENDULUM MEASUREMENTS ON YBCO SUPERCONDUCTORS

We report low temperature torsion pendulum measurements on two types o f YBa2Cu3O7-delta superconducting samples made from different precurso rs. The internal friction and modulus of the samples show in some aspe cts different behaviour than reported in the literature so far: a broa d maximum of the internal friction around room temperature and a stron g reduction of the anomalies near T-c. In addition, in both samples a peak becomes apparent when the samples are cooled below a temperature around T-c. The maximum for this peak is at 145 and 200 K for the two samples (for a frequency of about 10 Hz). Possible mechanisms for the peaks are discussed; however, details of the exact mechanism for these anomalies remain rather unclear.