Magnetic Field- and Frequency-Dependent Study of the AC Susceptibility of High-Tc YBCO Single Crystal

The temperature dependence of AC susceptibility (ACS) has been measured for a very high-quality plate-like slightly overdoped YBCO single crystal for different frequencies and AC magnetic fields. Frequency dependence of the ACS is weak irrespective of the magnetic field orientation, but significant effects of field orientation with respect to the CuO2 planes and field magnitude on real and imaginary components of fundamental ACS are observed. We have found that the magnitude of the ratio of the loss peak to the full diamagnetic signal is slightly larger in perpendicular field than that in the parallel field arrangement. The height of the loss peak saturates as full penetration of magnetic field is achieved. This saturation is achieved at a lower field amplitude when HIIc. The peak temperature, T-p, in chi '' shifts to lower temperatures with increasing magnetic field amplitude for both HIIc and HIIab. The value of T-p depends on the orientation of the magnetic field with respect to the crystallographic axes, illustrating the anisotropy in the magnetic flux dynamics. The superconducting onset transition temperature obtained from the diamagnetic shielding component of the ACS, on the other hand, does not depend on the orientation of the magnetic field. The superconducting transition width increases weakly with increasing magnetic field; the rate of this increment is more prominent for HIIc case. The Cole-Cole plot [chi ''(chi ')] shows qualitatively and quantitatively almost identical features for HIIc and HIIab, with small difference in the peak positions with respect to the orientation of the magnetic field and shielding current paths. The general features of chi ''(chi ') imply that there is no flux creep for the range of frequencies and AC fields employed in this investigation. The maximum value of the loss peak and its position with respect to chi ' in the Cole-Cole plot are largely consistent with the Bean critical state model. Slightly increased peak value in comparison to the predicted peak value within the Bean critical state model is probably due to a weak field dependence of J(c). The results obtained here are compared with various theoretical models and experimental findings. Notable differences are highlighted and discussed in details in this study.