Studies on ac losses in $Bi_2Sr_2CaCu_2O_8$ single crystals

Abstract

Using a novel technique of measuring absolute ac losses at radio frequencies, the temperature, magnetic field and angle dependence of rf losses in superconducting single crystals of $Bi_2Sr_2CaCu_2O_8$ is studied. An unexpected, larger than normal state ac loss is observed below $T_c$. We provide an explanation for this observation in terms of the energy required to decouple the intrinsic Josephson junctions in the crystals. The studies are carried out both at zero magnetic field and at different values of magnetic fields applied at different angles θ between the field and the c-axis of the crystal. We observe two maxima in ac loss as a function of temperature. The amplitude, width and the position of the first maximum (peak A) change with the magnitude and orientation of the applied magnetic field. These results are discussed in terms of the critical current density of the Josephson junctions, which decreases with the increasing field and changes with the orientation. The second maximum (peak B) appears just below $T_c$ only in the presence of the magnetic field and has been attributed to the vortex motion. The amplitude of this peak as well changes with the magnetic field and with the angle of orientation. The analysis shows that the loss is due to pancake vortices