Microwave and ac power absorption and ac susceptibility measurements of the high-Tc superconducting Tl-Ba-Ca-Cu-O system near zero field.

Abstract

The low-field dependence of electrical power absorption and ac susceptibility--temperature measurements are performed on three ceramic superconductors of the ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{\mathit{L}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{L}}$${\mathrm{O}}_{4+2\mathit{L}}$ family using modulated and direct microwave and ac methods. Below a critical temperature all samples exhibit a large increase in ac and microwave absorption near zero field, a Meissner effect, and a diamagnetic shielding phenomenon. The microwave signal recorded with a modulation technique has three components, out of which, one with hysteretic character is mostly the product of the normal cores of intragrain fluxons. The observed deviations of the signal from the real absorption behavior are explained. The measured ac susceptibility reflects mainly the moving magnetic flux vortices induced by intergrain Meissner and diamagnetic shielding currents. Our results suggest that the variation of magnetic field induces at low and microwave frequencies the same flux exclusion and penetration processes characterizable macroscopically mostly through the change in diamagnetic surface impedance.