LOW-FIELD MICROWAVE SIGNAL IN UNDOPED AND Sb-DOPED Bi-Pb-Sr-Ca-Cu-O

Non-resonant low-field microwave signal in undoped Bi-Pb-Sr-Ca-Cu-O (2223) sample inches monotonously with decreasing temperature, like that in many bulk samples of high-T_c superconductor. But the signed of a Sb-doped sample shows a peak at 103K, and declines gradually and tends to stabilize when the temperature is further reduced. A distorted 2223 phase (i.e., 130K phase) which was an intergrowth of 2223 and insulating monoclinic phases in the Sb-doped sample, was unstable, and it might change into the stable insulating monoclinic phase and lost its superconductivity at lower temperatures.