EXPERIMENTAL-EVIDENCE OF A SHARP TRANSITION INTO A FROZEN STATE AT LOW-TEMPERATURE AND HIGH MAGNETIC-FIELDS IN GRANULAR HIGH-TC SUPERCONDUCTORS

We present a set of experimental results of the microwave magneto-absorption at various temperatures for granular samples of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O. The role of magnetic and thermal history is evident. In the frame in which the absorption, as a function of the magnetic field, is due to the decoupling of Josephson junctions (JJ), we show that the array of JJs is driven in a frozen state by the magnetic field at low temperature. The transition to the frozen state occurs in a narrow range of temperature (15-20 K for Y-Ba-Cu-O and 25-30 K for Bi-Sr-Ca-Cu-O). The superconducting glass model for an array of JJs is able to qualitatively explain the experimental data