Low field magnetic response of the granular superconductor la_1.8Sr_0.2CuO₄

The properties of the low excitation field magnetic response of the granular high-temperature (HT_e) superconductor La_1.8Sr_0.2CuO₄ have been analyzed at low temperatures. The response of the Josephson currents has been derived from the data. It is shown that intergrain current response is fully irreversible, producing a shielding response, but that it does not carry Meissner magnetization. Analysis of the data shows that the system of Josephson currents freezes into a glassy state even in the absence of external magnetic field, which is argued to be a consequence of the d-wave nature of superconductivity in La_1.8Sr_0.2CuO₄. The macroscopic diamagnetic response to very weak variations of the magnetic field is shown to be strongly irreversible, but still qualitatively different from any previously known kind of critical-state behavior in superconductors. A phenomenological description of these data is given in terms of a newly proposed fractal model of irreversibility in superconductors.