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_c) superconductor La_1.8Sr_0.2CuO₄ have been analyzed at low temperatures. The response of the Josephson currents has been extracted from the data. It is shown that intergrain current response is fully irreversible, producing shielding response, but do not carry Meissner magnetization. Analysis of the data shows that the system of Josephson currents freezes into a glassy state even in the absense 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 the critical-state behaviour in superconductors. A phenomenological description of these data is given in terms of a newly proposed «fractal» model of irreversibility in superconductors.