Low field magnetic response of the granular superconductor La1.8Sr0.2CuO4

The properties of the low excitation field magnetic response of the granular high-temperature (HTc) superconductor La1.8Sr0.2CuO4 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 La1.8Sr0.2CuO4. 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.