AC Susceptibility of Superconducting La-Sr-Cu-O System

The complex susceptibility χ'-iχ'' of the superconducting La1.8-Sr0.2-Cu1-O4 was measured. The complete Meissner effect of the whole volume was observed below 20 K, though the transition temperature of the resistance was 36 K (at the midpoint). Both χ' and χ'' are found to be very sensitive to the magnitude of the a.c. magnetic field. The measurements were made on several specimens prepared in different conditions. From these measurements it was concluded that the boundary and the surface of grains play an important role in the superconductivity in this system.