Anomalous enhancement of fractional volume of superconductivity in La2CuO4−y due to field cooling process

Abstract The superconducting diamagnetism observed in undoped La 2 CuO 4−y has been measured as a function of the starting temperature (T fc ) of field cooling after zero-field cooling to 5 K. When T fc exceeds 160 K, the onset temperature of the superconducting transition started to increase from 36 K to 40 K, and the superconducting diamagnetism enhances abruptly around T fc =200 K by a factor 6. This is due to appearance of new 40 K superconductivity superposed on the 36 K superconductivity.