Emergence of two superconducting phases (33 K and 40 K) and their relation to structural phase transitions in crystallineLa2CuO4.011

Adiabatic calorimetry revealed three structural phase transitions existing in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.011}$ single crystal; the higher-order type at 287 K, the Martensitic type at 265 K, and the first-order type at 222 K. The phase stable between 222 and 265 K can be supercooled easily and shows superconductivity with onset at 33 K, while the phase stable below 222 K does that at 40 K. The annealing effects on the structural phase transitions are discussed in correlation with different appearances of the superconducting transition temperature and of the Meissner fraction.