Superconductivity in La-deficient and stoichiometricLa2CuO4

The effects of La deficiency and of impurities on the occurrence of filamentary superconductivity in ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ have been investigated by magnetic-susceptibility and electrical-resistivity measurements. Reducing the purity of the starting material degrades the superconductivity. La deficiency causes a fully reentrant superconducting-to-normal-state transition with decreasing temperature at temperatures below $T_c$. This behavior clarifies the incipient reentrance which is observed in nominally stoichiometric ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$. We associate the reentrant transition with the recently observed structural transformation from the orthorhombic to a semiconducting monoclinic phase.