Magnetic signatures of 120 K superconductivity at interfaces in La₂CuO_{4+: δ}

In self-assembled vertically aligned nanocomposite (VAN) thin films of La₂CuO_4+δ + LaCuO₃, we find from DC magnetic susceptibility measurements, weak signatures of superconductivity at ∼120 K. This compares to a maximum T_C of 40 K in bulk La₂CuO_4+δ. The 120 K signature occurs only when both c-axis and a-axis oriented La₂CuO_4+δ grains are present in the films. The superconductivity was lost after 3 months of storage but was recovered by annealing in oxygen. From lattice parameter analyses undertaken close to the c/a grain boundaries, it was determined that expansion of the La perovskite block in c-La₂CuO_4+δ enables the differently oriented grains to join at the interface. This expansion is consistent with the higher T_C interfacial region. The work shows a new direction for increasing T_C in cuprates-namely careful strain engineering of the crystal structure independently in-plane and out-of-plane.