Nanoscale coherent intergrowthlike defects in a crystal of ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ made superconducting by high-pressure oxygen annealing

Superconductivity with $T_c=53.5$ K has been induced in a large ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_6$ (La-2126) single crystal by annealing in a high partial pressure of oxygen at 1200 °C. Using transmission electron microscopy techniques, we show that a secondary Ca-doped ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (La-214) phase, not present in the as-grown crystal, appears as a coherent intergrowthlike defect as a consequence of the annealing. A corresponding secondary superconducting transition near 13 K is evident in the magnetization measurement. Electron energy-loss spectroscopy reveals a pre-edge peak at the O-$K$ edge in the superconducting La-2126 phase, which is absent in the as-grown crystal, confirming the hole doping by interstitial oxygen.

Figure 1

Temperature-dependent magnetic susceptibility ($M/H$) of a superconducting ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ single crystal, measured at a 2-Oe externally applied magnetic field. Measurements were performed both in zero-field cooling (ZFC) and field-cooling (FC) modes. Magnetic field is parallel to the $c$ axis of the crystal.

Figure 2

(a) ADF image along [110] of the as-grown nonsuperconducting sample. Inset: Atomic-resolution image. (b) Electron diffraction from the area shown in (a).

Figure 3

(a) ADF image along [110] of the superconducting sample after post-high-pressure oxygen treatment. (b) and (c) Atomic-resolution image expanded from the region shown in (a). Green and red labels indicate the different materials. (c) Lattice modulating between La-2126 and La-214 that allows for the intergrowth. (d) Electron diffraction from the area shown in (a). Some reflections are labeled for La-2126 and La-214 phases, respectively.

Figure 4

A typical EELS spectrum (a) and atomic resolution image (b) from the superconducting oxygen-annealed sample. Integrated intensity of (c) La-$M_{4,5}$ and (d) Ca-$L_{2,3}$ edge features shown in (a) along the scan path shown in (b).

Figure 5

Comparison of O-$K$ edge fine structures between the as-grown nonsuperconducting sample (black) and the remaining La-2126 of the superconducting sample after oxygen treatment (red).