The structure of oxygen-annealed La$_{1.9}$Ca$_{1.1}$Cu$_{2}$O$_{6}$ superconductor

Effect of annealing under high oxygen pressure on La$_{1.9}$Ca$_{1.1}$Cu$_{2}$O$_{6}$ (2126) is studied and structure change at the nanometer scale is investigated by using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The as-grown single crystal is non-superconducting, which is thought to be due to oxygen deficiency. With annealing under a high oxygen pressure, superconductivity is induced with T$_{\mathrm{c}}$ $\sim$ 53 K. While the as-grown 2126 shows homogenous structure at a large scale, after oxygen annealing, the sample develops a secondary phase La$_{\mathrm{2-x}}$Ca$_{\mathrm{x}}$CuO$_{4}$(214) at nanometer scale, which is evidenced by electron diffraction patterns together with EELS analysis. The content of 214 phase is estimated to be around 20{\%} based on the analysis of scanning TEM images. Magnetization measurements indicate that the 214 phase is also superconducting. Chemical change of the remaining 2126 phase after oxygen annealing will also be discussed.