Evidence for the existence of a sub-structure in YBa₂Cu₃O_7−δ single crystals

We have structurally studied the elaborated single crystal of YBa₂Cu₃O_7−δ using high-resolution x-ray diffraction, including φ-scan, rocking curve, reflectivity and small angle scattering. The results show that the single crystal is natively inhomogeneous. The rocking curve of 00l reflection indicates that the c-axis lattice constant of the crystal varies in a narrow range, clearly showing a multilayer characteristic. The reflectivity measurement supports this result; it also shows an obvious multilayer characteristic. The small angle scattering indicates that, although the c-axis lattice constant is little different in the crystal, it shows a long period of about 700 Å. The local work function measurement using a scanning tunnelling microscope demonstrates that the crystal is inhomogeneous, supporting the results of x-ray analyses. These facts tell us that so far we have not completely understood the structure of high-T_c superconductors.