Optical and electron microanalysis of cuprate superconductors

Individual anisotropic grains in heterogeneous and opaque cuprate-based materials, when viewed in a reflected-light optical microscope through crossed polarizers, often have characteristic colors, when a 'daylight' source is used. Of the cuprate superconductors, regardless of charge carrier type, examined so far, only one characteristic color has been observed. The presence of color is studied and a strong correlation with the existence of superconductivity is found. The change in color from insulator to metal to superconductor and the compositions corresponding to these changes found by quantitative energy dispersive X-ray spectroscopy on superconducting Tl-Sr-Ca-Cu-O and metallic but not superconducting La-Sr-Cu-O materials is discussed.