ABSTRACT
Once superconductivity had been clearly established in the new ceramic oxide materials, such as YBCO, the race was on to determine their composition, structure and basic properties. Few materials have ever been subjected so intensively to experimental study, using every conceivable technique. Scientists and engineers have a habit of saying “Nothing is easy”. This is particularly true of work on the cuprates. It has proved singularly difficult to make good clean material. After fabrication, samples always contain many defects, including grain and twin boundaries, vacancies and dislocations. The atomic arrangements are by no means perfect; the layers are not always stacked in the ideal sequence. In addition the constituent elements are often not in the exact atomic proportions suggested by ideal chemical formulae; a deficiency in oxygen is not unusual, and this can completely change the electronic properties, including superconducting behavior.
