1. 1. Wu, M. K. et al. Phys. Rev. Lett. 58, 908–910 (1987). 2. van Duzer, T. & Turner, C. W. Principles of Superconducting Devices and Circuits (Elsevier, New York, 1981). 3. Muller, K. A., Takashige, M. & Bednorz, J. G. Phys. Rev. Lett. 58, 1143–1145 (1987). 4. Williamson, S. J. & Kaufman, C. / Magnetism magn. Mater. 22, 129 (1981). 5. Gough, C. E. et al. Nature 326, 855 (1987).