Since 1986, when lanthanum barium copper oxide (LBCO) was found to superconduct above 30 K, many other such high-critical-temperature (high-T c) superconductors have been found, all of them copper oxides having layered structures. The superconductivity appears to arise from the planes of copper and oxygen atoms common to these compounds.
The undoped parents of all high-T c superconductors have one valence electron per copper atom. The ground state is an antiferromagnetic insulator, in which the spin of each electron is aligned opposite that of its neighbors. Once the compound is sufficiently doped with holes or electrons, it goes superconducting below T c, and the electrons move in coherent pairs throughout the sample. Just how the material makes the transition between these very disparate states has been the subject of intense study.
Of particular interest is the nature of any spin or charge ordering, either in the...
