The mysterious spin gap in high-temperature superconductors: New NMR/NQR studies

We review work, done mainly at the author’s laboratory, on the spin gap which is the NMR manifestation of the pseudogap observed in the normal state of high-temperature superconductors. The relation of the spin gap to an electronic crossover in YBa₂Cu₄O₈ is discussed. A possible explanation of both effects by assuming a charge density wave transition is presented. This suggestion is supported by measuring the isotope dependence of the spin gap in YBa₂Cu₄O₈.