Mind the pseudogap

Abstract

The discovery of predicted collective electronic behaviour in copper-oxide superconductors in the non-superconducting state provides clues to unlocking the 24-year-old mystery of high-temperature superconductivity. See Letter p.283 The pseudogap phenomenon, a discontinuity in the energy level of a material's electronic spectrum, is a universal characteristic of the high transition temperature (Tc) copper oxides. The nature of the pseudogap has been a central question in condensed matter physics for more than a decade, but many of its properties remain unexplained. Recent studies have pointed to the universal existence of an unusual magnetic order below T*, the temperature below which the anomalous properties associated with the pseudogap become apparent. If confirmed, this would have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. The results of inelastic neutron scattering experiments on the superconductor HgBa2CuO4+δ (Hg1201) now reveal a fundamental collective magnetic mode associated with the unusual order, providing further support for this picture.