Hidden magnetic excitation in the pseudogap phase of a high-T-c superconductor

Abstract

The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-T-c) copper oxides-a set of anomalous physical properties below the characteristic temperature T and above T-c-has been a major challenge in condensed matter physics for the past two decades(1). Following initial indications of broken time-reversal symmetry in photoemission experiments(2), recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point(5). Here we report inelastic neutron scattering results for HgBa2CuO4+delta that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode's intensity rises below the same temperature T and its dispersion is weak, as expected for an Ising-like order parameter(6). Its energy of 52-56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies(7-10).