The elucidation of the pseudogap phenomenon of the high-transition- temperature (high-Tc) 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. Following initial indications of broken time-reversal symmetry in photoemission experiments, 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. Here we report inelastic neutron scattering results for HgBa2 CuO4+δ́ 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. Its energy of 52-56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies.
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Acknowledgements We thank T. H. Geballe, S. A. Kivelson, E. M. Motoyama and C. M. Varma for discussions. This work was supported by the US Department of Energy and the US National Science Foundation, and by the National Natural Science Foundation, China. Y.L. acknowledges support from the Alexander von Humboldt Foundation during the final stage of completing the manuscript.