  The elucidation of the pseudogap phenomenon of the cuprates, a set of
anomalous physical properties below the characteristic temperature T* and above
the superconducting transition temperature Tc, 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*. 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 HgBa2CuO4+x (Hg1201) that
reveal a fundamental collective magnetic mode associated with the unusual
order, and that 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 and its enormous integrated
spectral weight render it a new candidate for the hitherto unexplained
ubiquitous electron-boson coupling features observed in spectroscopic studies.
