Browsing by Author "Steffens, P."

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).

There exists increasing evidence that the phase diagram of the high-transition temperature (T-c) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO2 square. Below the 'pseudogap' temperature T (T > T-c), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa2CuO4+delta the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description.