Collective modes in high-temperature superconductors

The role of collective modes in various experiments on the cuprates is investigated. We calculate the neutron scattering, photoemission [angle-resolved photoemission spectroscopy (ARPES)], and Raman-scattering intensities below T-c within the fluctuation-exchange approximation for the two-dimensional Hubbard model. It is shown that the large peak in the dynamical spin susceptibility arises from a weakly damped spin-density-wave collective mode. This gives rise to a dip between the sharp low-energy peak and the higher binding energy hump in the ARPES spectrum. Furthermore, we show that the collective mode of the amplitude fluctuation of the d-wave gap yields a broad peak above the pair-breaking threshold in the B-1g Raman spectrum. [S0163-1829(98)00142-8].