One of the leading issues in high-Tc superconductors is the origin the role of antiferromagnetic (AFM) spin fluctuations in cuprates. Over the years, inelastic neutron measurements have evidenced a magnetic resonance peak in various cuprates with crystal structure comprised of CuO2 monolayer units (like Tl2Ba2CuO6+d) and bilayer units (like YBa2Cu3O6+x (YBCO)). This excitation is then largely considered as a generic excitation of the superconducting state as its intensity vanishes above Tc. Detailed momentum dependences reveal a downward and an upward dispersion of the resonant excitation from the antiferromagnetic wave-vector, yielding incommensurate-like excitations at a fixed energy transfer. Upward and downward dispersions exhibit distinct temperature dependence which might indicate a different origin. Two different types of models have been developed to account for these findings. Either, in a localized spins picture, it points towards dynamical one-dimensional charge stripe arrays separating fluctuating AFM domains. In contrast, it can be described by a spin-exciton collective mode in the superconducting state within Fermi-liquid-based theoretical scenarios. Both approaches present appealing features but none is entirely satisfactory. Further, a second distinct magnetic resonant mode (with an even symmetry with respect to exchange between adjacent copper oxide layers) is observed in bilayer systems. From the relative spectral weight of both resonant modes, one can infer the manifestation of the Stoner electronic continuum of d-wave superconductors favoring the spin-exciton model. Its energy is in good agreement with the electronic energies directly measured with angle resolved photoemission, electronic Raman scattering or scanning tunelling microscopy.
Recent data in CuO2 monolayer system , HgBa2CuO4+d (Hg1201), shows that the resonant feature and the related downward dispersion is almost absent in the superconducting state. Instead, the spin excitations spectrum is dominated by a large susceptibility peak at about 51meV that onsets at the much higher temperature T*≈300K below which the Fermi surface develops a partial ‘pseudogap’ and does not evolve across Tc. The temperature dependence of this excitation matches the one the Q=0 magnetic order observed in the pseudogap phase of underdoped cuprates using polarized elastic neutron diffraction. The combined results for Hg1201 and YBCO reveal a profound universal connection between the AF fluctuations, Q=0 magnetism and pseudogap formation.
 Antiferromagnetic fluctuations and the pseudogap formation in the moderately underdoped high-temperature superconductor HgBa2CuO4+δ M. K. Chan, Y. Tang, C. Dorow, L. Mangin-Thro2 Y. Ge, M. Veit, X. Zhao, A. D. Christianson, J. T. Park, Y. Sidis, P. Steffens, D. L. Abernathy, P. Bourges, and M. Greven, Preprint