The effect of collective spin-1 excitations on electronic spectra in high-Tc superconductors

We review recent experimental and theoretical results on the interaction between single-particle excitations and collective spin excitations in the superconducting state of high-T-c cuprates. We concentrate on the traces that sharpen features in the magnetic-excitation spectrum (measured by inelastic neutron scattering) and imprint in the spectra of single-particle excitations (measured, e. g. by angle-resolved photoemission spectroscopy, tunnelling spectroscopy, and indirectly also by optical spectroscopy). The ideal object to obtain a quantitative picture for these interaction effects is a spin-1 excitation around 40 meV, termed 'resonance mode'. Although the total weight of this spin-1 excitation is small, the confinement of its weight to a rather narrow momentum region around the antiferromagnetic wavevector makes it possible to observe strong self-energy effects in parts of the electronic Brillouin zone. Notably, the sharpness of the magnetic excitation in energy has allowed these self-energy effects to be traced in the single-particle spectrum rather precisely. Namely, the doping and temperature dependence together with the characteristic energy and momentum behaviour of the resonance mode has been used as a tool to examine the corresponding self-energy effects in the dispersion and in the spectral line-shape of the single-particle spectra, and to separate them from similar effects due to the electron-phonon interaction. This leads to the unique possibility to single out the self-energy effects due to the spin-fermion interaction and to directly determine the strength of this interaction in high-T-c cuprate superconductors. The knowledge of this interaction is important for the interpretation of other experimental results as well as for the quest for the still unknown pairing mechanism in these interesting superconducting materials.