  Recent observation of a "kink" in single-particle dispersion in photoemission
experiments on cuprate superconductors has initiated a heated debate over the
issue of a boson that mediates the pairing in cuprates. If the "kink" is indeed
caused by interaction with a bosonic excitation, then there are two possible
candidates: phonons and spin fluctuations. Here, the role of anti-ferromagnetic
spin fluctuations in shaping the phase diagram of cuprate superconductors will
be discussed. By using the local (momentum-integrated) dynamic spin
susceptibility, recently measured in neutron scattering experiments to high
energies, the electronic self-energies are calculated that agree in many
aspects with those measured directly in angle-resolved photoemission (ARPES)
and optical spectroscopies. The spin fluctuations therefore seem to play a role
typically played by phonons in renormalizing single particles. The key question
emerging from this picture is whether the coupling detected in ARPES reflects
the mediating boson, i.e. whether the spin fluctuations may be responsible for
superconducting pairing.
