Recent inelastic neutron scattering (INS) experiments on La$_{ 2-x}$Sr$_x$CuO$_4$ observed a { \it magnetic coherence effect}, i.e., strong frequency and momentum dependent changes of the spin susceptibility, $\chi''$, in the superconducting phase. We show that this effect is a direct consequence of changes in the damping of incommensurate antiferromagnetic spin fluctuations due to the appearance of a d-wave gap in the fermionic spectrum. Our theoretical results provide a quantitative explanation for the weak momentum dependence of the observed spin-gap. Moreover, we predict { \bf (a)} a Fermi surface in La$_{ 2-x}$Sr$_x$CuO$_4$ which is closed around $(\pi,\pi)$ up to optimal doping, and { \bf (b)} similar changes in $\chi''$ for all cuprates with an incommensurate magnetic response.
Categories: cond-mat.supr-con cond-mat.str-el