Relation of Spin Susceptibility and a.c. Conductivity in Nearly Antiferromagnetic Fermi Liquid

Coupling of the charge carriers through the magnetic spin susceptibility which is strongly peaked at (, ) in the two dimensional CuO ₂ Brillouin zone leads to strong momentum anisotropies and energy dependencies in the inelastic optical scattering rates. The a.c. conductivity is isotropic and can be used to extract an effective, isotropic spin-charge carrier spectral density which reproduces almost exactly the optical scattering rates. The technique is then applied to data in superconducting, optimally doped YBCO and used to measure the charge carrier coupling to the 41 meV peak observed in spin polarized neutron scattering.