On the theory of the spin susceptibility of yttrium cuprates in the two-plane model of frustrated antiferromagnets

A spherically symmetric approach is developed for the two-plane frustrated Heisenberg model with the inclusion of finite damping in the dynamical spin susceptibility χ(q, ω) at finite temperatures. For yttrium cuprates, the theory allows the description of the neutron experimental results for the acoustic, χ_ac (q, ω), and optical, χ_opt(q, ω), branches of the susceptibility in a wide doping interval. The problem of the reconstruction of the exchange constants, the stripe scenario for forming the 40-meV peak in the q integrated susceptibility χ _2D ^″ (ω), strong renormalization of the spin-excitation spectrum (due to finite damping), and differences of the approach from the linear-spin-wave method are discussed.