Dielectric instability and local anharmonic model in the Theory of high- Tc superconductivity

The possible instabilities in high T_c superconductors are analyzed in the frame of the model considering the electron Hubbard correlations as well as the local anharmonic vibrations of apex oxygen O _IV. The calculation of the dynamic susceptibility which is performed in the generalized random phase approximation detect, besides the instability with respect to charge ordering at q = ( {π}/{a}, {π}/{a}) the dielectric instability at q = 0. The type of instability depends on the electron concentration and Hamiltonian parameter values.