Dynamical charge susceptibility in layered cuprates: Beyond the conventional random-phase-approximation scheme - art. no. 214512

The analytical expression for a dynamical charge susceptibility in layered cuprates has been derived in the framework of a singlet-correlated band mod el beyond the random-phase-approximation (RPA) scheme. Our calculations per formed near optimal doping regime show that there is a peak in the real par t of the charge susceptibility chi (ch)(q,omega) at Q=(pi,pi) at strong eno ugh intersite Coulomb repulsion. Together with strong maximum in the Im chi (ch)(Q, omega) at low frequencies, it confirms the formation of low-energy collective excitations or charge fluctuations. This justifies that these e xcitations are important and together with spin fluctuations can contribute to the Cooper pairing in layered cuprates. Analyzing the charge susceptibi lity with respect to the instability we obtain a new plasmon branch, w., al ong the Brillouin zone (BZ). In particular, we have found that it goes to z ero around Q approximate to(pi,pi).