Spin susceptibilities for the Hubbard model in a band approach

The properties of spin excitations superposed on a uniform ground state wit h antiferromagnetic (or spiral) spin structure are studied in a 2D Hubbard model. Expressions are derived for the spin susceptibility in the random ph ase approximation (RPA) using split Hubbard bands as a zeroth approximation . The calculated collective modes with dispersion omega(Q)=c\Q-(pi,pi)\ nea r Q similar to(pi,pi) reproduce well the characteristics of the spin excita tions observed in undoped cuprates. For doped systems with an antiferromagn etic structure of the ground state, calculating chi"(Q,omega --> 0) gives t he same mode with a peak at Q similar to(pi,pi), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure, chi"(Q,omega --> 0) peaks occur with incommensurate quasim omenta Q that are coupled to the spirality vector. (C) 1999 American Instit ute of Physics. [S1063-7761(99)02109-5].