Using a generalized random phase approach quantum fluctuations beyond the helical mean-field states of the Hubbard model off half filling are discussed. The collective mode spectrum is shown to exhibit spin-charge mixing and is found to be strongly renormalized by coupling to low-energy incoherent particle-hole excitations. Results for the collective mode dispersion which displays a significant anisotropy are presented for selected helical phases. Moreover phase space regions of ‘missing’ modes are identified with a finite wave length instability of the helical states suggesting novel inhomogeneous phases of the doped Hubbard model.
P.A.C.S. : 71.27.+a, 75.30.Fv, 75.40.Gb