Charge dynamics and spin order in doped Hubbard models

Hole motion in an antiferromagnetic (AF) environment is accompanied by the emission of spin wave excitations. Spin-wave shakeoffs are responsible for incoherent contributions to the dynamics of propagating holes. Using a spin-density-wave polaron scheme we calculate the optical conductivity () and show that the incoherent part of the hole spectrum contributes to the low-frequency part of ().Separately, we discuss the possible formation of spiral spin patterns upon doping of the half-filled one-band Hubbard model. In particular, we consider the influence of band structure effects arising from nearest- and next-nearest-neighbor hopping processes on a square lattice. Differences in the ground state spin patterns for hole and electron doping are obtained offering a possible explanation for the persistence of AF order in low electron-doped cuprate superconductors.