  The ground state of a hole-doped t-t'-J ladder with four legs favors a
striped charge distribution. Spin excitation from the striped ground state is
known to exhibit incommensurate spin excitation near q=(pi,pi) along the leg
direction (qx direction). However, an outward dispersion from the
incommensurate position toward q=(0,pi) is strong in intensity, inconsistent
with inelastic neutron scattering (INS) experiment in hole-doped cuprates.
Motivated by this inconsistency, we use the t-t'-J model with m x n=96 lattice
sites by changing lattice geometry from four-leg (24x4) to rectangle (12x8)
shape and investigate the dynamical spin structure factor by using the
dynamical density matrix renormalization group. We find that the outward
dispersion has weak spectral weights in the 12x8 lattice, accompanied with the
decrease of excitation energy close to q=(pi,pi), being consistent with the INS
data. In the 12x8 lattice, weakening of incommensurate spin correlation is
realized even in the presence of the striped charge distribution. For further
investigation of geometry related spin dynamics, we focus on direction
dependent spin excitation reported by recent resonant inelastic x-ray
scattering (RIXS) for cuprate superconductors and obtain a consistent result
with RIXS by examining an 8x8 t-t'-J square lattice.
