Magnetic excitations in La-based cuprate superconductors:: Slave-boson mean-field analysis of the two-dimensional t-J model

Motivated by recent inelastic neutron scattering experiments up to a high-energy region for La-based cuprates, we compute (q,omega) maps of the imaginary part of the dynamical magnetic susceptibility chi(q,omega) in the slave-boson mean-field approximation to the two-dimensional t-J model. While the strong spectral weight appears at incommensurate positions, namely, at q not equal Q equivalent to(pi,pi), for low energy, the incommensurate signals disperse with increasing omega and finally merge into a commensurate signal at a particular energy omega=omega(Q). These features are seen in both the d-wave pairing state and the normal state. In particular, the incommensurate signals below omega(Q) in the normal state are due to the Fermi surface geometry, which we expect for La-based cuprates because of a tendency to d-wave type Fermi surface deformations. Above omega(Q), strong signals appear to trace an upward dispersion especially for a low doping rate in the d-wave pairing state while typically broad spectral weight is obtained around q=Q in the normal state. Salient features of magnetic excitations in La-based cuprates are thus naturally captured in terms of particle-hole excitations. Global understanding of magnetic excitations in high-T-c cuprates is discussed through a comparison with magnetic excitations in YBa2Cu3Oy.