Detailed structure of the low-energy magnetic dispersion of the diagonal incommensurate phase in La_1.975Sr_0.025CuO₄

Inelastic neutron scattering experiments have been performed on lightly doped La_1.975Sr_0.025CuO₄, which contains a hole concentration slightly higher than the critical concentration for three-dimensional long-range antiferromagnetic order. We previously found that the magnetic excitation spectrum in the insulating phase with a diagonal incommensurate spin modulation has similarities to that in the superconducting regime, where the spin modulation is bond parallel. In this study, we investigate the excitations in detail around E_cross, at which the excitations become most nearly commensurate. It is found that both the magnitude and the anisotropy of the momentum width of the excitations change abruptly at E_cross. Our experimental results suggest that the magnetic excitations rising from the pair of (diagonally) incommensurate wave vectors merge at E_cross into isotropic excitations.