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

Abstract

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 neq Q =(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-Tc cuprates is discussed through a comparison with magnetic excitations in YBa_2Cu_3O_y.