Magnetic ground state and spin waves in A(A = K, Rb or Cs)Fe1.5Se2

We present theoretical results on the ground state phase diagram, spin waves and dynamic structure factor on the J1–J2 model. In the reasonable physical parameter region corresponding to AFe1.5Se2, the A-collinear antiferromagnetic phase is stable. The spin wave spectra have two acoustic branches and four optical branches for this phase on the rhombus-ordered vacancy lattice, and each of them is twofold degenerate. However, they have one nondegenerate acoustic branch and two nondegenerate optical branches on the square-ordered vacancy lattice. To offer the theoretical guidance for the further experiments, we also discuss the magnetic excitation spectra and the inelastic neutron scattering pattern based on linear spin wave theory. -- Highlights: ► We have obtained the ground state phase diagram of the J1–J2 Heisenberg model. ► The A-collinear antiferromagnetic phase is stable for AFe1.5Se2. ► Spin wave and dynamic structure factor are investigated with the spin wave theory. ► The obtained dynamic structure factor can offer some guidance for future experiments.