Triangular XXZ antiferromagnets near the Ising limit: excitation spectrum in plateau and supersolid phases
by
WHGA/121
Recent experiments have uncovered that the cobaltite K2Co(SeO3)2 realizes a spin-1/2 triangular XXZ model with an extremely strong easy-axis anisotropy, close to the ideal limit of a quantum Ising model. The experimental observation of neutron scattering spectra in this compound have stimulated an extensive interest on the theory of magnetic excitations in the corresponding quantum Ising model. This presentation will report on a theoretical analysis of magnon excitations in the "up-up-down" and in the low-field phase of the model. In the “up-up-down” plateau phase, the magnon spectrum is analyzed via a perturbative expansion in the anisotropy parameter α = Jxy/Jzz ≪ 1. It is shown that the linear-spin wave (LSW) approximation is exact at leading order in α, but severely underestimates the coefficients of the higher-order corrections in α. The second-order term in α, in turn, are shown to contribute significantly to the spectrum observed experimentally. The presentation will then discuss the spectrum at zero field in the framework of nonlinear spin wave theory. In the limit α≪ 1, S-> ∞, the spin-wave expansion reduces to a soft-core boson model with quartic interactions on the honeycomb lattice. The spectrum is analyzed using a renormalized one-loop approximation and is discussed in connection with the anomalous spectral features observed experimentally in K2Co(SeO3)2.
Laboratory for Theoretical and Computational Physics