Achille Mauri (EPFL)

Salle 523, couloir 12-13, 5è étage

Excitations in triangular-lattice antiferromagnets near the Ising limit

Recent experimental studies have stimulated an extensive interest on the excitation spectrum of spin-1/2 triangular magnets with XXZ magnetic anisotropy. Among the materials which were recently investigated, the cobaltite K2Co(SeO3)2 was demonstrated to realize an XXZ triangular magnet with an extremely strong degree of easy-axis anisotropy, implying that the compound is located very close to the limit of a quantum Ising model with transverse exchange. Motivated by the inelastic neutron scattering studies on this compound, this presentation will report on a theoretical analysis of the excitation spectrum in the "up-up-down" and in the low-field phase of the model, focusing on the Ising limit in which the longitudinal exchange Jzz is much larger than the transverse exchange Jxy. In the “up-up-down” phase, stabilized by a c-axis oriented field, we study the magnon excitations, both in the framework of spin wave theory and within a perturbative expansion in the anisotropy parameter α = Jxy/Jzz ≪ 1. We show that the linear-spin wave (LSW) approximation, although exact at leading order in α, severely underestimates the coefficients of the higher-order corrections in α. It will be shown that this discrepancy explains the deviations between LSW and scattering data observed in the up-up-down phase. The presentation will then discuss the spectrum in the case of zero field, for which the system is characterized by "spin-supersolid" long-range order. We analyze the first-order non-linear spin wave corrections to the linear spin wave theory, and show that the 1/S nonlinearities do not provide a simple framework for explaining the anomalous spectral features observed experimentally in K2Co(SeO3)2.