ENERGY SPECTRUM OF THE TRIANGULAR XXZ HEISENBERG MODEL REALIZED IN BA_3COSB_2O_9

GHIOLDI, E. A.; A. MEZIO; L.O. MANUEL; J. OITMAA; R. R. P. SINGH; A. E. TRUMPER

Conferencia; 27th international conference on low temperature physics; 2014

The compound Ba_3CoSb_2O_9 is an experimental realization of the triangular-lattice antiferromagnetic XXZ Heisenberg model. Motivated by recent neutron scattering experiments that show an unusual extended continuum above the spin wave excitations [1] we investigate the energy spectrum of the model using series expansions and mean field Schwinger bosons. The low energy part of the spectrum computed with series expansions gives a strong downward renormalization with respect to linear spin wave theory along with the appearance of rotonic excitations at the middle edges of the Brillouin zone. These excitations, already found in the isotropic case J_z/J=1, persist for all values of anisotropy $0leq J_z/Jleq 1$; while a flattening of the high energy part of the relation dispersion is observed in the rest of the Brillouin zone. In order to study the high energy part of the spectrum we compute the dynamical structure factor within the mean field Schwinger boson approach. Within this approximation the spectrum is described by mean of a continuum of pairs of spin-$rac{1}{2}$ spinons [2]. In particular, we find that to recover the unusual extended continuum observed in the experiments of $Ba_3CoSb_2O_9$ it is required, at least, an amount of frustration to second neighbors. Our suggestion is supported by the very good agreement between the series expansions and the mean field Schwinger bosons.