Eur. Phys. J. B (2012) 85: 187
https://doi.org/10.1140/epjb/e2012-20512-x

Regular Article

Excitations in a modified Shastry-Sutherland model

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China

^a e-mail: htwang@aphy.iphy.ac.cn

Received: 25 June 2011
Received in final form: 26 March 2012
Published online: 6 June 2012

Abstract

A modified Shastry-Sutherland model with fourth-nearest-neighbor dimerization is studied with strong coupling perturbation theory. One-triplet excitations are calculated up to the second order. Bose-Einstein condensation of one-triplet excitations may induce different ordered states, i.e. collinear antiferromagnetic state, Néel state or incommensurate state when the gap closes by changing the physical parameters or by an external magnetic field. Two-triplet bound states in the S = 1 and S = 2 channels are shown to exist around the symmetric lines of K_x =  ±K_y and K_x ± K_y =  ±π. For ferromagnetic dimer-dimer interaction, the energies of the bound states in the S = 1 (S = 2) channel lie above (below) the two-triplet continuum. By calculating the existence region, the degeneracy, the symmetry and the dispersion relation, a whole image and main features of the bound states are obtained. Experimental results of (CuX)LaNb₂O₇ with X = Cl, Br are interpreted.