In a recent experiment on CoNb_2O_6, Coldea et al. [Science 327, 177 (2010)] found for the first time experimental evidence of the exceptional Lie algebra E_8. The emergence of this symmetry was theoretically predicted long ago for the transverse quantum Ising chain in the presence of a weak longitudinal field. We consider an accurate microscopic model of CoNb_2O_6 incorporating additional couplings and calculate numerically the dynamical structure function using a recently developed matrix-product-state method. The excitation spectra show bound states characteristic of the weakly broken E_8 symmetry. We compare the observed bound state signatures in this model to those found in the transverse Ising chain in a longitudinal field and to experimental data.
@article{arxiv.1008.3534,
title = {Bound states and E_8 symmetry effects in perturbed quantum Ising chains},
author = {Jonas A. Kjäll and Frank Pollmann and Joel E. Moore},
journal= {arXiv preprint arXiv:1008.3534},
year = {2011}
}