Bound states and E8 symmetry effects in perturbed quantum Ising chains

In a recent experiment on CoNb2O6, R. Coldea et al. [Science 83, 177 (2010)] found 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 CoNb2O6 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.