Neutron scattering study of novel pseudogap excitations in the high-temperature superconductor HgBa$_{2}$CuO$_{4+\delta}$*

Following the discovery of universal novel magnetic order in the pseudogap phase of the cuprates [B. Fauqu\'{e} et al. PRL 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008)], our inelastic neutron scattering measurements of HgBa$_{2}$CuO$_{4+\delta}$ revealed two weakly-dispersive excitation branches that appear to be associated with this ordered state [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012)]. Here we report new results on the doping and momentum dependence of the excitations. The momentum-transfer dependence of the mode intensities is inconsistent with traditional magnetic or structural form factors. Polarized neutron scattering results suggest a possible dual magnetic and structural nature of the excitations