``Forbidden'' phonon in the iron chalcogenide series*

Recently, we uncovered evidence for the formation of a bond-order wave (BOW) leading to ferro-orbital order at low temperature, acting to stabilize the bicollinear AFM order, in the iron-rich parent compound, Fe$_{1+y}$Te [D. Fobes \textit{et al}., Phys. Rev. Lett. 112, 187202 (2014)]. Investigating the inelastic spectra centered near (100) in Fe$_{1+y}$Te, a signature peak for the BOW formation in the monoclinic phase, we observed an acoustic phonon dispersion in both tetragonal and monoclinic phases. While a structural Bragg peak accompanies the mode in the monoclinic phase, in the tetragonal phase Bragg scattering at this \textbf{Q} is forbidden by symmetry, and we observed no elastic peak. This phonon mode was also observed in superconducting FeTe$_{0.6}$Se$_{0.4}$, where structural and magnetic transitions are suppressed. LDA frozen phonon calculations suggested that this mode could result from a spin imbalance between neighboring Fe atoms, but polarized neutron measurements revealed no additional magnetic scattering. We propose that this ``forbidden'' phonon mode may originate from dynamically broken symmetry, perhaps related to the strong dynamic spin correlations in these materials.