Forbidden phonon: Dynamical signature of bond symmetry breaking in the iron chalcogenides

Investigation of the inelastic neutron scattering spectra in ${\mathrm{Fe}}_1+y{\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{Se}}_x$ near a signature wave vector $\mathbf{Q}=(1,0,0)$ for the bond-order wave (BOW) formation of parent compound ${\mathrm{Fe}}_1+y\mathrm{Te}$ [D. Fobes et al., Phys. Rev. Lett. 112, 187202 (2014)] reveals an acoustic-phonon-like dispersion present in all structural phases. While a structural Bragg peak accompanies the mode in the low-temperature phase of ${\mathrm{Fe}}_1+y\mathrm{Te}$, it is absent in the high-temperature tetragonal phase, where Bragg scattering at this $\mathbf{Q}$ is forbidden by symmetry. Notably, this mode is also observed in superconducting ${\mathrm{FeTe}}_0.55{\mathrm{Se}}_0.45$, where structural and magnetic transitions are suppressed, and no BOW has been observed. The presence of this ``forbidden'' phonon indicates that the lattice symmetry is dynamically or locally broken by magneto-orbital BOW fluctuations, which are strongly coupled to lattice in these materials.