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

TL;DR

This study uncovers a 'forbidden' phonon mode in iron chalcogenides, revealing dynamic local symmetry breaking linked to bond-order wave fluctuations, even in phases without static order or structural transitions.

Contribution

It demonstrates the existence of a forbidden phonon mode across different phases, indicating dynamic symmetry breaking due to magneto-orbital bond-order wave fluctuations in iron chalcogenides.

Findings

Forbidden phonon observed in all phases.

Mode absent in high-temperature tetragonal phase.

Presence of mode in superconducting FeTeSe without static order.

Abstract

Investigation of the inelastic neutron scattering spectra in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ near a signature wave vector $\mathbf{Q} = (1,0,0)$ for the bond-order wave (BOW) formation of parent compound Fe$_{1+y}$Te [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 Fe$_{1+y}$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 FeTe$_{0.55}$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.