Phonon spectrum of single-crystalline FeSe probed by high-resolution electron energy-loss spectroscopy

Highlights: • The phonon spectrum of β–FeSe(001) is probed by high-resolution inelastic electron scattering. • All three A_1g, B_1g, and A_2u phonon modes are observed at energies of about 20.5 and 25.6 and 40 meV, respectively. • The results suggest that, similar to the other Fe-based materials, magnetism has a profound impact on the lattice dynamics of β–FeSe. - Abstract: Utilizing high-resolution electron energy-loss spectroscopy (HREELS) we measure the phonon frequencies of β–FeSe(001), cleaved under ultra-high vacuum conditions. At the zone center ($\overline{\Gamma }$–point) three prominent loss features are observed at loss energies of about ≃ 20.5 and 25.6 and 40 meV. Based on the scattering selection rules we assign the observed loss features to the A_1g, B_1g, and A_2u phonon modes of β–FeSe(001). The experimentally measured phonon frequencies do not agree with the results of density functional based calculations in which a nonmagnetic, a checkerboard or a strip antiferromagnetic order is assumed for β–FeSe(001). Our measurements suggest that, similar to the other Fe-based materials, magnetism has a profound impact on the lattice dynamics of β–FeSe(001).