Phonon dispersion relation of single-crystalline β-FeSe

摘要

We report on the phonon spectrum probed at the β-FeSe(001) surface by means of high-resolution electron energy-loss spectroscopy (HREELS). Single crystals of β-FeSe are cleaved under ultrahigh-vacuum conditions and are subsequently measured below and above the nematic transition temperature. In total we observe five phonon modes and a phonon cutoff energy of about 40 meV. We identify the origin of each phonon mode based on the selection rules of HREELS and by comparing the experimental results to those of ab initio density functional calculations. The most prominent phonon modes A_(1g), B_(1g), and A_(2u) appear at energies of about 20.5 and 25.6 and 40 meV, respectively. These phonon modes disperse rather weakly while changing the momentum from zero up to the zone boundary, indicating that they are mainly of optical nature. A comparison between our results and the results of ab initio calculations indicates that there must be a mutual interplay between magnetism and lattice dynamics in this compound, similar to the other Fe-based superconductors. Finally, we comment on the role of temperature on the phonon modes probed at the f point. It is observed that both the B_(1g) and A_(2u) phonon modes undergo a downward shift while increasing the temperature from 15 to 300 K. In the case of the A_(2u) mode this shift is about 1.5 meV.