Phonons in doped and undopedBaFe2As2investigated by inelastic x-ray scattering

Abstract

We measured phonon frequencies and linewidths in doped and undoped BaFe 2 As 2 single crystals by inelastic x-ray scattering and compared our results with density functional theory calculations. In agreement with previous work, the calculated frequencies of some phonons depended on whether the ground state was magnetic or not and, in the former case, whether phonon wave vector was parallel or perpendicular to the magnetic ordering wave vector. The experimental results agreed better with the magnetic calculation than with zero Fe moment calculations, except the peak splitting expected due to magnetic domain twinning was not observed. Furthermore, phonon frequencies were unaffected by the breakdown of the magnetic ground state due to either doping or increased temperature. Based on these results we propose that phonons strongly couple not to the static order, but to high frequency magnetic fluctuations.

Phonon Spectra of PrFeAsO1-yvia Inelastic X-ray Scattering

“…The presence of only small changes in phonon spectra resulting from either temperature or doping, is in agreement with work by many other authors using various methods including IXS and inelastic neutron scattering, [14][15][16] nuclear resonant inelastic scattering, 17) Raman, 18) and infrared spectroscopy. 19) The discrepancies exists between the observed data and the ab-initio calculations is smaller in the 'magnetic' calculation than the 'original' non-magnetic one.…”

Phonon softening near the structural transition in BaFe2As2observed by inelastic x-ray scattering

“…This E g mode is a doubly-degenerate mode impacting Fe and As, and the contention is that the split is too large to be accounted for by the orthorhombic distortion and instead is indicative of spin-phonon coupling. Measurements of phonon behavior conducted with inelastic x-ray spectroscopy are replicated well by theory when magnetism is explicitly taken into account 34 , indicating the presence of spinphonon coupling.…”

“…Since the acoustic phonon frequency goes as the square root of the elastic constant, we expect a softening of the lowenergy acoustic phonon modes of BaFe 2 As 2 when passing through the structural transition temperature. Phonon calculations predict a linear dispersion in the low-energy, long wavelength regime 30,34 . The acoustic phonon modes in BaFe 2 As 2 can be characterized with energy transfers of less than 10 meV, and the small and constant momentum resolution offered by IXS allows precise measurements of the phonon dispersion as a function of temperature.…”

Electron–phonon properties of pnictide superconductors

“…[38]. It was later shown that the inclusion of a static AFM order leads indeed to a softening the Fe-As spring constant, and improves the agreement of the predicted crystal structure and phonon frequencies with experiment [42]; however, this cannot explain the softening of c-polarized As modes, which form a distinct peak at 20 meV, at an energy 20 % lower than predicted by calculations, which has been observed both in 1111 and 122 samples, and has been attributed to anomalous e − ph coupling [39,41].…”