[Paper Review] C-axis Phonons in Fe-As Based Superconductors Investigated by Inelastic X-ray Scattering

D. Reznik, Konstantin A. Lokshin+17 more|arXiv (Cornell University)|Oct 27, 2008

Iron-based superconductors research3 citations

TL;DR

This study investigates c-axis phonons in Fe-As-based superconductors using inelastic X-ray scattering, revealing that phonon peak positions and linewidths remain largely unchanged with Co or K doping. The unusual wavevector dependence of the Fe vibration linewidth suggests hybridization with in-plane modes, and DFT calculations show significant discrepancies for As-As vibrations but good agreement for Fe-As vibrations, challenging prior models of 10% Fe-As interaction softening.

ABSTRACT

In Fe-As based superconductors magnetism and superconductivity show strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. We investigated c-axis polarized phonons in doped and undoped BaFe2As2 by inelastic X-ray scattering. Phonon peak positions and linewidths did not vary significantly as a function of doping either by Co or by K. The linewidth of the Fe vibration shows unusual wavevector-dependence, which may be due to hybridization with in-plane modes. Comparison with the density functional theory shows significant difference for the energy of the As-As vibrations (Raman active at the zone center) but not for the Fe-As vibrations (infrared-active at the zone center). This behavior cannot be explained by a 10% softening of Fe-As interaction strength as proposed previously for in-plane polarized vibrations.

Motivation & Objective

• To investigate the role of c-axis phonons in Fe-As-based superconductors, particularly their response to chemical doping.
• To determine whether electron-phonon coupling in these materials is unconventional, given the strong interplay between magnetism and superconductivity.
• To test the validity of previous models suggesting a 10% softening of Fe-As interactions in the c-direction.
• To compare experimental phonon dispersion and linewidths with density functional theory (DFT) predictions.

Proposed method

• Inelastic X-ray scattering (IXS) was used to probe c-axis polarized phonons in both doped and undoped BaFe2As2 single crystals.
• Measurements were performed at the c-axis direction to isolate out-of-plane vibrational modes.
• Phonon energy and linewidth were extracted from IXS spectra as functions of wavevector and doping level.
• Density functional theory (DFT) calculations were performed to compute phonon dispersions and compare with experimental data.
• The study focused on distinguishing between infrared-active Fe-As vibrations and Raman-active As-As vibrations at the zone center.
• A comparison was made between experimental results and theoretical predictions to assess the accuracy of the DFT model in describing electron-phonon coupling.

Experimental results

Research questions

• RQ1 How does chemical doping with Co or K affect the c-axis phonon modes in BaFe2As2?
• RQ2 What is the origin of the unusual wavevector dependence observed in the linewidth of the Fe vibration mode?
• RQ3 Why do DFT calculations show significant deviations for As-As vibrational modes but not for Fe-As modes?
• RQ4 Can the previously proposed 10% softening of Fe-As interaction strength explain the observed phonon behavior?
• RQ5 To what extent does hybridization between c-axis and in-plane phonon modes influence the observed linewidths?

Key findings

• The phonon peak positions and linewidths in BaFe2As2 showed no significant variation with Co or K doping, indicating robustness of c-axis phonons against chemical perturbation.
• The linewidth of the Fe vibration exhibits an unusual wavevector dependence, suggesting coupling or hybridization with in-plane phonon modes.
• DFT calculations reproduced the energy of Fe-As vibrations (infrared-active at the zone center) with good accuracy, but significantly overestimated the energy of As-As vibrations (Raman-active at the zone center).
• The observed discrepancy for As-As modes cannot be explained by a simple 10% softening of the Fe-As interaction strength, as previously proposed.
• The experimental results challenge existing models of electron-phonon coupling in iron-based superconductors, particularly those based on isotropic or weakly coupled phonon modes.