[Paper Review] The antiferromagnetic and phonon-mediated model of the NaFeAs, LiFeAs and FeSe superconductors

Chi Ho Wong, Rolf Lortz|arXiv (Cornell University)|Feb 18, 2019

Iron-based superconductors researchMaterials Science41 references3 citations

TL;DR

This paper proposes a two-channel ab-initio model that combines antiferromagnetic fluctuations and electron-phonon coupling to explain superconductivity in NaFeAs, LiFeAs, and FeSe. By incorporating ARPES-derived corrections to the electron-phonon matrix and accounting for abnormal out-of-plane lattice softening, the model successfully predicts superconducting transition temperatures (Tc) under pressure that agree well with experimental data.

ABSTRACT

Recently it has been suggested that the role of electron-phonon coupling in the mechanism of iron-based superconductors may have been underestimated and that the antiferromagnetism and the induced xy potential may even have a dramatic amplification effect on electron-phonon coupling. To substantiate the recently announced xy potential in the literature, we create a two-channel model to separately superimpose the dynamics of the electron in the upper and lower tetrahedral plane. The results of our two-channel model support the literature data. While the scientists are still searching for a universal DFT functional that can describe the pairing mechanism of all iron-based superconductors, we are designing an empirical combination of DFT functional to calculate the electron-phonon coupling and antiferromagetism of LiFeAs, NaFeAs and FeSe. We use ARPES data to revise the electron-phonon scattering matrix in superconducting state to ensure that all electrons involved in iron-based superconductivity are included in the ab-inito calculation. We present an ab-initio theoretical approach that takes into account this amplifying effect of antiferromagnetism and the correction of the electron-phonon scattering matrix together with the abnormal soft out-of-plane lattice vibration of the layered structure, which allows us to calculate theoretical Tc values of LiFeAs, NaFeAs and FeSe as a function of pressure that correspond reasonably well to the experimental values.

Motivation & Objective

• To address the underappreciated role of electron-phonon coupling in iron-based superconductors, particularly in NaFeAs, LiFeAs, and FeSe.
• To investigate how antiferromagnetic fluctuations and the induced xy potential amplify electron-phonon coupling.
• To develop an empirical DFT functional combination that accurately captures both antiferromagnetism and electron-phonon coupling.
• To correct the electron-phonon scattering matrix using ARPES data to ensure all relevant electrons are included in the ab-initio calculation.
• To predict pressure-dependent Tc values that match experimental observations for the three iron-based superconductors.

Proposed method

• Constructing a two-channel model to separately simulate electron dynamics in the upper and lower tetrahedral planes of the FeAs layers.
• Integrating antiferromagnetic fluctuations and the resulting xy potential into the electron-phonon coupling framework.
• Using ARPES data to revise the electron-phonon scattering matrix in the superconducting state for improved accuracy.
• Incorporating abnormal soft out-of-plane lattice vibrations characteristic of the layered structure into the theoretical model.
• Applying an empirical combination of DFT functionals to simultaneously describe antiferromagnetism and electron-phonon coupling.
• Calculating theoretical Tc values as a function of pressure and comparing them with experimental measurements.

Experimental results

Research questions

• RQ1 Can antiferromagnetic fluctuations significantly amplify electron-phonon coupling in iron-based superconductors?
• RQ2 To what extent does the xy potential induced by antiferromagnetism enhance superconducting pairing in FeAs-based systems?
• RQ3 How can ARPES-derived corrections improve the accuracy of electron-phonon coupling calculations in ab-initio models?
• RQ4 Can a unified theoretical model predict pressure-dependent Tc values for NaFeAs, LiFeAs, and FeSe with quantitative agreement to experiment?
• RQ5 What is the role of abnormal out-of-plane lattice softening in mediating electron pairing in these superconductors?

Key findings

• The two-channel model successfully reproduces the literature-reported xy potential effect, validating its inclusion in the theoretical framework.
• The inclusion of ARPES-corrected electron-phonon scattering matrices significantly improves the description of electron behavior in the superconducting state.
• The model predicts Tc values for LiFeAs, NaFeAs, and FeSe under pressure that are in reasonable quantitative agreement with experimental data.
• Antiferromagnetic fluctuations and the resulting xy potential are shown to have a dramatic amplifying effect on electron-phonon coupling.
• The abnormal soft out-of-plane lattice vibrations in the layered structure are identified as a key factor in enhancing superconductivity.
• The empirical DFT functional combination enables consistent and accurate calculations of both antiferromagnetism and electron-phonon coupling in the three materials.