Preliminary Tc Calculations for Iron-Based Superconductivity in NaFeAs, LiFeAs, FeSe and Nanostructured FeSe/SrTiO3 Superconductors

Abstract

Many theoretical models of iron-based superconductors (IBSC) have been proposed, but the superconducting transition temperature (Tc) calculations based on these models are usually missing. We have chosen two models of iron-based superconductors from the literature and computed the Tc values accordingly; recently two models have been announced which suggest that the superconducting electron concentration involved in the pairing 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. We use bulk FeSe, LiFeAs and NaFeAs data to calculate the Tc based on these models and test if the combined model can predict the superconducting transition temperature (Tc) of the nanostructured FeSe monolayer well. 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 scientists are still searching for a universal DFT functional that can describe the pairing mechanism of all iron-based superconductors, we base our model on the ARPES data to propose an empirical combination of a DFT functional for revising the electron–phonon scattering matrix in the superconducting state, which ensures that all electrons involved in iron-based superconductivity are included in the computation. Our computational model 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. This 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. More importantly, by taking into account the interfacial effect between an FeSe monolayer and its SrTiO3 substrate as an additional gain factor, our calculated Tc value is up to 91 K and provides evidence that the strong Tc enhancement recently observed in such monolayers with Tc reaching 100 K may be contributed from the electrons within the ARPES range.

Phase diagram simulations incorporating the gap anisotropy with AFM spin and charge density wave under spin-orbital coupling in Fe-based superconductors

“…When compared to our old IBSC model, 34 our current IBSC model has significantly upgraded. The old IBSC model 34 was a preliminary version that represented our best effort available at that time. However, the model has been undergoing development and gaining insights over time.…”

“…However, the model has been undergoing development and gaining insights over time. There were five limitations in the old IBSC model 34 : (1) it did not take into account the electrostatic screening effect when revising the electron concentration within the ARPES range; (2) it did not optimize the combined effects of antiferromagnetic fluctuation and SDW; (3) it also assumed s-wave pairing without conducting a thorough analysis of the momentum dependence of the superconducting gap that could arise from nematicity; (4) the effects of spin-orbital coupling and higher-order spin fluctuations were not considered either; and (5) it involved a multiplication of the exchange factor by the renormalized pairing strength empirically where the exchange factor should have been multiplied by the original pairing strength before renormalization. In contrast, here, our current IBSC model has addressed these 5 limitations.…”

Upper Critical Field and Tunneling Spectroscopy of Underdoped Na(Fe,Co)As Single Crystals

“…Since electron-phonon coupling seems rather weak in Fe-based superconductors [53], we did not use the phonon renormalization term (1 5d) for a moderate Coulomb repulsion with a reasonable µ * e f f = 0.134 (this value is similar to that calculated in [54] for NaFeAs). Free choice of µ * > 0.2 seems hardly eligible unless there is a specific reason.…”