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 (T_c) calculations based on these models are usually missing. We have chosen two models of iron-based superconductors from the literature and computed the T_c 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 T_c based on these models and test if the combined model can predict the superconducting transition temperature (T_c) 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 T_c 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 T_c value is up to 91 K and provides evidence that the strong T_c enhancement recently observed in such monolayers with T_c reaching 100 K may be contributed from the electrons within the ARPES range.

Keywords: iron-based superconductivity.

Figure 2

(a) The theoretical and experimental [44] T_c values of NaFeAs. (b) The antiferromagnetically assisted electron–phonon coupling on the Fermi surface and the AF energy as a function of pressure. The DFT parameter can be found in Table 1.

Figure 3

(a) The theoretical and experimental [47] T_c values of LiFeAs are consistent. (b) The antiferromagnetically assisted electron–phonon coupling and the AF exchange Hamilton under pressure. $R_{SDW}^2$ equals 1.75 at 0 GPa.

Figure 4

(a) Both theoretical and experimental [24] T_c values increase with pressure. (b) The pressure dependence of the antiferromagnetically assisted electron–phonon coupling and the AF interaction. $R_{SDW}^2$ at 0 GPa is 1.59.

Figure 1

The theoretical T_c of NaFeAs varies slightly with the Coulomb pseudopotential. Our calculated µ*-value of the uncompressed NaFeAs is 0.13.

Figure 5

The local region of the unit cell (11 atoms per unit cell). A FeSe monolayered film is deposited on a SrTiO₃ monolayer to form a composite. The vacuum distance D above the composite is ~52 Å. Our theoretical T_c values are shown after the amplifications of interfacial F-K phonon, Coh factor and ARPES (or R_g factor) [21,26,31].